He said I should be good by boring .20 over so is that 100 mm? From the standard size?

If you're doing forged pistons you need to order the pistons, or have the shop doing the work order them before having the engine bored over. Actually I would do the same for any oversized piston, not just forged, so the piston to wall clearance can be set properly to the piston manufacturer's specifications. Too much PWC will result in a loud running engine that burns oil, too little PWC and you could destroy the engine completely with a piston getting stuck in the bore.

Uhmm but what's the difference? if you have 100 mm pistons and he bores to 20 over how is it going to make any difference if he had the pistons before hand or not? Yes I understand the piston expands when up to temp but it seems like it will have to be slightly more then 100 mm?

Because that is how it is done if you want an engine to last. You do realize that Subaru engines have slightly different votes from the factory even on the stock sleeves right. Without having the pistons you're going to use in hand there is a very low chance of getting the proper PWC by just punching the bores out 0.020".

Pistons are fit to the bore exactly when finished. Some machine shops will do it without but its a guess at that point.. A good shop wont touch a block without pistons in hand.. Forged need to be run a little looser but still need to have pistons... The tolerances on pistons are not perfect, and when were talking about tolerances that are nearly exact. It matters.

so pretty much buy the pistons then tell him to bore them to this size and he should be able to get the PTW needed.

Right,order pistons, take pistons and case halves to machine shop along with the spec sheet that will come with the pistons. The shop will measure the pistons, add the recommended PWC clearance and bore accordingly.

Well back from vacation I got my pistons before I left now I just need to save up after spending so much