70.4 Crankshaft, what fits what?
 

In 72, Porsche moved to a counter balanced crank with a 70.4mm stroke. According to the chart in the Tech Info Center, Porsche used the 70.4mm throw through 77. I see in the Porsche manual, cranks changed in 78.

So, my question is, what, for sure, will fit my 72? Any 2.4/2.7 crank, 72-77? Were there production differences making one 70.4 crank more desirable than others? Were there production differenced that made some 70.4mm throw cranks incompatible?

And does anyone have (a good) one available?

All 2.4/2.7 cranks are the same.
Bruce

So the early 3,0 SC fits aswell then, its has the same 70,4 stoke as the 2.4/2.7?

No. The stroke is the same but the flywheel end is different. The 2.4/2.7 crank works through the 3.0 Carrera of 1977. Once you go to 1978 SC engines you have a new crankshaft that has a bigger bearing on the flywheel end.

-Andy

Thx Andy, can the block be machined for it or is there specila bearing avalibale, Wayne has a 2.7 crank in 3.0 block in a build so it should work both ways?

Can you make the later aluminum case 70.4 crank engine into a 2.4 liter? ie 86 mm pistons?

The 2.7 crank is also used in the 75 to 77 3.0 Carrera and Turbo Carrera which uses a 6 bolt flywheel.
Bruce

To add what Andy is saying, it's all of the main bearings that are larger. The switch from the 6 bolt 70.4mm crank to the 9 bolt 70.4mm crank introduced larger mains for all eight bearings- journals on 1 thru 7 increased from 57mm to 60mm and number 8 nose bearing got MUCH larger. So to fit an SC crank into an earlier case, the main bores in the case need to be enlarged, rear main seal (flywheel) bore needs to be enlarged to accept the "928" seal . Lotsa work... Not an expert by any means, but i've not heard of anyone doing it. Just better off using a 6 bolt crank.

I think a "930" case could go smaller in bore pretty easy. 86mm custom pistons are no big deal. Custom cylinders would be the bigger challenge (cost) I think? Using the typical 95mm cylinders as a starting point, they would have a wall thickness increase of 4.5mm. That's a beefy cylinder!

So I think the bottom of the cylinders would have to be "mooned" to ensure clearance from the rods? Benefit of the beefier cylinder is increased thermal mass and strength. Seems like a nice way to go. You get the benefit of the better SC crank and have very sturdy cylinders.

But the problem is in the heads. You have combustion chambers that have too large of a diameter and need to be filled/welded-up. Actually, the valves are so far outboard in the heads, I think you may have difficulty with the valve clearing the edge of the reduced 86mm bore?

While not inexpensive by any stretch, its possible to build a smaller engine on an SC case & crank. Several years ago, we did a 2.8 litre motor on an SC bottom end that made very good power as well as being quite durable.

Custom P/C's were made and 935 heads were used to make this 325 BHP 2.8.

With a realistic budget, I think one could make a smaller one as well. :) :)

Nice Steve!

Are 935 heads the only way to approach this from a factory part perspective? I suspect if 935 heads are hard to obtain, we could look to Xtreme for a set of heads with the late 97mm stud spacing and a custom downsized combustion chamber to match up with the smaller bore?

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We did this LONG before Xtreme was doing new heads,...:)

The 935 heads simply offered a good solution with the best flow that fit this combination. Having them on hand, helped as well!

Bill@Xtreme can do almost anything you want so thats a good option, depending on budget.