What Causes Piston Ring Gaps?

In Episode 55 of Lincoln “Tech Tips,” brought to you by Lincoln Tech and Hot Rod Garage, Tony talks about piston ring gaps and how piston rings affect the four strokes of a four-cycle engine.

Tony: Welcome back to another Lincoln Tech tip. Today we're going to talk about piston rings, piston ring gaps, and what they do.

Now the piston ring performs a unique function for each one of the four strokes of a four-cycle engine. So, on the intake stroke, the rings seal up the combustion chamber and make sure that no air or oil gets in there.

Next, during the combustion stroke, the rings make sure that the air and oil mixture stay in the combustion chamber and are properly compressed before they're ignited.

During the power stroke, the rings make sure that the burning mixture doesn't get past the rings and into the crankcase as those gases are pushing the piston down.

Finally, during the exhaust stroke, the rings make sure that everything that's been spent inside the combustion chamber is pushed out and none of it makes it into the crankcase.

Onto piston ring gaps. Piston rings will expand when warm and you will set that end gap to take into account the thermal expansion and aim for best sealing once that engine is warm. A gap that is way too tight can actually close up once the engine is warmed up, causing a ton of friction in the cylinder. And if it's really tight, it can even break a piston. Now, if the gap is too big, the engine will have too much crankcase pressure, have a ton of blow-by, burn oil, and lack proper cylinder sealant.

Different engine setups need different piston ring gaps and it's mostly dependent on how much fuel is being burned. The more fuel, the more heat, and the more those rings are going to expand – the bigger the gap that will be needed. Modified engines will need bigger piston ring gaps than stock, and turbo, blower and nitrous engines will need an even larger gap.

Now, all piston ring manufacturers will give you a ring gap specification and that's the best route to follow. Now that we're all experts on piston rings and gaps let's get back to the shop.

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