Torque plate hone etc, Tom at BD4's said to me they build em with 2 thou or less piston to bore clearance (HKS pistons).
I can only think they could go that tight if they are a high silicon content piston, and barrel shaped when cold like TOMEI pistons.
Also made to suit jap style tight engines, not yankee 10 thou or so clearance lol.
Correct me if I'm wrong guys...
Cheers.