944M3

I’ve been researching the difference between 2618 and 4032 piston alloy, and I get it.

My question is how long does it usually take for 2618 pistons to expand to proper tolerance? How loud is it actually?

Looking at Mahle 104.5 turbo pistons for 16v build. But they are 2618 so I’m hesitant.

Btw if anyone has a set of pistons for a 16v build, I’m interested.

alex_cristocea

If you will not use the engine only for racing or track use I don't recommend using it. I use them for a few years and on normal day to day use if I open the window I can hear the slap at idle.

944M3

Thanks for the tip.

V2Rocket

related but not direct experience -
I am a fan of Subaru cars.
they build their engines with some extra piston/bore tolerance for whatever reason and piston slap is a known characteristic of the EJ/FA/FB engines.
on a cold start they sound like a box of rocks but it goes away before i get out of my neighborhood. a minute or 2 at most before the engine has warmed up enough.

four0four

I think a very large amount of the advice around 2618 is cargo cult - just people repeating what they've heard/read, up to some manufacturers. This leads to relatively huge ptw clearance, which leads to noisy pistons. Mahle's current recommendation for 2618 pistons is (for 2618) extremely tight (2.0 - 2.8 thou iirc). I called them up over this and the support engineer I chatted with was pretty helpful: Older versions of their drawings specified more "typical" clearances, but within the last...8 or so years, they changed to the current spec with no increase in failure reports. Highly suggest doing the same for your questions!

Anyway, I also grabbed a bunch of 2618 and 4032 alloy CTE characteristics and messed around with the expansion numbers a bit. Couldn't make it a problem, so I shot for the tighter clearances. I haven't turned the engine on yet (and I'm just some dude on the internet), so take all this with quite a bit of salt.

944M3

Thanks. I’ll give them a call.

blade7

In Alusil or iron liners? I have a set of unused custom 104mm Wossners, supplied a few years back by Racers Edge. Without checking the box I think the clearance is 4 thou.

four0four

Refinished Alusil at first oversize - struck a balance between wanting to stay all aluminum and not shelling out the extra for Millennium's nikasil process. Probably would've been more cost effective now that I'm on the far side of that, but euh.

blade7

I hope it goes well. I always fancied going with Alusil, if I ever got around to using my 2.7 block. But now it would have to be liners or plating.

944M3

ok I’ll bite.

I’ve read several posts that you’ve written suggesting that today’s pistons won’t work on Alusil. Why?

To be clear, I have no idea if Alusil is better than any other material. I’m just asking.

blade7

I don't know that there aren't any pistons manufactured today that won't work in Alusil, but it seems there have been failures of the coating recently used by the popular choices. Personally I would be taking advice from experienced Alusil engine builders on here about clearances. As 2 thou might end up being too tight one day.

Droops83

That is interesting to hear. I have a Jensen-Healey roadster with the Lotus 907 engine, had early 1990s Venolia forged pistons of the high-compression Euro-spec, also the old-school 2618 alloy. I rebuilt the engine, so I called Venolia and asked what the piston to bore clearances should be. The guy said 0.005-0.007." I almost spit out my coffee and asked him to confirm this three times. I went with 0.005," piston slap is hilarious while cold, but the engine has run great for 20K+ miles and doesn't use much oil (also despite lacking valve guide seals as from the factory).

Metallurgy has made great strides since the early 1990s, so I think piston slap shouldn't be an issue in any case in terms of a current performance engine build, unless you are building a drag or land speed record engine.