Question about solving piston slap.

94.GT.Wagon · Post by **94.GT.Wagon** » Thu Jul 02, 2009 6:10 pm

My wife's 99 Outback (ej25d, 105k mi) has more piston slap than I can tolerate so I intend to replace all 4 pistons. Do you guys recommend using new OE pistons or something from the aftermarket? When I asked some questions at the dealer they said that Subaru sells replacement pistons that are a couple of thousandths larger in diameter that solve the problem. Thoughts?

SLODRIVE · Post by **SLODRIVE** » Thu Jul 02, 2009 8:30 pm

I wouldn't do it at all. If it's just piston slap, your motor will more than likely live a long (albeit a bit noisy) life as it is. If you throw different pistons in without boring & honing the cylinders, there's absolutely no guarantee the pistons will fit correctly, nor can you be sure the cylinders are perfectly round and straight.

gijonas · Post by **gijonas** » Thu Jul 02, 2009 9:33 pm

Not to mention all the other crap that can be screwed up in the process - unless you want to go ahead and be thorough the whole way,which will cost too much to justify.

suba · Post by **suba** » Fri Jul 31, 2009 11:08 pm

Also (someone correct me if i'm wrong here) I believe part of the reason those engines get piston slap is that they don't have skirted pistons. Replacing the pistons will quiet it down....for a while. So far I haven't found any aftermarket pistons for that engine. I have found that supposedly Wiseco makes performance lower and higher compression pistons for the ej25 which are supposed to be skirted however the kit for pistons alone is upwards of $700.

wtdash · Post by **wtdash** » Sun Aug 02, 2009 7:07 pm

If you replace the pistons....

Your EJ25 Phase I is about due for its head gaskets too.
TD

subytech · Post by **subytech** » Sun Aug 16, 2009 3:46 am

Traditionally the dreded 2.5 piston slap happens on cylinder number 4, sometimes number 2. Now I'm not sure about the phase I motors but I do know that, as long as the bore is ok, subaru offers 0.0002" larger pistons that are supposed to cure this problem. All you need to know is what pistons you already have (A's and or B's) No boring or honing required (or so subaru says). So if your not wanting to do a whole rebuild I'd look into this and maby slap some pistons in your noisy cylinders.

94.GT.Wagon · Post by **94.GT.Wagon** » Sun Aug 16, 2009 3:30 pm

Good info, Subytech, thanks. How do I find out what pistons I already have and what's up with the A and B pistons anyway? What differentiates A from B?

fasttrifive · Post by **fasttrifive** » Mon Sep 07, 2009 10:09 am

I thought I heard that the a and b pistons are very slightly different in size and were determined by the temperature, humidity, etc of when the block was bored and pistons installed. Aka to make up for tollerance issues at the factory.

Post by **Legacy777** » Mon Sep 07, 2009 5:00 pm

Yeah there are slightly different sized stock pistons, A, B, & C.

The block will be stamped with the size pistons it was put together with.

beatersubi · Post by **beatersubi** » Thu Apr 15, 2010 3:32 am

Legacy777 wrote:Yeah there are slightly different sized stock pistons, A, B, & C.

The block will be stamped with the size pistons it was put together with.

Where might one find this stamping?

subytech: how does one go about ordering the larger pistons?

Thanks guys.

Post by **Legacy777** » Fri Apr 16, 2010 6:32 pm

It's on the top of the block halves, right where they come together under the alternator. There will be a machined section and if you look closely, it will have the letters there.

You would need to order them from Subaru. Honestly.....if you're tearing down the motor to put in larger pistons, the rings will likely need to be replaced, which really means the bore needs to be rehoned so the rings seat.

At that point, you may want to look at buying a used or new short block....or rebuild the motor. However if it's a daily driver, I'd try and find a used motor.

beatersubi · Post by **beatersubi** » Sat Apr 17, 2010 7:55 pm

Found it, thanks.
Block is already torn down and rings were in the plan anyway. Anyone have access to a parts catalog w/ part numbers for the oversized pistons?

Post by **Legacy777** » Mon Apr 19, 2010 12:23 am

Email or PM your VIN, and I'll look them up on the electronic parts catalog I have.

Bheinen74 · Post by **Bheinen74** » Mon Apr 19, 2010 10:50 pm

this would kinda be bad if the factory installed the "c" pistons in your engine already due to their AKA tolerance in the factory....if they put largest in then, then what, find some "d" pistons... or sol.rolleyes

Post by **Legacy777** » Tue Apr 20, 2010 5:37 pm

Pretty sure there's only A, B, & C size pistons. If you need larger, you would need to have the engine over bored.

IronMonkeyL255 · Post by **IronMonkeyL255** » Fri May 14, 2010 7:20 am

A, B and C for our motors (2.2), and A or B for the Phase 1 2.5s, IIRC.

For the 2.5, A pistons are 3.9167 to 3.9171, while the B pistons are 3.9163 to 3.9167

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