Piston Technology - Racers' FAQ

Q: What is gas porting? Is it helpful in a stock car race engine?

Gillis: gas porting is a series of holes drilled in either the top of the piston or the side just above the top ring land. they allow cylinder pressure to get inside the ring and push the ring out against the cylinder wall as well as down against the ring land. the effect is to improve ring seal.

Gas porting will always make more horsepower. the only negative to gas porting is as you increase loading against the cylinder wall, you will also increase wear on the rings. So, if you are talking about an engine that will be expected to run all season, it may not be necessary. but if you are talking about a piston that has low-tension rings, then absolutely. Remember our earlier question about low-tension rings-they always need gas porting to help improve their seal against the cylinder walls in high-performance applications.

Q: I've seen the new ultra-light piston designs with really small slipper skirts. they look nice, but I can't afford to be rebuilding my engine all the time. Will they last?

Gillis: Yes. the learning curve on slipper skirt pistons has been pretty steep. We have learned a lot about how the piston needs to be turned in the skirt area to make them durable and to make them not stick or scuff. And we've come a long way in a very short period of time. We feel that the designs we are putting out today are as durable as a fully skirted piston.

Q: how do I measure my pistons to know if the 4.030-inch set I ordered is really what I received?

Gillis: on JE forged pistons, we measure at 1.2-inch up from the very end or bottom of the piston. We design the minimal amount of clearance between the cylinder bore and the piston at that point on the skirt. the amount of clearance depends on several factors, including horsepower and block material. more horsepower means more heat, and the more heat you have, the more the piston expands. this requires more clearance between the cylinder wall and the piston. but that clearance is clearly illustrated on our spec sheet that's included with every set of pistons.

Q: If I haven't damaged anything, how do I determine how long (how many rebuilds) I can run a set of pistons before they must be replaced? How about rings?

Gillis: the first thing you should do is turn the piston upside down and look for heat discoloration. You do not want to run a piston that has been annealed-or softened-from high heat. the underside of the piston will turn a purplishblue color at the first stage of excessive heat. If there is no evidence of heat, you can bypass this test. So the first stage is a purplish color, and if you see this, I would consider maybe running the piston one more season, or a half season, depending upon your rebuild schedule. the next stage is a gold or a rainbow color. If you see this, then the piston is too soft and you should not use it. the same thing goes if you see brown or have any kind of black staining from either oils or extreme heat. throw those pistons away.

The next thing you want to check is the ring lands to make sure that they are still within the manufacturer's recommendations. A ring groove for a 1.16 package is normally 0.064-inch. If it measures 0.065 or more, then the ring land has been beaten so much that it will never maintain ring seal.

The third step is a visual inspection of the skirts. these ride on the cylinder walls. If they have a lot of scratches, or you see a lot of debris or trash in the skirts, I would replace the pistons. that's because the skirts, if you put the pistons back in the engine, will destroy the bore finish of the block and you will never attain good ring seal.