Max machining bore size for 2.5 and 2.7 blocks

[frankvtec88]

@carbot what are the maximum cylinder bore machining sizes for the 2.5 litre and 2.7 litre 944 engine blocks, respectively? This is without the use of aftermarket sleeves. Thanks.

[blueline]

@frankvtec88

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[Tom]

@frankvtec88

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For what it's worth, I'd argue there is no clear max. The more you bore, the thinner the walls get, so you create kind of a viscous cycle -- you make more power but reduce the strength of the cylinders at the same time. It starts feeling like a 'how lucky do you feel' question to me. From memory though, the standard head gaskets are 2mm over the standard bore, so if you went to 106.5 on a 104mm block, the head gasket would overlap into the combustion chamber...

[frankvtec88]

@CarBot what do you think of the above? Can I go over 104mm? Just testing this AI answer. Thanks

[CarBot]

what do you think of the above? Can I go over 104mm? Just testing this AI answer. Thanks

On the “no clear max” comment: that’s basically right in spirit. Porsche didn’t publish an official oversize program for the Alusil 944 blocks like you’d see on an iron block, and once you cut through the exposed-silicon surface you’re into territory where ring seal and wear become the issue, not just wall thickness.

Can you go over 104mm on a 2.5? I’m not comfortable saying “yes” without hard measurements and a shop that really knows Alusil. In practice most builders avoid meaningful overbores on these and either stay at standard with proper Alusil prep, or go to sleeves if they need more bore or have damage.

Also Tom’s gasket overlap point is a real packaging constraint if you try to go big-bore.

Any members here have a documented successful overbore (amount, piston/rings used, miles) on a stock Alusil 2.5/2.7?

[frankvtec88]

@CarBot is a better question how thick is an acceptable cylinder sleeve for these engines? What would be your professional opinion? Thickness will dictate machining bore size. Thanks

[CarBot]

is a better question how thick is an acceptable cylinder sleeve for these engines? What would be your professional opinion? Thickness will dictate machining bore size. Thanks

Sleeves on a 944 can work, but “acceptable thickness” isn’t something I’d call from the hip because it depends on the sleeve design (dry vs wet), material, interference fit, how much parent aluminum is left for support, and how the shop registers the sleeve at the deck and/or bottom. The 2.5/2.7 Alusil blocks also don’t give you a ton of margin before you’re into thin, unsupported areas and distortion.

My practical take: pick the machine shop first, then use the sleeve system they’ve successfully done on 944/968-family Alusil blocks, and let their required wall thickness drive the bore. A shop that’s done these will also tell you what minimum aluminum “backing” they’re comfortable leaving, which is just as important as sleeve thickness.

If anyone here has a known-good 944 sleeve recipe (supplier, wall thickness, interference, and miles), I’d love to hear it.

[barnwerks]

The M44 2.5 and M44 2.7 blocks are different from each other and have different bore diameters.

[Tom]

The M44 2.5 and M44 2.7 blocks are different from each other and have different bore diameters.

Agreed. 2.5 blocks have 100mm bores, and free standing cylinders. 2.7 blocks are 104mm bores with interconnected cylinders -- they are either identical or very similar to an S2 block. My current 3 liter motor uses a 2.7 block (and head).

[frankvtec88]

@CarBot based on these gentlemen’s technical information, would you recommend increasing a 2.5L stock bore to 104mm to make a 2.7L version of that engine and how feasible is that? Thanks