Ok here we go.....
First about rebuilding an engine to use stock pistons:
Honing and boring are two different things.
Honing is the process of refinishing the cylinder walls to create an ideal surface finish to break in a newly installed set of rings and then create a good seal. It removes a small amount of material. Equate to sanding with a abrasive if that works for you
Boring is the process of resizing a cylinder by cutting away larger amounts of metal with a cutting tool. Boring does not create the necessary finish. Boring is always followed by honing.
Subaru does make oversized pistons. You can go to the dealer today and order .25mm oversized pistons and .25mm oversized rings.
The part number for the left pistons is 12018aa520 $87.35 each you need 3
The part number for the right pistons is 12013aa520 $87.35 each you need 3
The part number for the rings is 12033aa460 $351.05 for the complete set
You will find the pistons are listed as piston sets. That means they include the pins and circlips not that you get more than 1. So a complete set of new pistons and rings will cost you $875.15 which is less than a custom piston set will set you back.
.25mm is basically 10 thousandths of an inch. This means your cylinders will have to be bored..so both bored and honed.
So the way it works out is buying new parts from the dealer is a little less expensive than going with custom pistons and you know ahead of time they will be right. Custom pistons aren't always right.
There is another option which can offer a little better performance for a little less money but it requires more access to used engine parts than most people have. Standard size Subaru pistons come in 3 fitments (A,B,C). A size pistons are the biggest. Your engine will have an assortment of these sizes in it. You can collect together a complete set of A size pistons and send them to Calico Coatings or Swain Coatings to get performance coatings. They will bake and then chemically clean the pistons so they are nice and perfectly clean and prepped before coating them. Then they can apply an anti-scuff/anti-friction coating on the sides and a thermal barrier coating on the tops. Your engine builder can then lightly hone your cylinders to perfectly fit the pistons. Here is just such a set I've collected together. If you want it I'll give it to you for $150. It includes an extra set of lightly used genuine Subaru rings you could use with no problem.
