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Engine Building for Forced Induction

 - All upgrades to an engine will require careful planning.
 - Serious power increases will always require engine modification.
 - Engine reliability should always come before power on a road car.

Upgrading a forced induction engine or converting an NA engine into a forced induction unit requires careful planning and will involve some form of engine modification. The higher the boost levels the more consideration that needs to be given to the engine.  Engine reliability and integrity are a major factor and can’t be underestimated. When Toyota add a blower to their engines, even with mild boost wheels, they strengthen their engines in various ways despite the need to build to a budget.

With forced induction, small engines are producing higher output of power and are subject to higher mechanical and thermal stresses. Even when the heat and pressure in the cylinders isn’t much higher, they stay to the peak for much longer on the crank stroke. Parts like the cylinder head, valves, head gasket, pistons, rings and upper cylinder area will be subject to a greater heat load as there will be less time to shed combustion heat. Con rods are the exception. They are generally put under more load on naturally aspirated engines. But on excessive boost, these parts must also be considered. 

These same components will suffer the normal distortions, but the distortion will be held for longer so they may be inclined to remain distorted. An out of shape piston creates more friction against the cylinder wall. So along with the additional heat due to heat soak, it is also generating heat by contacting the cylinder wall. It may appear to be easy to fix these type of problems by reverting to stronger, heavier internal engine components but this will introduce its own set of problems. If we retain the same rev limit, then the heavier components will place heavier loads on the piston pins, con rods and con rod bolts and to a lesser extent on the crank.

A lot of supercharged engines suffer detonation. This is normally blamed on excessive boost when in fact it is caused by ineffective piston ring seal. The blow-by caused due to ineffective rings will blow into the crankcase and disturb the lubricating oil on the cylinder face, spraying a mist of oil into the inlet track, thus lowering the octane rating of the mixture, leading to detonation. It also forces the rings away from the cylinder wall causing a further build-up of heat on the piston, further encouraging detonation. 

If the engine is designed and built properly, with attention to every detail then reliability won’t be an issue. 

Cylinder Block Preparation and Crank Testing.

The cylinder block must be addressed first and foremost. Inspect the water passages for rust and scale. Boil and clean in a chemical bath with all the Welch and oil gallery plugs removed. Finish the oilways and stud holes with brushes. Clean and dry with high pressure air. Green blocks (new) should get the same treatment, ensuring the water passages are free of casting sand.

After this visually check for cracks and do a pressure crack test with the Welch plugs attached and the head in place. All water passages should be blocked off. Gradually build the air pressure to 40psi, watching out for Welch plugs or other items flying out under pressure. Hold the pressure for 5min. If its holding firm then go to 50psi. Anything after this can be dangerous, but if you are using a heavy duty, thick wall race block and you’ve taken the right precautions (protecting yourself from projectiles) then continue up to 100psi. Slowly spray WD-40(or soapy water) over the entire block, looking for air bubbles.

If you find a crack that is small and not in a critical area then it can be repaired easily. Otherwise you will have to junk the block and get another one. However, some blocks routinely crack in certain areas and its better to have a repaired block rather than having one that hasn’t cracked, but probably will as soon as its subjected to competition use. If you are aware of a point on the crank that does give trouble during competition use then it’s a good idea to a pre-emptive fix by reinforcing this area.

After successful pressure testing, clean the head stud and main bearing cap threads of dirt and burrs using a plug tap. This is essential so that you get a correct tension reading when rebuilding. Chamfer any holes that need it so that threads don’t pull up, causing head gasket sealing problems. A pulled main bearing cap thread can let a bearing spin. Check the depth of each hole to make sure the bolt won’t bottom out. Grind away any casting slag, especially around the main bearing webs, the sump pan deck. This will minimise the risk of localised expansion and contraction and therefore, cracks.

Main Bearings.

Next the block must be squared. The main bearing holes should be perfectly aligned otherwise the bearings and even the crank will be destroyed, and at best it will rob the engine of power due to frictional losses. So not assume that the alignment is correct, especially if you plan to exceed the previous rpm limit.Any miss-alignment can be corrected by line boring. This will also be necessary if the main bearing caps are replaced by heavy duty items. The main bearing caps should be numbered and have the front marked. This will help to correctly align them every time the engine is stripped.
After this check that the top of the block (deck) and the crankshaft centreline are identically spaced from left to right (that they are parallel). If the block is out, then milling will be required. 

Cylinder Boring.

Obviously the cylinder walls must be exactly 90deg to the centreline of the crank and must not be canted to the front or the read, to keep frictional losses at a minimum and to maintain good ring sealing. There cant be any steps of tapers at the top or bottom of the bore.

Boring and honing to 0.0003in tolerance must be carried out. Small boring machines that bolt onto the deck are not acceptable. The machinist cannot setup the boring bar based on the existing position of the cylinders. A bore index plate must be bolted the block as a reference to locate the bore centres correctly. This ensures that the cylinders are centred over the crank throws. This also allows different heads to be swapped between blocks with the combustion chambers correctly matching the bores. Only 0.01in should be removed with each cut. This will cost more in labour, but it will be easier to accurately hone because the boring tool marks won’t be as rough.

Cylinder Walls.
Pressure and heat will warp the cylinder walls if they are overbored too much. This will lead to blow-by past the rings. At high power outputs the main bearing webs can break away from the bottom of the cylinders.

Normally we only want to bore the cylinders to rectify damage or sloppy factory boring. We are not seeking to increase the capacity of the engine. There is no point in gaining a few CC’s if it means the power will be robbed through wall flexing and warping. Most modern cast blocks only have a wall of 0.1-0.15in when finished with a standard bore. However, due to casting the wall thickness can be down to 0.07in in places. Added to this the fact that most manufacturers will use soft cast iron in their blocks, such thin walls cant afford to be bored any wider.
For good ring seal a wall of 0.1in is needed if making 70bhp per cylinder. This is especially true at the top half of the cylinder, where ring seal is critical, first to make compression and then to retain cylinder pressure during combustion. Towards the bottom of the cylinder we can afford to drop 0.02in of thickness if necessary. But there are other considerations such as operating rpm, crank stroke, main bearing web strength, detonation inclination.

At 75bhp per cylinder wall thickness should be 0.11in to 0.13in.

At 85bhp per cylinder wall thickness should be 0.15 to 0.18in.

At >100bhp per cylinder wall thickness should be 0.22 to 0.25in.

If you want an engine to seal well at 8500rmp for long periods of hard use then these thicknesses should be used. Some tuners get away with less, but they tend to scrap blocks more frequently and the risk of main bearing web falling away is higher. 

Measuring Cylinder Wall Thickness.
Checking for wall thickness and core shift (due to casting) can be difficult. You can measure the thickness of the web between cylinders and then measure the thickness of the water passage between the cylinders using feeler guages, but this won’t allow for cast bore shift so it is meaningless. Sonic testing can be used to give a reasonably accurate estimate of wall thickness. Care and good judgement must be used to measure at the right spot. Also, this method wont detect porous areas or areas of dirty metal. The measurements taken may also be 0.02in less than indicated. You have to so pressure crack testing along with the sonic test. It is also wise to backup your measurements with basic measuring tools. 

Strengthening Open-Deck Blocks.

Some blocks have more problems than wall thickness. Open deck designs with the top of the cylinder unsupported or lightly supported will experience problems handling extra horsepower and rpms. Cylinder and block flexing can become excessive. For small Hondas, you can fit a block guard by inserting into the water jacket to fit between the block and the cylinder. Some Subaru’s with an open deck can require expensive mods to switch to a closed deck setup. The Rover K-series has a different problem. Even before supercharging, the block can experience problems at 200bhp and above. The cylinder liners can split. The solution is to shrink a steel sleeve around the outside of the liner up towards the top. 
Cylinder Honing.
The last task on a block is to hone the cylinder bores to their finished size. A 2in honing plate should be fitted and torqued to normal head torque levels to the block with a head gasket in place. This is needed to ensure perfect honing (get it made when you are getting the bores bored). The block is then distorted to the same level as when the head is fitted. The main bearing caps should also be bolted on with the correct torque because these also distort the block a bit. The bores should be honed to 0.004in of the finished diameter and then matched to each piston. Keep piston clearance to a tolerance of 0.0003in.

The honing method varies from block to block but the following general procedure should be followed: First use 220 grit stones, remove 0.003in of material to bring the bore to 0.001in of final size. Use 280 grit stones, remove 0.0005in. Use 400 grit stones to bring the bore to its final diameter. The result is a very accurate bore with little break-in time required.

It is important to leave a cross-hatch pattern on the walls. A 45deg cross-hatch pattern must be left with a finish of 10-12 micron. This type of finish means you have to run the rings in, but they will last a long time and will seldom leak. A smoother finish will cause a glaze to form on the ring face and bore wall due to lack of lubrication. Oil consumption and power loss will result. A rougher finish will lessen ring bed-in time, but they won’t last as long and glazing might again be a problem because the rings grind off the walls and raise temperatures which contribute to glaze forming. 

Final Block Preparation.

The upper and lower lip of each cylinder should be lightly chamfered to remove sharp edges. Lightly dull the sharp edges of the main bearing caps and webs as well. The camshaft bearing bores must be in line and each tappet bore must be of the correct bore and perpendicular to the centreline of the crankshaft. A tappet tipped off centre may dig into the lobe of a racing cam causing wear or breakage (pushrod engines).

When available use high quality high tensile bearing studs. If standard main bearing studs are used they must be new. Never reuse. Usually the standard caps are OK, but for very high power engines use steep caps for reliability and peace of mind. In very hot engines use a main bearing support saddle. This is a one-piece item that supports all main bearings and sometimes replaces the individual main bearing caps. It may extend to the oil pan deck and transfer the bearing load so that it is shared by the outside of the block as well as the main bearing webs.

After all the machining, the block needs another cleaning with warm soapy water. Blow the block dry with compressed air and spray all the cylinders, tappet bores, and bearing bores with WD-40. 

Crankshaft Material.

Most engines use cast nodular iron crankshafts. These are OK is low rpm limits and light internals are used. Some of them come from the factory with rolled fillets which adds fatigue resistance. Some are also heat treated by using Tuft riding. But when power and rpm limits increase so does the need for a forged steel crankshaft.
Forging increases the density of the metal-it is squeezed into shape and compacted to give a stronger core and a better fatigue resistance. But it is the type of steel used and the heat treatment received that determines a cranks suitability for competition use. The steel must have a high tensile strength (ability to resist breaking under high loads) and a high fatigue resistance (ability to resist repeated bending and twisting).

The production forged cranks made by Japanese and Euro makers are generally good enough for high output engines. Some will feature rolled fillets and some will be heat treated by the nitriding process. However, steel cranks made from 1053 grade steel need to be avoided, even if nitrided. Also note that some steel cranks are twisted during the manufacturing process (more common on v8 cranks). The front and rear throws are twisted 90deg to give the required crankpin orientation. But often the throws will not be at exactly 90deg and will be 2 or 3deg out. They can still be balanced, but the counterweights required to do this will place undesirable high loads in the high rpm engine. Therefore, non-twisted forged cranks should be used, where the rod journals are forged in place.

Note: most of these bad steel cranks are American (e.g. small block Chevy v8 steel).

Crank Preparation and Balancing.
If the standard crank is to be used it must be crack tested before working on it. If a magnetic type crack test is done, then it must be fully demagnetised. Next check its straightness. If its not perfectly straight then it must be discarded. It can be straightened but will likely revert once high loads are applied to it. Measure each main bearing and crank pin journal. Crank journals wear oval, so check the diameter at several angles. Measure at each end and in the middle of every journal. Ovality and taper should be less than 0.0003in out. If the wear is greater, the journals will have to be ground undersize. But this will weaken the crank so it may have to be replaced. Any casting slag should also be ground off to avoid cracks.

To reduce bearing inertia load the crank should be dynamically balanced. This will reduce the shock loading and vibration that any imbalance will cause. This is only necessary if planning to run the engine above 6000rpm for extended periods, or if heavier than standard internals are to be used. 

WRC and Group A Crankshafts.

Competition cranks are often forged from EN40B steel. In the US 4340 and the inferior 5140 steel is used. Just because high grade steel is used it is no guarantee of quality of strength. The steel may be dirty or the alloy might not be upto spec. Some heat treatments are better than others. The manufacturer may use an inferior heat treatment system so that more batches of cranks survive the process. Such cranks may be cheaper to buy, but they wont be cheaper in the long run if they fail under load.

Lighter cranks will also be more expensive, but they will pay dividends by reducing reciprocating weight and improving acceleration out of the corners and in reduced bearing load. The most expensive and best cranks are machined from a billet of high grade steel that has been hammer forged. After this it is shot-peened and heat treated to further enhance its strength and fatigue resistance.

WRC cranks should be fully counterbalanced – overall and at each crankpin. Balancing each crankpin doesn’t modify the overall balance, but achieves an internal balance for each throw. But if bearings can be replaced frequently and a crack test can be performed regularly, then it might be a good idea not to fully balance the crank when it involves adding counterweights. This will ensure maximum acceleration.

Oilway modifications are undertaken to prevent bearing failure.

From a purely performance standpoint, the crank throws should be of equal lengths and correctly indexed. Slight differences in length from cylinder to cylinder wont make much difference, but incorrect phasing will knock power. If a throw is out by 5deg this will have the same effect on performance as an ignition or camshaft timing error of 5deg. Most cranks will have perfect phasing or indexing, but check to be sure. 

Con Rods.

The rods experience alternating compression and tensile loads. They have a tougher job than any other internal part. A large number of failures are experienced in competition engines due to rod malfunction or failure. The highest load is experienced when the piston is at TDC on the exhaust stroke. The tension can reach 15,000lbs. This maximum load is experienced on the non-firing stroke and is caused by the inertia of the reciprocating assembly (piston, pin, small-end). At TDC the piston is suddenly stopped and reversedproducing the high tensile load. On the compression stroke the load is not as severe because pressure builds up slowly and because soon after TDC the load changes to a light compression load.
There high loads are applied and removed on every stroke. Which is much more severe than continually applying the same load. A rod has to survive millions of stress cycles in its lifetime. Most rods are made from carbon steel. Some are still cast and some are made from titanium or aluminium. Cast iron is to be avoided; titanium can be used if the budget is big. Aluminium rods can be used on drag cars and hill-climbers where they can be replaced regularly. They are light and reasonable strong and deliver less of a shock to the bearings when detonation occurs.

The best rods are made from 4340 or E4340 steel. The standard rod is called an I beam rod and it is very strong. It is manufactured by respected rod manufacturers such as Oliver and Crower. H beam rods are made by Carillo who are the masters in making rods. Their success lies in their use
of the best E4340 steel and in their heating and shot-peening techniques
(at great expense).

Con Rod Ratio.
The con rod ratio is the rod length vs. the crank stroke. Most rod ratios range from 1.5 to 2.1. The average is 1.7. A long rod will cause the piston to dwell at TDC longer and to move away from TDC more slowly. This does not affect the power output of engines with flat top or dish top pistons. It is only on nitrous and NA high compression engines with very high top pistons that combustion will be upset at high power and rpm range.

With shorter rods, engine wear is a problem because increased cylinder wall and piston loadings occur. For this reason, a ratio lower than 1.65:1 should be avoided. A ratio of around 1.8:1 is better. Fitting a long stroke crank will lower the ratio and will increase the tendency of the piston to rock in the cylinder. Pistons with a reduced compression height and shorter skirt must be used with a stroker crank. This aggravates wear and ring seal problems.

Con Rod Bolts.

The tensile strength of the bolt used must be 185,000psi minimum. High quality boron bolts can be reused several times, but standard bolts must never be reused. If the bolts are over tensioned they will fail. Of course they will loosen if they are not tensioned enough. Reaching this preload is not just a matter of using a torque wrench. The bolts and rod threads must be lubricated. The under head area needs to be lubed. Only use the oil recommended by the manufacturer.

If engine oil is recommended and you use diff oil you can end up over tensioning by up to 10lbs. Stick to the bolt tension of the bolt manufacturer even if this is different to the manufacturer’s figures. Use a good torque wrench and tighten the bolts to the correct torque and loosen the bolts twice before finally tightening the bolts… tighten-loosen-tighten-loosen-tighten.

To repeat, never reuse standard bolts and only reuse boron steel bolts three or four times.

Bearing Materials.

Bearings have to provide a low-friction wearing surface while absorbing tremendous shock loads. Only top quality trimetal bearings should be used for the main and bigends. White metal bearings can be used for the camshafts. Vandervall lead-indium and Clevite 77 bearings are best and can stand up to the worst conditions. These bearings are steel backed, with a copper-lead intermediate layer that gives the bearing good fatigue strength and load carrying capacity. The running surface of the Vandervall bearing is a precision overlay of lead-indium, the Clevite 77 overlay is lead-tin.

Bearing clearance is important. Too much clearance causes knocking and pounding and higher frictional losses and increased oil consumption. Excessive clearance at the big ends will lead to oil starvation at the main bearings. Too little clearance will cause rapid bearing failure due to high temps and too thin oil films forming. Generally we should have 0.0009in clearance for each inch of shaft diameter for main bearings and 0.0012in clearance for each inch of shaft diameter of rod bearings when running 15W-50 fully synthetic. With 0W-30 clearance can be a little tighter.

The following is a good guide for trimetal copper lead bearings.

Diameter of Shaft Clearance shaft-bearing Clearance side of bearing
1.5  0.0012 to 0.0017 0.004 to 0.006
2.0 0.0015 to 0.0020 0.005 to 0.007
2.25  0.0018 to 0.0025 0.005 to 0.007
2.5  0.0022 to 0.0027 0.005 to 0.007
2.75 0.0024 to 0.0028 0.006 to 0.008
3.0  0.0025 to 0.0028 0.007 to 0.009
3.25  0.0025 to 0.0030 0.007 to 0.009

With an aluminium alloy block, more expansion takes place so modified clearances must be used. A clearance of 0.002in cold will increase to 0.004in when hot. The engine should be assembled with tighter main bearing clearances when cold. With such tight clearances, the oil and water should be preheated before start-up – on race engines obviously.

Bearing Fitting.

A simple procedure must be followed when fitting bearings.

1. Measure the inside diameter of the bearings and big-end housing without the bearing. Do the same with the crankshaft main and crankpin journals.
2. Wash the bearings in solvent to remove the protective film. Do not damage the overlay, do not use any abrasive material.
3. Do not worry about a rough overlay – it will be flattened after installation.
4. Measure the bearing shell thickness and double it. The difference between the shaft and housing diameters and the doubled shell measurement is the space left for running clearance.
5. When you have calculated the running clearance for each bearing, some might have a little more clearance and others might have a little less clearance. If this is the case, the manufacturer might have 0.001in oversize and undersize bearings available. If you want even less you can mix one bearing with another and get a difference or 0.0005in. If this is the case use the smaller bearing on the top for a main bearing and on the bottom for a big-end.
6. Fit the shells and check oil hole alignment, any misalignment should be corrected with a small file.
7. Coat all the bearings with engine oil and fit the crankshaft.
8. Fit the main bearing caps in the right order and orientation
9. Gradually tighten the bolts.
10. Before final tightening, tap the crank to each side with a soft hammer to line up the bearing caps.
11. Check the crankshaft end float, it should be about 0.005in with a cast iron block. If its bigger than this fit bigger thrust washers.
12. Follow the same procedure for the big-end bearings.

Note the following… When the shells are pushed into their housings they should feel springy and snap into place. If not the bearing housing is distorted or oversize bearings are required to match housings that have been bored oversize. Also, use Loctite on the threads to avoid problems with loose bolts.

Piston Design.

High states of tune demand unslotted cast or unslotted forged pistons. To recognise the piston type… cast pistons have intricate under crown shapes around the gudgeon pin boss, while forged pistons are smooth inside and lack intricate struts and braces. Cast hypereutectic pistons can be a good choice for highly tuned NA engines because they are lighter than forged pistons, but for forced induction engines, forged pistons are the only choice. Forged pistons are much denser, leading to high tensile strength. The can withstand higher pressure  and heat loads and their higher density improves their thermal efficiency to the extent that they run 30degC cooler.

Piston Clearance.
Pistons appear round, but the skirt is actually ground oval and it tapers from bottom to top. Both of these features prevent seizure. In operation the top of the piston is twice as hot as the bottom of the skirt, so it expands more. Also, because of the heat in the pin bosses, the piston is elongated across the piston pin axis. Most pistons are 0.005 to 0.012in less in diameter across the pin axis. Only measure piston clearance across the thrust axis (at the bottom of the skirt or near the pin – check with supplier). High performance engines need more clearance than standard engines. Check with supplier for the correct clearance but in general use a min of 0.002in.

For road with cast pistons and bores of 80-110mm use 0.003in.

For road with forged pistons and use 0.0012 to 0.0015in per inch of bore.

For rally with forged pistons and use 0.0015 to 0.0018in per inch of bore.

However, some JE Forged pistons use just 0.003in clearance.

There should be a max of 0.0005in difference between the tightest and loosest cylinders. If its more than this, try swapping around the pistons in different cylinders. Then mark the position of each piston. Do not mark the crown incase it is machined later. 

Valve to Piston Clearance.

Minimum of 0.06mm vertical clearance. For push rod engines use 0.08in for the inlets and 0.01in for the exhausts. The cut-out diameter for safety is 0.12in greater than the valve head diameter. This can be reduced to 0.05in for engines with no carbon build-up on valves or pistons. 

Piston to Cylinder Head Clearance.

The crown of each piston must rise to the same point in each cylinder (deck height of the piston). A good engine with steel rods can run with 0.04in clearance between the piston and squish area of the head. If the head gasket is 0.03in when compressed, then we can run a deck clearance of 0.01in – all the pistons should then be machined to reach this clearance. When using aluminium rods, allow 0.07in clearance to allow for the additional expansion and stretch. Engines with steel rods, but with block flex or crankshaft whip will require 0.06in clearance. 

Piston Crown Shape.

The top of the piston is a part of the combustion chamber and the profile of the top of the piston influences combustion as well as maximising compression. Some profiles simply maximise compression without regard for compression, combustion and exhaust. Tuners worry about modifying the shape because of lost compression but the gain in flow and better combustion can far exceed the slight drop in compression ratio.

We want the flame to travel smoothly across the piston dome and back towards the squish area. Abrupt edges on the compression lump will disrupt  the flame leaving pockets of unburnt mixture. Rounding the edges of the compression lump helps alleviate this. The exhaust valve cut-out must not be laid back. The sharpness of the exhaust lump also affects flame propagation, but cylinder scavenging and exhaust flow are usually better with the sharp exhaust lump. Combustion completeness can be assessed by examining the coloration of the piston after a few hundred miles of use. The piston dome colour will indicate where flame propagation is being stifled (this method is extremely accurate). Machine the compression domes lightly until the carbon build-up is more even in quantity and colour. It will not be necessary to machine the entire uncoloured area of the lump. Just machine the area in front of and around the start of the uncoloured area – relative to the direction of the flame travel.

Blowers and turbos may require a lower compression ratio to avoid detonation. Conversion from NA will definitely need lowered compression. Dished top pistons should be used for best combustion control. They have a flat band around the outside of the crown that will come into close contact with the head and provide the required squish for good burning. 

Piston Rings.

Some low boost, high rev engines use two rings, but most engines use three rings. The first compression ring contains the combustion pressure and dissipates most of the piston heat. The second compression ring backs up the first, but its primary purpose is to carry more heat from the piston. It also assists the oil ring in scraping off excess oil from the cylinder wall. The bottom oil control ring scrapes oil from the wall and ensures enough oil remains behind to lubricate the upper rings and assist in sealing.

Piston rings cause a good deal of friction and therefore cost power. To give maximum flexibility in choosing rings, the manufacturers supply three grades of rings. Standard tension for normal engines, low tension for competition engines where almost the same control is retained but the frictional losses are greatly reduced, and high tension for the odd engine running at very high rpm and poor length to stroke ratio.

Piston Ring Frictions:

Pressure Sealing Oil Control Frictional Losses
Top Ring 78%  5% 30%
Second Ring 22% 45% 20%
Oil Ring 0%  70% 50%

There are lots of ways to get heat out of the pistons, but the most effective way is by using the piston rings. What we do with the piston rings has a big effect on piston heat dissipation.

Ring Placement.

The top ring land is the thickest and is 9mm from the crown. The second ring is 6mm down from the first and the oil ring is 3mm below the second ring. The further down the piston crown the cooler it gets, that’s why the rings are closer further down.

Ring Sealing.

The rings inherent radial tension holds it against the cylinder wall to a small extent, but it is the gas pressure behind the back of the ring that really forces it against the wall. Ring float or flutter can occur in high rpm engines which limits heat transfer from the piston and can lead to detonation. This occurs because the weight of the ring causes it to break contact with the walls at TDC. Radial tension is unable to prevent blow-by caused by the ring flutter. The soln is to replace the rings with new ones. The narrower the ring the higher the rpm that flutter will occur, however, reducing the ring width will also reduce the heat transfer from the piston. The top ring land may distort and at worst the piston will melt. Narrower rings also have reduced service life.

For the majority of performance engines running to 8000rpm the top and second ring should be 1.5mm thick. For 9500rpm engines use 1.2mm top ring and a 1.5mm second ring. Above 10500rpm use a 1mm top ring and a 1.2mm middle ring.

Piston Gas Ports.

Apart from reducing the width of the rings you can also drill some lateral gas ports above the top ring to maintain the pressure behind the ring. These ports help an ordinary ring work like a Dykes L ring by ensuring there is always an open path for gas to travel behind the ring and push it out against the bore wall.

Piston Ring Lifting.

The top ring can also be pushed up by pressure building between the top ring and the second ring. Gas escaping past the first ring cant easily get past the second ring and a pressure build up occurs. The top ring then breaks contact with the bottom of the top groove and combustion pressure is lost. We can combat this by increasing the end gap of the second ring or increase the space between the two rings.

Zero Gap Rings.

When we have an engine running extremely rich especially on methanol or nitro, cylinder lubrication is marginal. Also a huge volume of fuel can escape into the oil sump and dilute the oil. Zero gap rings appear to stem the flow of fuel into the oil. The rings and cylinder bore then bed-in better. The lube oil isn’t diluted as much and the engine retains more horsepower by keeping the bearings and cam lobes healthy. However, a better solution is to lean up the mixture. If the mixture is super rich to provide cooling then maybe another area is deficient and the rich mix is masking this problem.

Ring End Gap.

We have to be careful to fit each ring perfectly square in its bore and then measure the end gap and increase it if necessary by filing the ring ends. The min gap per inch of bore for supercharged engines is 0.006in. This is also suitable for light nitrous engines. Competition nitrous engines need 0.0065in per inch of bore. Oil ring rails must be 0.004in per inch of bore, but more wont hurt. When the engine is running, the gap of the top ring will reduce by 50%. If the engine gets close to detonation, the gap will be almost closed. Check your old rings to see if the ends are polished. If they are then they were touching and the gap should be increased by 0.002in. If you don’t the rings could break.

Groove Clearance.

A race engine should have groove clearance of 0.001 to 0.0015in. Ring breakage is rare and can usually be contributed to worn piston ring grooves which allow the rings to move around and break, or excessive taper of the bore, causing radial flutter and ring breakage. If new rings are fitted to the old work grooves they will break again.

Ring Design.

Ductile nodular cast iron is normally used to make high performance rings. Normal cast iron is used to make standard rings. For the top ring use moly-filled nodular cast iron rings. It has three times the strength of standard rings. It is ductile rather than brittle and can be bent without breaking. Molybdenum belongs to the same chemical family as chrome. It has a lower coefficient of friction and high resistance to abrasion. Its thermal conductivity is several times higher than that of cast iron or chrome-plated cast iron. Its porosity acts as an oil reservoir, reducing scuffing and cylinder wear. For oil control use the multi-plate pressure back type. This minimises frictional losses to the min while at the same time maintaining adequate oil control. Only low tension oil control rings should be used for competition engines.

Fitting Rings.

Avoid fitting rings the wrong way up and avoid damage by incorrect installation. They should be expanded sufficiently to fit over the top of the piston and be allowed to drop into the groove. Expander tools are available for this, but a better way is to use two feeler guage blades placed between the ring and piston which prevents scraping. Taper face rings are marked TOP or have a dimple mark to indicate the top. The bottom of the ring should be wider than the top.

The torsional twist ring is fitted with the inner chamfered edge uppermost.

The Multo seal type second ring is fitted with the serrations in the ring face downwards.

Always position the gap ends to minimise oil and gas leakage(although this doesn’t really matter because of eventual ring rotation). Dip each entire piston in engine oil before fitting into the engine.

Bedding In.

The initial bed-in is done be giving the engine a full throttle burst for a few seconds(this forces the rings out against the bore walls), followed by snapping the throttle shut(this causes a vacuum in the cylinders drawing oil up) and coasting for a few seconds. This is repeated 12-15 times with the engine at normal operating temperatures. Accelerate in top gear from the slowest speed it will pull in that gear. This minimises the risk of glazing and allows the ring face and cylinder wall to cool. After initial bed in, the engine can be driven at 80% of power, but constantly vary the engine speed, otherwise, the rings may still glaze. After a half hour of this on the dyno the rings are bedded in.

A road engine with chrome rings need 200 miles preferably in one session to bed in. Avoid constant high speed until 700 miles are completed. The engine should really be initially run in on the dyno. When the engine cools, the tappets are adjusted and the head retensioned. Following warm up, full load tests are done to determine ignition advance, fuelling etc. By the end of the tests the engine is fully run-in having spent four hours on the dyno.

Testing Ring Seal.

A blow-by guage should be connected to the crankcase and tested at full power. This will give a quick overall indication of the ring seal. If the power is down and the blow-by guage is reading high then the rings aren’t sealing properly. A leak-down tester will reveal which piston rings are leaking. Run the engine to normal operating temp, remove all the sparkplugs and bring piston 1 upto TDC on the compression stroke, connect the leak-down tester to the spark –plug hole and compressed air to the tester. Ideally we want 2-3% leakage. 5% or more and there is a problem. Do the same test on each piston at TDC. A leak-test should be carried out regularly. In a competition engine it should be carried out after every event. If there is a problem then the engine should be stripped, the walls lightly honed and a new set of rings used, touch up the valve seats and replace the valve springs and bearings also.

Harmonic Balancer.

The harmonic balancer is attached to the nose of the crankshaft and extends the durability of the engine. Most engines use a pressed metal drive pulley spot welded or bolted onto the crank. These are liable to break-up at much above 6000rpm so should be replaced with a cast or machined pulley.

The balancer dampens crank torsional vibrations.

Before fitting the harmonic balancer, spray it matt black and file a large groove at TDC to match the engine timing case. Paint the groove white or silver. Also paint the area around the timing marks black and paint the timing marks themselves silver. This will make timing adjustment and checking much easier.


The flywheel is attached to the other end of the crank. Two good dowels and retaining bolts coated with Loctite are used to hold the flywheel in place. Any reduction in the engines reciprocating mass or rotating mass will improve acceleration. Lightweight pistons, rods, cranks, clutches and flywheels all benefit performance.

Lightening the flywheel also increases crank service life by reducing the twisting load on the end of the crank. There is less risk of flywheel explosion due to less inertia load. For road use, the flywheel should only be marginally lightened, for full performance the flywheel should be lightened as much as possible while retaining the required strength to function safely.

The added weight of a road flywheel minimises lumpy idling and low speed surging. A heavy flywheel better absorbs the uneven torsional impulses coming through the crank and keep the engine turning smoothly at low engine revs. A highly tuned engine with a heavier flywheel will be down a little on overall acceleration and response, but will be much more pleasant to drive.

To lighten the flywheel you can machine your existing flywheel or choose an aftermarket lightened flywheel. There may also be a lighter flywheel available from the same type of engine fitted to a smaller car, or an engine of smaller capacity made by the same manufacturer, but fitted with a lighter, compatible flywheel.

When lightening your own flywheel, only remove metal from the outside. Never remove metal from the centre or close to the centre. You will improve the overall flywheel strength and durability by removing some metal from the very outside of the flywheel. This reduces stress on the centre. To prevent distortion, don’t overdo it. Don’t remove so much metal on the clutch side, such that clutch slip happens due to overheating.

Cylinder Block Deck.

The deck needs to be perfectly smooth, free from rust, grime or any foreign matter. Take care not to allow dislodged material fall into the oil or water passages. Never use emery paper or light sandpaper. Abrasive material will lodge in the cylinders and the cam and lifters. Compressed air will only drive the grit further into the engine.

Head Gasket

The head gasket will have to withstand pressures up to 1800psi. The stock gasket will not do on an up rated engine. Even a light rise in boost will require a better head gasket. Its not he boost in itself that wrecks the gasket, it is detonation. No matter what the boost level, the gasket will be OK if detonation never occurs(excluding head lifting etc due to boost). Copper asbestos gaskets are too soft. Stainless steel sheet gaskets are too hard to deform into minute imperfections. Steel/copper/asbestos types work a bit better. High performance composite gaskets use a cylinder fire ring which can better withstand detonation. A round wire steel ring is OK for cast iron heads, but for aluminium heads they are too aggressive, so a pre-flattened steel ring gasket or a round wire hard copper ring gasket may work better.

For more specialised sealing methods a stock composite gasket or a thick full copper gasket can be used with special O rings fitted around the cylinders. With softer composite gaskets, the W ring is preferred, followed by the O ring, the mild steel round wire O ring may also be preferred with solid copper type gaskets. The usual setup is to machine the block to accept 0.041in mild steel wire so that is stands proud of the deck by 0.007in. It therefore exerts a high compressive force against the gasket and head.

Head Gasket Clamping.

The other big destroyer of head gaskets is insufficient clamping force, allowing the gasket firing ring to flap during violent combustion. Poor design of the head or block or ineffective clamping (read, head or stud bolts) is usually the cause.

Replacing the bolts with head studs will improve the clamping force and reduce distortion and pulling of the block deck. When initially fitting the studs, torque them to 30ftlb. Note that after fitting studs, it may be more difficult to remove the head when its in the engine bay. Some factory blocks have bolts removed to make manufacturing easier. Re-inserting these bolts and torquing them to 20ftlbs can eliminate leakage problems.

If you still have a problem, then check the integrity of the deck of the head. If the deck of the head is so thin and lacking in internal bracing then it will bow under pressure and not clamp tight against the head gasket. An aftermarket head might have stronger, thicker deck. Otherwise, try to strengthen the existing head. The best bet here is to find a race team that uses this head and ask them how they deal with the problem.

If the deck of the block is bowing then we have to switch to a dry deck block. All water passages and oil return pipes in the block should be blocked with threaded plugs. Any holes that cannot be plugged must be welded shut. This strengthens and braces the block. An external water tube must then be devised to carry the coolant into the head and an oil return to the sump (wet sump).

Another was to strengthen the deck is to glue and screw a 8-12mm steel plate to the deck. After bonding and curing, the deck plate is surface ground and the cylinder bores honed. Longer con-rods can then be used and/or more crank stroke can be used. We have to use a piston design which keeps the top of the top ring away from the new deck plate.

Head Bolts.

A lot of engines was stretch-to-yield head bolts. These can never be reused. If they are reused they will continue to stretch and not provide the clamping force necessary. If sticking with a factory setup, then follow the manufacturer’s instructions to the letter when fitting the head. The head bolts should have their threads buffed and oiled(along with under the head of the bolt) and be tightened according to the right sequence. Reverse sequence to remove the head. Use four or five progressive steps to avoid head warpage. After the gasket has time to bed-in(15-30min) tighten again to the final torque. if the head is alloy that’s it. If it’s a cast iron head then warm-up the engine and retension. Do 300 miles and retension again. If the head has long bolts on one side and short on the other then tighten the longer bolts 10% more.

Cylinder Head Heat Treatment.

If you have an aluminium head from an engine that has previously overheated then it might be annealed and no gasket, studs or good decks will give a good seal. The hot gases destroy the heat treatment on the head and it goes soft and cant hold its shape and allows leakage on the deck. It also gets shorter and narrower. If the studs don’t fit well, or the bolts are hard to get started then the head has been annealed and needs to be replaced.

It can be checked with a Brinell or Rockwell hardness tester. On a Brinell scale the head should read 95 or more, anything under 75 is too soft. On the Rockwell-B scale the hardness should be between 48-60. Anything under 38 is too soft. The tests must be carried out in several places close to where the head gasket contacts the head… upto 20 tests.