Re: Contemplating a new project - Building an Engine
because Subaru mass produces these cars on a factory scale. not every bearing is measured and fit to exact tolerance. Some builders have stated they've seen Factory OEM case halves that have never been ran have out of round main journals.
likely you will want to increase clearance over factory since the FSM for my 2012 states:
Rod clearance: .00007 to .0018
Main clearance: .00039 to .0012
These are tight for an engine that is going to be abused.
It depends on how the engine will be used on how you decide the clearances, but a "general" spec is .002 on rods and .0015 on mains.
Piston slap is due to increased piston-to-wall clearance. Subaru specs this at:
-0.00039 to .00039
If you just drop in forged pistons, you're PTW clearance will likely be .0025 to .0045 or worse. it will slap bad on cold start(ask me how i Know. my PTW clearance is .0025) and "can" eventually oval the bores causing oil consumption.
to remedy this, you get your pistons sized with the bores. by taking it out to 99.55mm or 99.75mm.
Stock cylinder diameter is:
A bore: 99.505 to 99.515
B bore: 99.495 to 99.505