When should they be replaced if at all.I'm rebuilding a engine and should I replace them ? 2 need to be replaced from detonation.

I am replacing my JE's at only 140 hours. Normally would go much longer, but I had a problem with porosity in my Merlin heads and had water seeping into the combustion chambers, turning into steam, and superheating the oil. The engine builder saw some signs of premature metal fatigue in the skirts and recommended replacement.

I would replace them all. The price of 6 pistons is not that much in the scheme of things when you are talking a full rebuild. Not worth taking a chance.

I thought so but I needed someone to tell what to do since wife useally does it but she can't help with this one.Thanks

If 2 are needing replacement, then I agree to do them all. Chances are that you'll need some overbore for the 2 new ones and will need to do them all anyway.

Rene