Certain hypereutectic pistons like the line of Keith Black (KB) pistons need a wide top ring gap to perform properly. KB pistons call for 0.0065 inch per inch of bore size for the top ring gap for street-driven engines running on gasoline. Forged or cast pistons for gasoline engines often call for a much tighter clearance of typically 0.004 to 0.0045 inch per inch of bore. For a 4.030-inch bore small-block for example, the typical 0.0045-inch spec would demand 0.018 inch of top ring end gap. The KB piston’s larger top ring gap of 0.026-inch for a 4.030-inch bore is very close to a 50-percent larger clearance gap.
The main reason for the wider end gap spec is that KB moves the ring land closer to the top of the piston. This imparts more heat into the top ring which expands the ring more than it would in a lower position and therefore requires more of an end gap spec. This is true of any piston, forged or cast, that places the top ring close to the top of the piston. This is why it’s important to read the instructions that come with new pistons, as technology is constantly changing. Of course, using a supercharger, turbocharger, or other power adder also requires that top ring end gap be increased due to higher cylinder temperatures.
To emphasize the point, one of our favorite anecdotes is a time when we stopped by to drop off parts at Jim Grubbs Motorsports, in Valencia, California. There was a hypereutectic piston sitting on the counter that had broken just above the wrist pin boss. The piston looked brand new. Then-owner Jim Grubbs told us that a customer had not paid attention to the wider end gap spec and when the rings stuck in the bore with barely five minutes on the engine, the result was a spectacularly broken piston. It’s better to learn from that guy’s mistake than to be another victim.