Oil clearances and a surprise

PRH said
Pistons on backwards?

As long as there isn’t much variation in piston or bore sizing, you can just swap the pistons/rods side to side, if you want the rod chamfer, piston notch orientation “correct”.

Having them on backwards is an old racing trick anyway.
Put 1 in 2, 2 in 1, etc.
Supposedly reduces the skirt loading from the pin offset(which is only there to reduce noise at cold start up).

If the issue is race type pistons and the valve notches....... that’s a different story.

What pistons are you working with?

Pressing pistons on backwards is an old racing trick for dead stock pistons. The pin bore in stock pistons is off center to make the engine quieter. Reversing them actually frees up a little power by reducing friction and loads but at the expense of a little piston slap.

However, performance pistons have the pin centered, so unless he is running a stock or stock replacement piston, pressing them on backwards will do nothing for performance.