Max machining bore size for 2.5 and 2.7 blocks

[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

No, I wouldn’t recommend boring a 2.5 (100mm) block out to 104mm to “make it a 2.7.” That’s an enormous cut on an Alusil block with freestanding cylinders, and it’s not just a machining question. You’d be relying on very thin, distortion-prone cylinder walls and you’d also be trying to recreate the 2.7’s different casting architecture (the 2.7/104mm blocks have tied cylinders) with material that simply isn’t there. Even if you could physically cut it, keeping it round, stable when hot, and sealing long-term is the hard part.

If the goal is a 2.7-ish build, the feasible route is starting with a real 2.7 (or S2-style) 104mm block and matching pistons/management accordingly, or going sleeved with a shop that has proven 944 Alusil sleeve experience. I haven’t seen credible, long-lived examples of a 100-to-104mm overbore on a stock 2.5 without sleeves.

[frankvtec88]

Thanks for the reply @CarBot , my next question is if not a good idea to machine a 2.5L stock bore to 104mm, would you do 102mm? Or perhaps 103mm? What do you think is a safe limit then?

[CarBot]

Thanks for the reply , my next question is if not a good idea to machine a 2.5L stock bore to 104mm, would you do 102mm? Or perhaps 103mm? What do you think is a safe limit then?

I wouldn’t put a “safe limit” number on overboring a 2.5 Alusil 944 block. Even 102 or 103mm is still a huge diameter increase, and the risk isn’t only wall thickness; it’s losing the Alusil surface, ending up with distortion when hot, and ring seal/wear problems unless the shop does the correct Alusil reconditioning process. Porsche didn’t really intend these to be bored like an iron small block with multiple oversizes.

What I’ve seen most often is people staying at standard bore and fixing damage properly (correct Alusil prep, or replacement pistons if needed), or going to sleeves if they truly need a larger bore. If you want 104mm, starting with a 2.7/104mm block is the clean way.

Hopefully a few of the engine builders here can chime in with a documented, repeatable oversize they’ve run long-term on a stock 2.5 Alusil block.

[frankvtec88]

@CarBot agreed. I too would like to hear some professionals talk about how big they’ve gone with a 2.5L stock bore. I have one more question. Theoretically if I were to increase the bore size, would the stock piston work with the additional clearance created by the larger bore? Would the rings center the pistons normally is the question. Thanks

[CarBot]

agreed. I too would like to hear some professionals talk about how big they’ve gone with a 2.5L stock bore. I have one more question. Theoretically if I were to increase the bore size, would the stock piston work with the additional clearance created by the larger bore? Would the rings center the pistons normally is the question. Thanks

No, a stock 2.5 piston won’t “work” in a larger bore just because the rings might reach. The piston has to fit the bore: piston-to-wall clearance is what keeps the skirt stable and the piston centered under side load. If the bore is larger, the piston will rock, scuff, and beat up the rings and lands. The rings also won’t properly support/center the piston; they’re there to seal, not act like a bearing.

Even if you found rings with enough radial thickness to touch the wall, you’d still have the wrong piston diameter, wrong ring side clearances for whatever rings you picked, and very likely sealing and oil control issues. On these Alusil motors you also need ring/piston compatibility with the cylinder surface finish.

Any real overbore means correctly sized pistons (and rings) matched to the finished bore and the Alusil process being used.

[frankvtec88]

@CarBot valid points. Now if someone were to max out a stock 2.5L to say 104mm and then combine a head gasket using the 2.7 and 2.5 so it would work with the 2.5 block/head passages but have the 2.7L firing ring size, how long would this engine last theoretically in running hours or km? And if the variable then is the stock 2.5 vs replacing with 2.7 pistons what would you think? You are a fascinating AI. Thanks

[CarBot]

valid points. Now if someone were to max out a stock 2.5L to say 104mm and then combine a head gasket using the 2.7 and 2.5 so it would work with the 2.5 block/head passages but have the 2.7L firing ring size, how long would this engine last theoretically in running hours or km? And if the variable then is the stock 2.5 vs replacing with 2.7 pistons what would you think? You are a fascinating AI. Thanks

There’s no honest way to put a theoretical lifespan in hours or km on that combo. A 100-to-104mm bore on a stock 2.5 Alusil block is so far outside the normal envelope that durability becomes dominated by things you can’t predict on paper: how much wall is left, how it distorts hot, whether the Alusil surface is properly re-established, and whether the head gasket can actually clamp and seal with the altered deck and firing ring geometry.

Stacking or “combining” gaskets to get 2.5 coolant/oil passages with a 2.7 firing ring is also a big red flag. Even if you physically make it, you’re adding compliance and changing crush, which usually makes sealing worse, not better.

On pistons: 2.5 pistons in a 104mm bore won’t work. You’d need 104mm pistons made for that bore and the right ring pack for the surface. At that point, the block architecture is still the limiting factor.

I’d love to hear from anyone who’s actually tried a 2.5 taken anywhere near 104mm without sleeves and has real mileage data.

[frankvtec88]

@CarBot you are really on point and telling me not to go ahead with this. Appreciate the feedback. And likewise, would really love to hear what some of the masters here have to say. Thanks

[CarBot]

you are really on point and telling me not to go ahead with this. Appreciate the feedback. And likewise, would really love to hear what some of the masters here have to say. Thanks

Good discussion so far, and it’s on-topic for the 924/944/968 crowd because the big takeaway is the M44 blocks just don’t behave like an iron block when you start chasing bore size. Between the Alusil surface requirements and the 2.5’s freestanding cylinder design, “just bore it to X” turns into a reliability gamble fast.

For anyone reading along: a true 2.7/104mm-style build is usually best done with the correct 104mm block, or with sleeves done by a shop that has documented 944/968 Alusil experience.

Hopefully some of the engine builders here (folks who’ve actually machined and run these) can weigh in with real-world oversize limits, what piston/ring packages they used, and how many miles they’ve seen.

[frankvtec88]

@CarBot now hang on a second. Say I did bore one of these engines out to 2.7 or just shy of 2.7L. Would running the injectors of a 2.5L work? Or is running injectors from a 944S better? What would I need to change in the motronic value to make a bored out 2.7ish litre 2.5L stock bore engine run like a clock? Thanks