Years of research and development by piston manufacturers, top engine builders, and dedicated racers have developed a wealth of good information concerning piston design and application for small-block Chevys. In fact, there is so much information available that it’s easy to become confused when trying to make a piston selection.
Factory pistons offer a good starting point for high-performance applications. There have been several types of pistons used in the smallblock engine: permanent-mold cast aluminum, modern Hypereutectic, and forged or “impact extruded” aluminum pistons. Prior to 1962, all small-block pistons were cast aluminum with either flat tops or 1/8 -inch pop-up domes. The 1962 340-hp, 327-ci engine received the first factory- designed forged pistons, and thereafter they were offered in highperformance 327, 302, and 350 engines. The 327-type dome was used through 1967 on all high-performance 327 engines and in the 1967 302-cid Z28 engine. Later forgings featured a larger dome used in 1968 and later 302 and all high-performance 350 engines.
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Small-block pistons from different displacements are generally not interchangeable. Other than differences in bore size, the major difference between pistons is the compression height (also called pin height: the distance fromthe centerline of the piston pin to the deck surface of the piston). Rod length and block height are standardized (except in the 400 and in talldeck racing blocks), so changes in stroke length are accommodated by raising or lowering the position of the pin in the piston. However, there are some notable exceptions: the 400 uses a shorter rod, therebymaintaining the same compression height as the 350; and the 307 piston has the same compression height as the 327 piston because it has the same stroke as the 327, though it has a 283 bore size.
Your piston selection must always complement the stroke of the crankshaft; otherwise you’ll end up with the piston sticking out the top of the block or only partly up the cylinder at TDC. With properly matched components, the piston deck will always be very close to the deck surface when the piston is at TDC. It should appear flush with the deck surface if you’re eyeballing it, but in actuality it must be a slight distance down the bore when checked with proper measuring equipment. This distance is the deck clearance specified in factory blueprints.
When selecting pistons, it’s important to honestly evaluate your requirements. Your favorite Pro/Stock racer may use special high-compression forgings that are ultralight and double trick, but that doesn’t mean you need the same thing for your street performance car. Remember that many of the factory high-performance cars provided exceptional performance with nothing more than cast, flattop or newer hypereutectic pistons. You can often save big bucks by choosing a low-compression piston that is compatible with lowerquality fuels, but is still quite capable of delivering exhilarating performance when combined with the proper power components.
Factory (or factory-type) cast or hypereutectic pistons have inherent advantages over forged pistons, including greater dimensional stability. This prevents the piston skirts from expanding greatly during normal operation and allows a relatively small “cold” piston-towall clearance (something on the order of 0.001 to 0.002 inch). The tighter piston fit reduces cold-start engine noise, ring wear, and oil consumption.
These pistons are also designed to use press-fit wrist pins, eliminating the potential problem of pin retainer failure. And the pins in these pistons are offset by 0.060 inch to help minimize piston slap, reducing engine noise and improving ring life.
Factory pistons provide excellent results when engine speeds are kept below 6,000 rpm. However, skirt clearance should be increased slightly (to 0.0025 to 0.003 inch) if higher engine speeds are anticipated. Remember that a good street engine should produce the broadest possible torque curve, allowing it to develop substantial power at or below peak engine speeds. At higher engine speeds, tight clearances invite engine seizure because of increased friction and higher temperatures. In nearly every case, it’s best to keep clearances tight, peak speeds below 6,500 rpm, and religiously avoid detonation that can crack rings and distort ring lands.
Another point of concern is piston balancing and lightening.Weight removal is something that can help engine performance in some respects although there is substantial evidence that this is not always the case. While the ability of the engine to rev faster is improved when the reciprocating mass is lessened, most modern racing pistons are now about as light as you can make them anyway, so further machine work is usually not recommended.
Current Factory Pistons
You’ll find that GM currently has cast, hypereutectic, and forged pistons for 350 performance engines. Some dealers may still have stock replacement pistons for other engines such as the 327, 305, and 400, but you may be better off purchasing special aftermarket pistons for these engines.
GM Performance also offers a large selection of special racing pistons for 4.00-inch bore 350s. They are all designed to produce 12.5:1 compression, and are forged aluminum for severe-duty applications. They are available for both 5.7- and 6.0-inch rods in standard and oversize bore diameters, and they’re machined for 1⁄16- inch top and second rings. Each piston comes with round wire pin retainers and a pin machined specifically for these locks. These pistons come in sets with asymmetrical domes designed to fit Chevy combustion chambers. Piston ring sets are also available from GM or some aftermarket manufacturers.
There is also a wide variety of specialty pistons available from several manufacturers. Some of these are remanufactured and should be avoided, but many provide an excellent base from which to construct a true budget engine. The thing you really want to avoid is used pistons or locally reconditioned pistons with knurled skirts and spacers to reduce the ring side clearance. Used pistons are always going to have excessive ring side clearance and very often they will have too much back clearance behind the ring because the engine rebuilder has been in there with his ring groove cleaner and removed another 0.010 to 0.020 inch of metal. With good pieces available at reasonable prices, there is no reason you should ever have to resort to used pistons.
Hypereutectic pistons now available from GM and from aftermarket manufacturers such as Keith Black are another great alternative. These pistons were developed specifically to provide a stronger piece that would withstand punishment without requiring an expensive forging. They are more expensive than replacement cast pistons, and they definitely are not bulletproof, but they offer an added measure of strength and performance in engines where cylinder pressure, detonation, and fuel quality are controllable.
When your plans include serious racing or hot street engines, forged pistons provide the additional strength and performance you need. Forged pistons are made by forming aluminum slugs into pistons with enormous presses (forging). This procedure creates a dense metal grain structure, making the pistons stronger without incurring a substantial weight penalty.
Consider for a moment the incredibly tough job a high-performance piston must perform. Few mechanical pieces of any sort operate under such severe conditions. The piston has to accept the full force of combustion and transmit it to the rod and crankshaft assembly with little or no distortion. It must further withstand the additional ravages of pre-ignition and detonation, while maintaining efficient cylinder sealing and preventing excess oil from reaching the combustion chamber. Moreover, it has to cope with speeds up to and sometimes exceeding 4,500 feet per second. To do this it has to maintain structural integrity so that the piston rings maintain their seal against the cylinder wall. In addition, the piston has to dissipate a significant proportion of the heat of combustion through the rings and skirts.
A forged piston is more able to handle these conditions because of denser grain structure and lack of porosity. This is especially important when it comes to ring land distortion. The piston must precisely retain and stabilize the pistonrings if the engine is to make good power. Forged pistons are generally considered superior in this respect, but the degree of success depends upon the specific design of the piston. A great deal of investigation has gone into piston alloys and skirt design. While a forged piston is much tougher, it’s still aluminum and therefore has difficulty dealing with heat. Because of the greater density of forged alloys, expansion from heat is greater, resulting in a piece that has to be fitted with significantly greater skirt clearance.
The cam-shaped skirt and the barrel contour of the skirt face are typical characteristics of TRW/Speed Pro pistons and slipper skirt pistons in general. The shape of these pistons is elliptical (as viewed from the top of the piston), which reduces expansion and concentrates thrust against the strongest part of the skirt. The barrel contour (as viewed from the side, along the pin axis) reduces friction and stabilizes the piston by reducing the amount of skirtmaterial in contact with the cylinder wall. Only a small portion of the skirt contacts the wall, but it’s enough to stabilize the piston, while the ring package keeps the piston centered in the bore. This technique offers significant reductions in friction, which translates to improved power and lower temperatures. This leads back to our original requirement: To be an efficient piston, it must effectively support the rings so they can seal the cylinder and prevent charge contamination as the piston moves in either direction. In a nutshell, fuel mixtures and cylinder pressure must be contained above the rings, and oil must be kept out of the combustion chambers.
When you get down to selecting the pistons for your engine, it becomes a matter of obtaining the desired compression ratio for the engine application, given the selected cylinder heads and the octane rating of fuel. First of all, remember that the very act of increasing the bore size will increase the compression ratio. This occurs because you are squeezing a larger volume into the same combustion chamber. You should also consider that flattop pistons promote efficient combustion and increase engine efficiency. Domed pistons may still have a place in the overall scheme of things, but many racing engines are now built with flat tops and combustion chambers sized to control the compression ratio. For a general performance street engine, 9:1 compression is a great place to start.
Top Super Stockers run pretty hard with the stock compression ratio and flattop pistons, but they are allowed to snug everything up to minimum specs. With the allowable overbore and minimum combustion chamber sizes, the true compression ratio is usually higher than the figure quoted by the factory.
In some cases, high-dome pistons must be used, even at the expense of flame travel efficiency. The wedge design of the small-block engine makes use of a significant amount of quench area to increase turbulence in the combustion chamber. If you examine the small-block head, you’ll see that the combustion chambers are not completely round, but more like a bathtub with a wedge-shaped chamber. Nearly a third of the cylinder bore is capped by the flat surface of the head. This area matches the flat pad or quench surface of the piston. It is this area where piston-to-head clearance is often measured, and it is also the area that creates turbulence in the chamber. When the piston rises rapidly in the bore during compression, mixture squeezed out of the quench area creates vortices of turbulence inside the chamber. This usually improves mixture quality and flame travel. When the piston has a dome that can interfere with this process, it has been found that careful shaping of the piston dome and a generous fire slot across the dome are the straightest paths to horsepower.
For a hot street engine or a bracket racer, you don’t need a gargantuan dome that matches the shape of the combustion chamber. If you start with a standard TRW/Speed Pro piston and lower the dome by machining 1⁄8 inch from the peak, you’ll still achieve a substantial compression ratio. Cut a slight trough in the area of the spark plug leading across the dome. This will ensure that the dome does not mask the flame front. After altering the dome, blend and smooth all sharp edges. The trough should be carefully blended into the general shape of the dome. Consider having the piston crowns glass beaded. This leaves them with an excellent finish, but make sure the rest of the piston is masked with heavy tape. Thoroughly clean each piston with solvent and compressed air when you’re finished.
For most street engines, piston-tovalve clearance will not be a problem, but it’s something you should always check as a matter of course. It is more of a concern in a bracket-type engine where you may be using a healthy camshaft with lots of lift. Checking piston- to-valve clearance involves assembling the actual pieces to be used in the engine and using light replacement springs on the valves or modeling clay on the piston tops to determine the distance from the valve to the piston when the piston is in the vicinity of TDC. For absolute assurance, the clearance should be checked at all points for 10 degrees on either side of TDC. The closest proximity may occur at a point just before or after TDC, depending on the phasing of the cam in relation to the crankshaft.
If you determine that the clearance is less than 0.100-inch on either valve, the pistons will have to be flycut to deepen the valve reliefs. This operation will have to be performed for you by a competent shop with the right equipment. Your job is to accurately determine where and how much material needs to be removed. On a car with an automatic transmission, you may be able to reduce the intake clearance to about 0.080-inch—but no less. Take care toremovenomorematerial thanis necessary, because you’re also reducing compressionratio. Inanycase, thisprocedure should be followed by sandpaper blending and glass beading.
Piston pin retention is another matter of wide debate.For most performance engines, it’s safe to say that a pressed pin is the only way to go. Full-floating pins have not been found to have significant advantages over pressed pins. They make engine assembly easier, but in a street engine they are just another potential trouble spot. In most cases, pressed pins are your best bet.
If you choose toworkwith floating pins, other factors need to be considered. One of themost important things to check is connecting rod side clearance. If side clearance exceeds 0.018 to 0.022 inch, too much movement may subject the pin to greater stress. Even more important is floating-pin end clearance. If this clearance is too great, it can allow the pin to hammer the retaining locks right out of the pin bores! Make sure end clearance falls between 0.001 and 0.008 inch. In any case, you should ascertain that there is not an interference fit, which would actually place constant pressure against the locks and just about guarantee removing themthe hard way.
Pin clearance should be 0.0006 to 0.0008 inch in the piston and 0.0008 to 0.0010 inch in the rod. These clearances have to remain tight to prevent the pin from knocking in the rod and causing virtually instant failure. Although floating pins have been successfully operated without using bronze bushings in the rods, there is always a danger of pin seizure. They rarely give trouble when a bronze bushing is pressed in the rod small end. The most important aspect of all this is proper pin lubrication. If the piston does not have a hole leading from the oil ring groove to the wrist pin bore on either side, you’ll have to provide it by drilling intersecting holes in the ring groove and the pin bore. Some engine builders prefer to oil the pin with a small groove around the piston just above the oil ring. Oil collects in this groove and is drawn to the pin through oiling holes by a properly functioning vacuum system. Most of these features are now included in modern racing pistons.
Oiling the pin bushing in the rod is also a matter of considerable debate. It has long been accepted that the pin oiling hole should enter from the top of the rod. Then many respected engine builders began drilling two smaller holes from the bottom of the pin boss, on either side of the rod beam. This has become popular, but many people still cling to the older single-hole method. It’s difficult to say which is better, but you have to consider that there probably is more oil located below the pin than above it. We have also seen rods fitted with holes on both the top and the bottom, which seems to have no affect on rod strength, at least in most performance applications.
An interesting fact that you may have observed concerns the subtle difference between various pins. Close inspection of most pins will show that they are bored in one operation. This yields a smooth notch-free bore through the center of the pin. However, some pins appear to be bored from both ends and the bores never match up correctly. There is often a step right in the center of the pin, exactly where the greatest loading occurs. This presents no particular problems in a street or bracket-racing application, but any severe service application, such as a supercharged or turbocharged engine, should be fitted with smooth-bore pins.
Piston ring selection can make or break your project. Ideally, you want a ring set that provides excellent cylinder sealing, good oil control, and minimum frictional drag. This is a tall order when you consider the operating environment. On one hand, you have the heat of combustion trying to melt the top ring and leak by it, while the second ring and the oil ring are trying desperately to prevent oil from entering the combustion chamber and contaminating the mixture. On the other hand, you have the cylinder walls, which are trying to grab the rings and bring everything to a standstill.
Many experimental ring designs have been used to meet the specific requirements of different racing environments. For the average street or bracket-racing engine, ring combinations are fairly standardized, and the best plan is to stick with proven pieces and procedures. The conventional three-ring layout works exceptionally well when the rings are gapped and installed correctly in properly honed cylinders.
Stock Chevrolet pistons generally have 5⁄64-inch top and second rings with either a 1⁄8-inch or 3⁄16-inch oil ring. The factory ring sets are an excellent choice for street performance, as are TRW and Speed Pro replacements. Engines that aren’t required to operate at very high engine speeds work extremely well with wider rings, because they stabilize the pistons and the wide face provides a better seal at low engine speeds. For high-performance work, you should consider lighter and thinner rings. They are more able to seal the engine at high speeds because they resist ring flutter and the subsequent loss of seal. Thinner rings also reduce frictional drag, which skyrockets at high engine speeds.
The choices are clearly defined: A wider factory-type ring will seal the cylinder better at any engine speed up to 5,500 rpm; above this, thinner 1⁄16- inch rings do a better job until engine speed approaches 8,000 rpm; beyond that, you’ll need extra thin rings and special sealing techniques that exceed the spirit of even most bracket-racing engines. This is pro-racer stuff, and you probably don’t need any of it!
There is no reason to use anything other than a single-moly or plasmamoly ring set in any hot street engine or bracket racing powerplant. Chrome rings may still have a place in environmental conditions where engines are prone to ingesting a lot of dirt and grit, but the moly ring is the absolute best choice in nearly every other case. For a turbocharged engine, plasmaspray moly rings are the hot tip since they resist flaking under severe detonation conditions.
The conventional three-ring system for general street and high-performance use consists of a high-strength ductile iron top ring with a molycoated surface. The temperature resistant moly is usually embedded in a thin channel on the face of the ring. Second rings are standard or low-tension (for racing only) cast iron. They have a tapered face and no coating. The second ring acts primarily as a backup to the top ring and helps transfer combustion heat to the cylinder walls. Three-piece oil rings have stainless steel rails and an internal expander to provide good oil control with a minimum of friction.
There is also a double-moly ring set where the second ring also has a moly inlay. These rings work well in a competition environment, but they are not necessary in a street engine since they do not provide a substantial benefit for day-to-day operation, and the low-tension second ring used in most double-moly sets does not provide good, long-term oil control.
All of the rings available from TRW/Speed Pro are offered in standard oversizes and also in special sets that have an additional 0.005-inch oversize to allow for individual fitting to the bores. Careful ring fitting to the individual bores is an important procedure that can virtually guarantee more horsepower from your engine. In a well-sealed engine, the end gap is the only path through which combustion gases can escape to the crankcase. It is therefore desirable to close the gaps as muchaspossible toprevent thepassage of gases in one direction and oil in the other direction.
Standard ring sets are manufactured so they may be easily fitted with little consideration given to the end gaps. Generally, they’re pregapped slightly on the wide side so that rebuilders can stay out of trouble while flat rating their way through your engine. The special oversize sets require that each individual ring be gapped to its particular cylinder. It’s important that the gap be kept small enough so that there will be minimum gas leakage when the engine reaches operating temperature. You have to be careful here, because a ring that has insufficient clearance will butt in the cylinder, scuff the bore, and immediately end the possibility that your engine will deliver topnotch performance and longevity.
It’s also essential that the end-gapping procedure be performed properly. Not only do you have to set themat the right clearance, youmust keep the ends square and free of burrs or sharp edges. Furthermore, when you check a ring in its bore,make sure that it is straight and dead parallel with the deck surface. A false reading will be obtained if the ring isnotpositionedproperly. Ifyourengine is used for bracket racing and you tear it downfromtimetotime,besuretocheck the end gaps for signs of butting. If they are shiny after extended usage, it may indicate they have been in contactwith one another.
Pistons and rings are very important parts of your engine, whether it’s a pure stocker or a full-race piece. Their selection and preparation should be the subject of careful consideration. Don’t buy over your head, but don’t buy budget parts if you’re building a race motor. A little forethought here will save you money and grief, and go a long way toward ensuring that you build a strong, reliable small-block!
Written by John Baechtel and Posted with Permission of CarTechBooks