KB107 (0.30) Pistons undersized?

.004 is pretty much the limit of piston to wall clearance on 107's, not a huge problem, it will run there, might make a little noise due to piston slap. Being Hypers, they don't expand much when warm, which is why you can run them so tight.

You are doing a piston change.......just out of curiosity, what was the piston to wall clearance of the old pistons, what did you take out?

On off chance possibility, call UEM and see if you can get a set that measures 4.030? It's a possibility that you are a victim of manufacturing tolerances stacking up against you. A phone call doesn't hurt.