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This is an outstanding primer on the expectations, benefits and pitfalls of forged pistons that I read some time ago when researching the ringland issue. It's a very balanced approach that alerts a possible power enthusiast to the benefits, as well as pitfalls of forged pistons. Basically because forged pistons have looser tolerances, they will score the cylinder wall and wear down faster. So in the short term, forged pistons can take more abuse such as higher power levels or being run harder such as on the track, the overall lifespan of the motor may in some cases be shorter.

I am not steering anyone away from getting a built motor or not, but I think this should be read by everyone in the situation where they might be thinking about a built motor. Remember that in life happiness equates to expectations, if they are met you will be satisfied. If your expectations are not met, you will not be satisfied; if they are exceeded though, then you will be truly happy.

The key is then to have expectations that are reasonable, high enough so that you don't settle for the first lousy thing that comes your way, but not so excessive that you will always be disappointed. This thread on the Legacy Forum is a great place to start:

To those of you considering aftermarket forged pistons... - Subaru Legacy Forums
 

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This is why I went for the Cosworths. Closer to 4032, but not as brittle as something like the Mahle 4032 pistons.
 

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This is why I went for the Cosworths. Closer to 4032, but not as brittle as something like the Mahle 4032 pistons.
The real issue is that Manley, cosworth, etc won't release their material specs with hardness testing data for each piston. Meaning we don't know the true strength of the Pistons. I can almost guarantee they batch these and take a sacrificial piece to be tested. The problem is that we don't know the QA for these different companies. If we did, we could better choose a manufacturer.
 

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I'm looking for proven longevity so I'm interested in 4032AL pistons. Mahle makes the 4032AL OEM pistons for the EVOX so that's pretty much guaranteed to last (and hold 350 WHP out of a 2L). Pretty sure the (Mitsubishi) OEM has lots of test data for the material at hand and it was chosen as a good compromise between longevity and strength. I believe Subaru also uses 4032AL pistons on their JDM 2.0L STI's.

4032AL Pistons for the Subaru Platform = Cosworth, Mahle, Wossner, and Supertech. I want to snag Wossner 4032AL pistons coupled with a closed-deck block from Outfront (and the rest OEM) but they don't make them in 99.75 bore (closed deck process requires machining work so no 99.50 mm).

Want something reliable, doesn't need a rebuild for 100K miles - not building BIG power (as in, we are happy with our stock turbo's, or slightly larger than stock turbo like the DOM 1.5 XTR at most)
 

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Interesting info, sheds light on a different perspective.
 

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I can't imagine why you would need more than one reserved spot...

As far as forged pistons, I've found Subaru's seem to be on the loose side. When I built my SHO motor, the PTW clearance with Wiseco pistons seemed to be closer to OEM PTW clearance of a Subaru. On the spare motor I am going to build, I will do a closed deck setup that will include the hone. It looks like you can tighten up the clearances safely doing so.
 

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Discussion Starter #12
I can't imagine why you would need more than one reserved spot...

As far as forged pistons, I've found Subaru's seem to be on the loose side. When I built my SHO motor, the PTW clearance with Wiseco pistons seemed to be closer to OEM PTW clearance of a Subaru. On the spare motor I am going to build, I will do a closed deck setup that will include the hone. It looks like you can tighten up the clearances safely doing so.
I did it in a haste because I don't know what I want to do with it, but it's a good article to read; even though it's for Hondas the concepts carry over and it's a good primer to read if someone is considering a built motor.
 

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yamahaSHO - considering you are going closed deck, you'll be running a 99.7X piston then right? I don't believe they have closed deck blocks from any vendor that's at the stock 99.5 bore.

Per Outfront, the close-deck process distorts the cylinder so they bore it to the next size up (usually 99.7X)
 

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yamahaSHO - considering you are going closed deck, you'll be running a 99.7X piston then right? I don't believe they have closed deck blocks from any vendor that's at the stock 99.5 bore.

Per Outfront, the close-deck process distorts the cylinder so they bore it to the next size up (usually 99.7X)
Yes, I will have to step up to a larger piston.
 

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I'm looking for proven longevity . . . .

4032AL Pistons for the Subaru Platform = Cosworth, Mahle, Wossner, and Supertech. I want to snag Wossner 4032AL pistons coupled with a closed-deck block from Outfront (and the rest OEM) but they don't make them in 99.75 bore (closed deck process requires machining work so no 99.50 mm).

Want something reliable, doesn't need a rebuild for 100K miles - not building BIG power (as in, we are happy with our stock turbo's, or slightly larger than stock turbo like the DOM 1.5 XTR at most)

You shouldn't need a closed deck block for 1.5?
It's borderline for needing to be built at all?
 

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Sure, stronger has no drawback if you can afford it, and "recent" blocks do seem weaker. At least Element reinforces all their blocks too. Still not sure its necessary for a 1.5. Never heard that recommendation for a GD block.

Also bigger than a 1.5 is not a 1.5.
 

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Why Subaru blocks has problems with forged pistons, many OEM brands, honda for example, has forged pistons and theirs engines last 100K miles. I made 50K with SC S2000.

It is because the block's design and metal alloy? or the problems comes when making big power +400 WHP and driving the car very hard
 
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