Piston Ring Question

[slvrbrdfxr]

Hi All,
Had a problem assembling my Jackpiner top end over the weekend and wondered if anyone had encountered this before. I was reinstalling the 65mm Elco piston which takes an "L ring" on the top and a "square ring" on the bottom. I bought a brand new set of rings and did all the usual preassembly checks as far as piston to wall clearance, ring end gap checks and filed rings to the minimum end gap. When I went to install the cylinder, the piston would get stuck in the bore approx 1" up from the bottom of the sleeve. Thought maybe I had one of the rings positioned so that the end gap was not on the locating pin on the piston but every time they were good. After about 6 attempts to install the cylinder I knew something else had to be wrong. I took the piston back off the rod and then tried to fit it into the cylinder with just the bottom "square ring" installed and the piston slipped right through the bore. Next I tried to fit the piston into the cylinder with just the top "L ring" installed on the piston and found that it would get stuck about 1" from the bottom of the cylinder sleeve as I described earlier. That told me that the problem had to be with the top "L ring". Upon closer inspection, I noticed that the new "L ring" would not full compress into the ring groove on the piston and so the piston would get stuck in the bore. Did a comparison between the new "L ring" and the old ring removed at disassembly. I saw no visible differences and no difference when I measured them with calipers. I took a magnifying glass and compared the ID markings on both rings and found them to be slightly different. The old "L ring" was marked "Elco Z SGV"  and the new ring was marked "Elco Z K". Okay... now that your probably exhausted after reading all this here comes the question. What's the difference between the "SGV" and "K" designations on the two rings ?? Obviously there has to be something slightly different about the two or else the new ring would have worked for me. Please let me know if you have an answer to this one. Thanks !!

Dave McCullough

[454MRW]

Since you are re-installing a used piston with new rings, there is a possibility that the ring land in the piston has slight detonation damage and the new ring is getting tight in the groove. I would first install the old "L" top ring in the piston and try to install it in the bore. If it goes in easy and the new ring is uniform in thickness compared to the old one, and there are no traces of carbon, debris, or casting flash in the ring groove, the possibility of a tight ring groove may be your problem and may require replacing the piston or using a stack of fingernail files on the ring groove slightly in the area the ring is tightest, or a ring groove cleaning tool if available. I had a similar problem on my 74 250 Harescrambler and it led to eventually a ring land breaking completely out of the piston. Luckily the cylinder was not damaged.

Michael R. Winter
1974 250 Harescrambler
1975 250 HS Project
1977 250 MC5 Project
1978 250 MX6  
1979 250 KTM Project
1980 250 MX KTM

[slvrbrdfxr]

Michael,
Thanks for the reply. I installed the old "L ring" on the piston it slipped right into the cylinder like it should. I had cleaned all the carbon out of the ring grooves with a nail file prior to installing the new ring so I don't think that was part of the trouble. I still think the new ring with the "K" designation might be slightly tapered or something. The engine was originally torn down due to a bad crank bearing and had very little time on the top end so I ended up using the old "L" ring to assemble the engine. End gap was within tolerance on the old ring and bike seems to run fine now so still hoping to figure this one out. Just wanted to install a new set of rings since it was apart.
 
Dave McCullough

[Gavin Housh]

Could it be possible that you filed enough off the end of the ring so that the notch in the two ends of the ring is now not big enough to fit over the ring locating pin. Some builders use a ring one size over the bore and then file the end to get the minimum clearance. I have never done this but it could be a problem. Gavin

[slvrbrdfxr]

Gavin,
Thanks for the reply. Though of that scenario too and made sure the new ring had enough of an opening to fit around the locating pin on the piston. Had to be somethind else.

Dave McCullough