Bigmikey65

I was recently looking through a local junkyard for parts. While looking at an engine block I noticed that the #7 piston was changed - it was different from the other 7 pistons - which were all the same judging by their markings. I’m just curious - what would cause an engine problem that would lead you to change just 1 piston ? Is it ok to change just 1 piston and leave the other 7 be ? Thank you - hope all are healthy and safe.

Sugar Bear

Yes it is okay to change one piston if it is the same weight, height etc. It may have been damaged from detonation or water in the cylinder or a crack.

It seems peculiar though that it was different enough to be readily noticeable...maybe that led to the junkyard trip.

What was different about the piston that made you notice?

Bigmikey65

7 of the pistons had the same marking stamped on the top. If your motor was running with 1 bad piston - how would you be able to tell ? What are the warning signs ?

Sugar Bear

What was the marking? Can you describe it?
The symptom would most likely be rough running due to low compression, oil fouling of a spark plug or a noise from a cracked/broken piston.

Greg Rogers

I bought a 2002 Ford van with a loud knock. I narrowed it down to the #1 cylinder. Removed the head and saw a funny mark on the top of #1 piston and another mark on the head at the same spot. What the hell is that? Long story short, someone dropped a 1/4" socket down the spark plug hole, and tried to start it. They then got it back out with a magnet and finished changing plugs and gave it back to the previous owner! I don't know if the noise started right then or it developed later. I am sure of that as I later checked the mark against a 7/32 socket which is the size one would use to remove the coils on top of the plugs. Anyway original standard size pistons were not available so it was either rebuild entire engine with oversize pistons, or find a used piston. I did buy a used piston/rod assembly off ebay!! I installed it with new rod bolts, using the rings that came on the used piston. No noise and ran great. Then took off on a 10,000 mile trip across country trip after I retired. No trouble at all!
--Anyway there is your answer for one reason why would any one change just one piston!!

Sugar Bear

Had a Mercury in-line six outboard, only cylinder 3 was bored oversize. I asked two excellent, experienced Mercury techs about the piston weight difference. They said piston weight was the same oversize vs standard.
​​They also said and we're proven correct by a compression test that the PSI would be slightly higher due to boring leaving a flat lip in the quench area. This was a blind cylinder design so the head couldn't be removed for boring.
Piston needed to be replaced due to detonation because of too much timing advance which was my fault.

​​​​​​Curious, anyone know if the weight is the same for a GM factory oversize piston vs standard or was this unique to the Mercury outboard?

Bigmikey65

Sugar Bear the marking looked similar to a Pi symbol.

Tedd Thompson

In a place loong loong time ago I helped a friend in high school auto shop bore a single cylinder on a 292 Y block out till we could fit a standard 312 piston in it. It ran and didn't experience any major vibration nor did it overheat. Seems a wrist pin came lose and tore up the cylinder wall. He had bought this car (55 T bird) quite cheap and was the envy of our school After he got it running.....Tedd