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Subaru - EJ255 Build

General Engine Building Discussion

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Hey all,

I have a 2009 Subaru Forester SH XT 2.5L Turbo (EJ255), long story short the motor is out of the car it was mixing coolant/oil. It got hot at some stage and this was the result.

None the less I am thinking of building the motor with the goal of a streetable, reliable 300kw motor - give or take.

So far my planned build includes - Forged internals (Rods/Pistons), stock crank and closed decking the block, bigger turbo (Still undecided), E85 fuel setup - Injectors/Pump. I want to do the assembly myself and outsource the machine work.

Here is when my main question's begin.

1. What is acceptable piston to cylinder wall clearance for a forged engine?

2. Should/Does the machine shop do the measurements for me?

3. How do I know what clearance to run? Does the manufacture include the tolerance's or is there a generally accepted rule/formula ?

4. Are standard clearance's ok for the forged rods ?

5. Will using forged pistons cause piston slap when cold? Is this still a thing?

I don't currently own a dial bore gauge or micrometers....yet (Assuming these will be required if the machine shop does not measure everything up)

If anybody has built some EJ255 motors I would be very interested in hearing what you did and what you recommend for a everyday driver.

Hey Ben, I'll answer your questions below:

1. This is dependent on the particular piston you're using and the power level/boost level. I'd start by checking the manufacturer's recommendations as they will give you a required clearance to work to. For a 2618 forged piston you're likely to need 0.003-0.005" clearance.

2. Yes, this is done during the boring and honing process. You should be specifying your required clearance to the machine shop in writing though.

3. Start with the manufacturer's workshop manual for the engine. All the clearances and tolerances are listed for the stock engine. For a 300 kW build I'd be aiming for the loose side of the stock clearance range.

4. Yes, the rod doesn't actually have much to do with the clearance, but rather it's a function of the power and usage the engine will see.

5. Yes it can, however if your machine shop carefully controls the piston to wall clearance then this should be minimal. To some degree it's unavoidable because the forged piston requires much more clearance when cold than a stock cast piston.

Definitely go on the looser side of factory bearing clearances. 0.0015" for mains and 0.002" big ends are much better clearances for a high power motor. This is what i aim for with the forged motors i build but if you have a machine shop that does a lot of subaru motors they may be able to give you an idea.