1981 100 Mercury rebuild egg shape pistons

[CraigTD]

I just tore down my 100 mercury because of a bad knocking sound.I thought it might be a bad bearing, but turned out to be piston slop in the #2 cylinder. I took the piston out, it had worn it self egg shaped. of all the car engins I have rebuilt, I have never seen this before. Question: Can I bore out the cylinder or do I need to replace the sleve? And can I just do 1 piston or should I do all 4, I mic. the other three pistons, and they seem fine. Thanks Craig

[Fouled Plug]

Egg shaped? Yikes! How far out of round is the bore itself?

[CraigTD]

I don't have the specks with me right now. it just bugs me that the only piston egg shape is the #2 piston...

[BayStLouis]

If memory serves, Mercury advertised "Cam-Turned" pistons back then, described as OUT-OF-ROUND pistons which would normalize at operating temperature (essentially, the idea was to make the piston end up nearly perfectly round when running, rather than starting out round and distorting due to thermal growth).This may or may not explain what you've found. If the *other* cylinders make good compression, you might be able to just sleeve the "bad" cylinder.