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OEM pistons better for Daily?

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Hi there,

I have been tearing into an engine for a rebuild and was thinking, while im in there, might as well put in forged internals. I can get away with re-honing the bores and replacing rings, but this is a boosted application (Mazdaspeed3) and I want to leave headroom for fun down the road.

My only concern with forged internals right now is that some say that the material of typical aftermarket forged pistons require larger clearances, which result in "piston slap" on cold start, ultimately wearing out the engine. I live in Canada, and this will be my daily driver.

So, stick with OEM pistons and rods or is this a myth and its all in the quality of assembly (go forged)?



How long do you plan to keep the car and how many km's per year? The forged internals will help prevent it from going out with a bang, and the OEM pistons will help prevent it from going out with a whimper, if that makes sense. If there is a significant risk of blowing something up (and my understanding with the Speed 3 is that there is--low speed preignition concerns, high pressure fuel pump failures, plus the normal stuff that goes wrong on boosted project cars), then I'd bite the bullet and go aftermarket forged. I assume the aftermarket ones have a similar piston crown/piston bowl design compared to stock.

From a driveability perspective, typically pistons don't have a big impact unless there is a major change in compression ratio (requiring more retuning). It's changing the heads and cams that can affect driveability.

Piston slap shouldn't be a big concern if the clearances are on point. I find a lot of machinists tend to ignore the manufacturer's recommendations regarding piston to bore clearance and machine a little on the loose side. From their perspective it adds a little safety margin but it can result in piston slap when cold. JE have recently come out with their 'perfect skirt' coating which is specifically designed to help with this situation so I'd have a look into whether they have a suitable application for your engine. We are running these in both our SR20VE and our FA20 but neither are operational yet so it's hard to give you specific details on the results.

Thank you for the replies.

One other thing I would like to mention that its common to go to -25C (and colder) during winter months where I live. Not so great for starting.

@Raymond: This will be a daily driven car, to replace my current (tired) car. I strongly believe that most cars get a bad reputation because of abuse, usually oil maintenance or lack of supporting mods. I am most interested in longevity, but I do want to leave room for a little fun in the future.

@Andre: I appreciate the in-site/experience.

The machinist I dropped off my bare block and cylinder head at told me the following:

- Current piston (OEM) to wall clearance is about 0.004"

- Cylinder 1 out of round (minor)

- Need to bore all cylinders by 0.020"

Cylinder 1 was slightly lower on compression and these blocks are known for going out of round.

This machinist runs a very small shop, has not worked with these engines (he usually does subaru) and has a bit of an ego. Am on the fence about this shop. What are your thoughts guys?


Properly tuned, there's no reason why you can't get good reliability and a long service life out of the OEM pistons even with a reasonable boost in power. That being said, as mentioned above, I don't see a problem with forged pistons either. You decision is really down to how far you're likely to push the engine and whether aftermarket pistons are justified. From there you'd also need to consider costs because in some instances factory pistons can be very cheap but other times ridiculously expensive.

In general machining is machining so I wouldn't be put off just because the machinist specialises in Subaru. The bit about his ego is harder to advise on though. I'd be talking to his other customers to see what their experiences have been like.

Andre, thanks again for the advice, puts my mind at ease.

I remember in one of your videos you shared a thought on why lapping isn't very effective. I do not remember the exact comment from you, but this machinist told me that I do not necessarily need a valve grind, just lap the valves and be done with it.

What are your thoughts on that comment?

Lapping valves needs to be dealt with carefully. Lightly lapping the valves with a fine grinding paste is absolutely fine and can help show you the contact between the valve and seat as well as removing light chatter marks that are common from the valve seat cutter. The problem comes where you use a more serious amount of force and a coarse grinding paste to recover poor or damaged valve seats. When you do this you will end up with a concave valve seat and the valves will only seal properly when they are cold. So ultimately the solution depends on the condition of your existing valve seats.

Thank you for the information.

Regarding the original question of piston material.

A popular supplier (for my car) has a disclaimer on their website that states 2618 alloy pistons require a large piston-to-wall tolerance and therefore inherent to piston slap and ultimately cylinder bore wear (Rounding). They recommend 4032 alloy pistons to accommodate a tighter tolerance for longevity. They provide no testing or scholarly evidence of this, but provide theoretically possible information.

I was curious if this disclaimer could be based on outdated information? Maybe machining or piston manufacturing technology has advanced to eliminate these claims? May be difficult to answer, but would like to understand this topic better before investing into my engine.

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