Piston Technology - Racers' FAQ

Stock Car Racing visits regularly with many of the top organizations involved in motorsports to find out what questions they are most frequently asked by the average short track racer. This month we speak with Randy gillis of JE Pistons, one of the top manufacturers of high-performance pistons in the industry.

Q: Is there a big difference between a forged piston and a cast or hypereutectic piston? I'm in a Street Stock class and the power is usually somewhere between 350 and 400 hp. Is a forged piston necessary?

Gillis: Yes, there is a difference. The forged piston has the advantage of greater durability in a high-stress environment like racing. A secondary benefit, one that is pretty significant to most racers, is that a forged piston is a lot lighter than either a cast or hypereutectic piston that is built to withstand the same amount of power.

Q: I'm trying to minimize the size of my valve pockets to bring the compression up a bit (my track mandates flat-top pistons and combustion chamber size). how little piston-tovalve clearance can I safely get away with?

Gillis: With respect to a flat tappet camshaft which you usually see in stock car racing, typically, you can get away with 0.050 to 0.060 minimum clearance between the piston and the intake valve. the minimum clearance numbers for the exhaust valve will always be larger. The reason why the intake can always be tighter is because it gets loaded against the opening ramp of the cam when it is closest to the piston and is always under control. on the exhaust valve, I would like to see a minimum of 0.090-inch of clearance to a comfort zone of 0.100- 0.110-inch. this is because the exhaust is closing when it is closest to the piston, and the inertia creates a greater chance of the valve floating and coming into contact with the piston.

Q: I've heard people are improving durability with coatings on their pistons. Are they helpful? What types of coatings work best?

Gillis: Let's start with a coating for the piston skirt. Skirt coatings that minimize friction can allow pistons to run at slightly tighter clearances than normal. this is especially important on forged pistons that have a higher expansion rate than cast pistons. there is no downside to a properly applied piston skirt coating.

Thermal coatings to the crown of the piston are also popular. typically, we only see a benefit on crown coatings when the piston is made too thin or too fragile for the application. then the crown coating becomes a kind of band-Aid for a part that might prematurely fail in an uncoated situation.

Q: Can I gain much power by going with a set of pistons that use lighttension rings?

Gillis: Yes. You can always gain horsepower through reducing frictional losses. Low-tension oil rings are one of the easiest ways to pick up free horsepower. the power is already there, and when you get rid of the friction it just makes that power available to the rear wheels. but it feels like you are finding power. the only downside of lowtension rings is making sure you maintain adequate oil control.

Q: I overheated my engine and stuck a couple exhaust valves. When I tore the engine down I noticed a mark on a few of the pistons where the valves contacted the piston tops. Is there a way I can tell if these pistons are still useable, or are they automatically junk?

Gillis: If you are using cast pistons, I wouldn't trust them because they are more fragile. but it is possible to reuse the pistons if you are using stronger forgings. If the metal has been moved, I would be very concerned and would have them crack-checked before I considered reusing them. If you notice a raised layer around the area where the valve contacted the piston, or a dent, that is what I consider metal being moved. If you crack-check the pistons and they come back oK, then you can dress the affected area with a sandpaper roll and reinstall them.

If, however, all you have is a witness mark in the carbon on top of the valve and no impression left on the piston itself, you should be fine.