goodgollyjosh
XS650 Addict
I have personally not come across this problem but I am always reading up on weird occurrences just in case I have a hard diagnosis one day I can rely on some of these findings. Food for thought when re-ringing an engine from a tech specialist at Total Seal Rings:
Recently got a call from a well-known builder with a 555ci Super Comp engine who put a new set of the exact same rings that he'd been using all along. Calls me up and says the engine is down 40 horsepower and there's 8 CFM of blowby. We talked about honing, and break-in and everything's right on. Then he tells me it was the same piston and that's when the light went off. When re-ringing an engine, rings and piston grooves are like what camshaft and lifters do to each other. This is a sealing surface. I suggested getting some gauge pins and checking the top ring groove. Turns out the ring groove looked perfect to the naked eye but actually had about .0005-inch taper in the bottom of the land. He was putting a fresh ring with parallel sides into a groove worn only .0005, and now the ring is trying to seal on this little back area of the ring and out front it's flopping around like a teeter-totter. The builder got a new set of pistons, used those same rings, didn't touch the cylinder walls — got 1.5 CFM blowby and picked up 40 horsepower. Moral of the story: on rebuilds you have to really check the piston ring lands with gauge pins. The issue is that the groove doesn't wear even; it ends up like a wedge.
Recently got a call from a well-known builder with a 555ci Super Comp engine who put a new set of the exact same rings that he'd been using all along. Calls me up and says the engine is down 40 horsepower and there's 8 CFM of blowby. We talked about honing, and break-in and everything's right on. Then he tells me it was the same piston and that's when the light went off. When re-ringing an engine, rings and piston grooves are like what camshaft and lifters do to each other. This is a sealing surface. I suggested getting some gauge pins and checking the top ring groove. Turns out the ring groove looked perfect to the naked eye but actually had about .0005-inch taper in the bottom of the land. He was putting a fresh ring with parallel sides into a groove worn only .0005, and now the ring is trying to seal on this little back area of the ring and out front it's flopping around like a teeter-totter. The builder got a new set of pistons, used those same rings, didn't touch the cylinder walls — got 1.5 CFM blowby and picked up 40 horsepower. Moral of the story: on rebuilds you have to really check the piston ring lands with gauge pins. The issue is that the groove doesn't wear even; it ends up like a wedge.
