Weird Piston Rings

[VintageHondaGarage]

Someone local gave me their top end to look at due to low compression. I couldn’t really find too much wrong with it, but I still gave everything a once over.

The piston and rings on the left side threw me off.

The left cylinder measures 64.25mm. The piston is stamped 0.25. Obviously it’s on its first over bore.

The thing is, the top two rings on the left piston are stamped 50, which in my head means .5mm over bore.

It doesn’t make sense. The left rings, when put into the 64.25mm bore, have a rings gap of .019”, which is well within the wear limit.

The right side position is stamped 0.25 and its top two rings are stamped 25, which makes sense to me.

I am confused.

 

[ancientdad]

Did you check the rings stamped 50 in that cylinder to see what the end gap was? It's possible (though in this case it doesn't make much sense) that someone used 2nd over rings and filed the end gaps to use in a 1st over bore.

 

[VintageHondaGarage]

Ring end gap is .019, same as the right side

 

[ancientdad]

Ring end gap is .019, same as the right side

Which tells me that they might have done what I suggested. I don't have time ATM to check the FSM for spec, but since they're the same it's possible they broke a ring or two and used what they had lying around. Can you see if the ends of the ring look like they're not as square as others (due to being filed)?

 

[VintageHondaGarage]

Which tells me that they might have done what I suggested. I don't have time ATM to check the FSM for spec, but since they're the same it's possible they broke a ring or two and used what they had lying around. Can you see if the ends of the ring look like they're not as square as others (due to being filed)?

The ID gap is about .003” (.016”) smaller that the OD gap (.019”) so not perfectly straight.

 

[ancientdad]

The ID gap is about .003” (.016”) smaller that the OD gap (.019”) so not perfectly straight.

Then it seems they might have been filed to fit, possibly because someone broke one or two during the rebuild.

 

[VintageHondaGarage]

Then it seems they might have been filed to fit, possibly because someone broke one or two during the rebuild.

I’m having him just buy actual .25 rings so we don’t have to worry about it

 

[ancientdad]

I’m having him just buy actual .25 rings so we don’t have to worry about it

But I'd still check the end gap. Sometimes it's better to buy the next size up and file the ends to get the optimal end gap for longevity.

 

[boddy]

That low compression might be due to a bent exhaust valve. Looks like a witness mark on the piston.