Machine Early 964 Cylinder or Used 993 Cyliders?

[jamesjedi]

Thought I would just check for something I keep asking myself.

My engine is out and on a stand. I am addressing an exhaust port and replacing that head. Engine does not have any history of leaking at the cylinders. I am considering having the heads and cylinders machined for a groove to have a sealing ring.

However, would it be better to replace the cylinders with used ones from a 993 and then machine the head to have a better mating surface?


Thanks for any input.

[AVI_8]

Personally if it was me presuming that your cylinders and Pistons have been measured and found to be within spec I'd have the cylinders and heads machined flat and rebuild as is with new piston rings.
You said that you had no problems prior and it would be an unnecessary expense to ditch a perfectly good set of P's and C's

If they're marginal with regards to condition then I'd bite the bullet and get a new set and have the heads machined to accept the sealing ring.

[targa72e]

The issue with 3.6 cylinders is not really about the sealing ring. 3.2 and 3.3 engines did not have sealing ring and did not have the same problems because the stud spacing is smaller. With the 3.6 engines the stud spacing was increased to accommodate the 100MM cylinders. Because the forces on the head were now further from the edge of the cylinder this allowed the heads to "bend" as a result causing a sealing problem. FYI 3.0, 3.2, 3.3 3.6 cylinders all have the same outer bore diameter at the head. The change with the later 3.6 involved more than just a sealing ring. The later 3.6 cylinders are flat across the top all the way past the stud holes. Early 3.6 cylinders are like all 3.0-3.3 cylinders. To use the later 3.6 cylinders requires machining the heads flat as well. Because the clamping force was no longer concentrated on the sealing surface of the cylinder a gasket was required. Good explanation with pics at link below.



Understanding 964 Head Gasket Modification


john

[cgarr]

You are referring to technical service bulletin 9404 where the early heads are machined out to 145mm a fairly easy and straight forward task


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[Speedy Squirrel]

Great explanation and link. Thanks. I have been trying to explain this to people for years.

Originally Posted by targa72e

The issue with 3.6 cylinders is not really about the sealing ring. 3.2 and 3.3 engines did not have sealing ring and did not have the same problems because the stud spacing is smaller. With the 3.6 engines the stud spacing was increased to accommodate the 100MM cylinders. Because the forces on the head were now further from the edge of the cylinder this allowed the heads to "bend" as a result causing a sealing problem. FYI 3.0, 3.2, 3.3 3.6 cylinders all have the same outer bore diameter at the head. The change with the later 3.6 involved more than just a sealing ring. The later 3.6 cylinders are flat across the top all the way past the stud holes. Early 3.6 cylinders are like all 3.0-3.3 cylinders. To use the later 3.6 cylinders requires machining the heads flat as well. Because the clamping force was no longer concentrated on the sealing surface of the cylinder a gasket was required. Good explanation with pics at link below.



Understanding 964 Head Gasket Modification


john

[jamesjedi]

Thanks for the replies.

I think it is not worth replacing my cylinders. Mine are low mileage, however I am looking at addressing the "early design".

I think I am correct in saying that some shops machine a groove in the cylinder and fly-cut the head?

Another shop that is very well known machines the surfaces of the cylinders (fly-cut) and the heads very slightly to make them more flat and then they use a steel ring. This is the system that I am leaning towards.

[jamesjedi]

I have another question, hopefully I will not need to start another thread.

If I find later cylinders that are within spec, should I replace my good, low mileage early cylinders with later used cylinders? The price difference is not that much compared to machining the early head and cylinder.



Also, how does the machining of the head not bump up the compression ratio?

Thanks.