Strange one this:

New engine, new custom Wiseco pistons, freshly bored block. On start up there were no strange noises, engine sounded sweet, good oil pressure etc...

Was mapped on the dyno, made good power, did around 250 miles or so without any noises what so ever or any sign of a problem when due to some unlucky problems it got very hot and killed the headgasket. Since then it developed a light knock, which quietened after time. The car still drove well, didn't smoke and performed well.

We assumed the noise was the lifters being full of crap and it had got too hot, but after overheating again for the second time the noise got a lot worse, to the point where on a cold start up it sounded like it had spun a shell, but the noise got a lot better after some heat/time. A few people suggested it was piston slap, but it never piston slapped before so we discounted it.

We decided to pull the head off for a rebuild though and when we looked at the pistons all 4 have around 1-1.5mm of sideways movement I.e. Piston slap. They have lightly scored the bores too.

Now when building the engine it didn't have this movement, and it never made this noise before; so why now?

Machining error? Or related to the fact it overheated twice?

Properly confused!