Ring gap refers to the space (gap) between the ends of the piston ring.
Why ring gap is important
If the ring gap is too small, the ring will want to bulge into any available area. Usually, this means it will try to bulge into your ports. The more a ring bulges into the ports, the higher the likelihood that it will snag on the edge of the port and break the ring.
Alternately, having a ring gap that is way, way too wide can also be a problem, as it will allow gasses that should be pushing the piston downward to escape past the ring, in a phenomenon called blow-by. Blow-by is a much bigger deal in single-ring pistons than it is in dual-ring pistons where the lower ring serves as a guard against blow-by, should the first ring fail.
How to measure ring gap
- Gently push the ring into the cylinder bore so it is parallel and about 1/2” below the head surface and recheck it just below the transfer ports.
- Use the ring-less piston to make sure that the ring is square/level inside the cylinder
- Use a feeler gauge to measure the end gap. Larger is OK, smaller has to be corrected.
- Use a flat file or fine sand paper to remove material from the ends of the ring if needed.
Calculating proper ring gap
The correct numbers for setting the ring gap will vary depending on who you ask and where you look. Below is a sampling of answers found on the forums.
- .004 inches per inch of bore 
- .003 - .004 inches per inch of bore 
- .003 - .005 inches per inch of bore 
- 0.1 to 0.12mm per inch of bore 
- .006 inches per inch of bore for the first ring, .007 inches for the second ring 
- The lower ring gap needs be 0.002” bigger then the top ring. 
The following table shows the ring gap of different cylinders based on .004 inches per inch of bore.
|Bore (mm)||Bore (in)||Ring gap (in)||Ring gap (mm)|