Physical difference in crank carriers from 2.5 to 2.7?

[Bebbetufs]

As you probably have guessed I have never compared them, but from looking online I can see that the main bearings are mirrored, i.e. the tang is on opposite sides.

Apart from this, are there other differences?
I'm guessing the 2.7 is machined to accept the longer throw of the 2.7 crank, can anyone confirm?
It would be great if anyone has a photo of the two compared.

[78F350]

If you want a definitive answer, try contacting Jake Raby. https://flat6innovations.com/
Here's a post about mixing various M96 engines that I read a while back. Not an answer to your question, but similar topic:
https://rennlist.com/forums/996-forum/684465-is-there-a-diference-between-a-3-2-986-and-3-4-996-head.html#post9339329

I carry out this type of interchange frequently. I have included an interchange table in my assembly manual that should answer most questions.

Combustion chamber variances of 3cc are the biggest difference in the two heads as long as both are from the same era. Both engines must be "5 chain" to allow this to happen easily. There were no 3 chain 3.4 engines, and all were 5 chain. The 3.2 heads will increase the compression ratio of the engine due to their smaller chambers. The 3.2 and 3.4 share the exact same camshafts and the only difference between the 2 internally is the bore size, CR and intake arrangement.

It is possible to install 3 chain heads on a 5 chain engine and 5 cain heads on a 3 chain engine, according to what the combo is you are shooting for.. Hell, I have even used a 2.5 Boxster crank inside a 3.8 liter 997 crankcase for a short stroke, Big Bore HI RPM beast.
3.6 heads have vane cell adjusters for their vario cam + cam arrangement and NONE of these parts are interchangeable with the earlier engines to include the camshafts.

Its all in the combo.

[Bebbetufs]

Thanks for trying to help!

As you can see from the quote Raby keeps the details to himself while giving broad ansvers This is understandable, given that this kind of info is his bread and butter. As such, the answers in the quote can be seen as an advert.

I don't like asking business owners directly for free info. I would rather hear from someone in the forum , if possible.

[Homeoboxter]

The difference between the 2.5 and the 2.7 is the stroke, while the bore size is the same. So the cranks should be different, probably the bearings as well. I'll be splitting the crankcase carrier of my 3.2 and I will take pictures, and I have pictures of the 2.5 carrier, if that helps. I don't have anything for the 2.7 though. But I assume it's similar to the 3.2.

[Bebbetufs]

Pictures will be great. Thanks. Measurements would be lovely.

The 2.7, 3.2 and 3.4 cranks are interchangeable, so should use the same bearings. The rods are the same for all, so use the same bearings. The incompatibility lies in the chain drive to the IMS which is different for cars with the late updated type chain. The 2.5 has the early dual bicycle type chain.

I'm expecting the 2.5 crank to fit the 2.7, 3.2 and 3.4 crank carriers, but not the other way round. due to the longer throw.

According to some photos, the bearing tang on the 2.5 is different/mirrored from the later engines, but that should be fixable by cutting a new notch on the opposite side. My 951 has Supra rod bearings which have the notch on the opposite side from stock bearing shells. I machined notches on the opposite side of the big end bores to make them fit. Was relatively easy in a cheap Chinese lathe. I just needed to make a fixture for them on the cross slide.

I'm wondering if the 2.5 crank carrier can be machined to accept the longer throw of the 78mm cranks.

[Homeoboxter]

I'm not sure what you are trying to do, but here's a couple shots of a 3.2 carrier that's sitting now on my desk in the garage.
For the 2.5 you can look through similar pictures in this thread:

https://986forum.com/forums/show-tell-gallery/73917-blue-boxster-resurrection-project-6.html

Hope that helps. I can do measurements later in the weekend if you specify what you need.

[Bebbetufs]

Great.
Thank you.
So, what I'm trying to figure out is if a 2.7 crank installed in a 2.5 carrier would cause the rod bolts or big ends to contact something. Most likely the areas where the arrows are pointing. I'm specifically wondering if the castings are the same but the machining in this area is different. It looks from the photo like some of the casting has been machined to ensure clearance. Perhaps this is left untouched in the 2.5 carrier. If this is the case, all I need to do is to machine my 2.5 carrier in order to upgrade to a 2.7 crank.

[Homeoboxter]

I see. How are going to handle the chain sprocket difference? You'll change out the IMS as well?

Btw, you can buy empty carriers on ebay, perhaps that would be worth shipping to Europe, it's not as heavy as when it has the crankshaft in it.

[Bebbetufs]

I'll use an early 2.7 crank. It has the early chain sprocket.