MotoIQ Wrench Tip – How to Gap Piston Rings
Setting the gap on your piston rings is an important step to building an engine that performs at its best. Getting your piston rings to have the optimal seal is perhaps the most critical aspect of good engine building. Of course you want to spend time assuring that your machine work and cylinder wall finish is correct for the type of rings used but a lot of people neglect blueprinting the ring gap.
You can usually buy rings in two ways, off the shelf pre-gapped or slightly oversized rings that you have to gap yourself. Off the shelf pre-gapped rings typically have a conservatively large ring end gap in order to prevent the rings from expanding with heat and having the ends butt together. If this happens the rings will start to dig into the cylinder walls leading to scuffing and in severe cases breakage with severe damage to the cylinder walls and pistons.
The trouble with large ring end gaps is that a lot of combustion pressure can leak past the gap reducing your compression and power. So most professional engine builders get slightly oversized rings and hand set the gaps at their optimal gap width. This width is the one recommended by the piston manufacturer.
Oversized file to fit rings typically have a gap that is 0.005 to 0.015″ too small and you have to set your end gaps yourself. This is the preferable way to go when building your own engine.
Typically the top compression ring has a sightly larger gap than the second ring because it sees more heat. Forced induction and nitrous motors usually run slightly wider gaps as well. Different engine builders may also have their own favorite gaps and theories that they like to set their engines up with.
Gapping piston rings is pretty easy but we thought we would show you how it's done just in case you haven't done it before. It is a good extra step to take when building an engine just so you can be assured that you get every bit out of it and have a long service life.