Probe piston wall clearance.

You can check their site for suggested gap. That's about what they should be undersized. Forged depends on the material. 4032 expands less and will probably only have 1.5-3 thou built in the piston. 2618 expands more so 3.5-4.5 thou would be a good estimate there, imo.

2618 pistons are a stronger piston from what I've read. You can call Probe Tech dept and ask them for the skirt dim for that specific piston.