Home measurements #2

OK we need a re-bore. Damn. That means a new piston and new rings. Just for the heck of it I measured the wear on the piston rings. The rings sit in grooves in the piston (see an earlier post for a pic) and seal it to the bore so as it goes up and down gases don't escape around the edges. Very important. In the photo above I've removed a ring from the piston and have installed it in the cylinder by itself. The rings aren't solid like a ring on your finger, they are cut and spring open a bit when you hold them in your hand. When installed in the cylinder they compress and fit very snuggly. The end-gap, shown clearly in the photo above, is an indication of how much the ring has worn. This is an important measurement because you can imagine gases leaking by the little slot that this end-gap leaves open. The gap should be about the thickness of two sheets of paper put together (0.006") but it's more like 0.060" -- ten times too big. The photo below shows how this is measured using feeler gauges.

What does this all mean? A large end-gap like this means lots of gases can escape past the rings, and the engine doesn't perform as it should. Basically an internal combustion engine is a gas pump -- as the piston moves up and down it pumps gases around. Leaky rings means it isn't efficient and has less power than it could.