3-Ring Pistons for an O-235C

[N1230k]

I am restoring a Luscombe with a STC for an O-235C engine. In trying to decide do I overhaul the engine (a -C1B) so I wanted to put together a parts list, look up parts cost, etc. The Parts manual I have gives an obsolete part number for the pistons and rings (the 4-ring pistons). It appears, through my reading of Lycoming SB-642A that there is a conversion kit to go to 3-ring pistons. But what I do not know is if that conversion requires a new (ne' different) cylinder. To state it differently, I can see that Lycoming has produced a complete cylinder kit that includes the 3-ring pistons (the subject of the SB) but what I do not know is if the 3-ring piston is compatible with the original cylinders that came with 4-ring pistons.

This is really an exercise in logistics. My cylinders look good, I fully expect them to be reusable with new valve, valve guides, etc. If I am correct, rings only for a the 4-ring pistons are ~$550/cylinder. But a new 3-ring piston with the rings runs ~$480/cylinder. Every little bit counts, I would rather save the $280 and spend it on AvGas, but I am more thinking the 3-ring pistons should add value in the long run. For example, it is not a far stretch to think one cylinder may loose compression in the future - in that case swapping all 4 now saves a little, but saves a lot in a future repair. And if the 4-ring pistons are being obsoleted . . . well, that savings will exponentiate.

Ergo, for those that know - if my cylinders which originally had 4-ring pistons installed are good, is there any reason I should NOT just buy new 3-Ring Pistons and have them installed? (FYI the engine is completely disassembled, reworking the cylinders is going to happen no matter what, so cost associated with changing valves, guides, springs, etc. is not a factor. This is strictly a 4-ring piston vs. 3-ring piston issue.)

 

[Craig]

Piston used is going to change based on the cylinder. Look at Table 1 of SI 1037Y, which can be found here: https://www.lycoming.com/sites/defa...037Y_Approved_Pistons_Rings_and_Cylinders.pdf

 

[Dan Thomas]

The O-235 engines run notoriously cool, and so moisture tends to be a problem. Light pitting of the cylinder bores lets the rings eat away at the cylinder walls, and a ridge forms at the bottom of the bottom ring's travel. You can't see this until the engine is apart, but in some the aluminum piston pin plugs get chipped by that ridge and the flakes show up in the filter. If the fourth ring is below the piston pin on the piston, that ridge will form lower down and not chip the plugs. Maybe this is why Lycoming is going to four-ring pistons....

If the cylinder wear is bad enough you'll need new cylinders, or they'll need boring to .010" oversize, if they haven't been already.

 

[N1230k]

Piston used is going to change based on the cylinder. Look at Table 1 of SI 1037Y, which can be found here: https://www.lycoming.com/sites/defa...037Y_Approved_Pistons_Rings_and_Cylinders.pdf

Thank you. I had found this SI but only the first 6 or 7 pages, the table was missing. When I read it, and reached the end, I was left more confused than before.

 

[N1230k]

The O-235 engines run notoriously cool, and so moisture tends to be a problem. Light pitting of the cylinder bores lets the rings eat away at the cylinder walls, and a ridge forms at the bottom of the bottom ring's travel. You can't see this until the engine is apart, but in some the aluminum piston pin plugs get chipped by that ridge and the flakes show up in the filter. If the fourth ring is below the piston pin on the piston, that ridge will form lower down and not chip the plugs. Maybe this is why Lycoming is going to four-ring pistons....

If the cylinder wear is bad enough you'll need new cylinders, or they'll need boring to .010" oversize, if they haven't been already.

That is a good explanation of why to go to 4-ring, but this is a case of the inverse. The O-235-C was originally developed using 4-ring pistons. The O-235-L2 (much more common) has always had 3-Ring Pistons, at least that is the way I understand it. Because it was more common, parts are more plentiful and less expensive. I suspect the change to 3-Ring Pistons has more to do with logistics (lowering the part numbers in the catalogue) and production cost. But no one at Lycoming called and invited me to that meeting, so I am only making assumptions.