What's the deal on 964 Head Leakage? Is it important?
Here is a picture of a 964 with leaky head to cylinder mating surface: (Click to zoom)
and here's how Porsche fixed it:
First - a little history.
- All 911 SCs had these "Ring" head gaskets. it was basically a hollow or spring filled "C" section that inserted into the groove on the cylinder at the head mating surface.
- The cylinders worked well with the 3.2 Carrera, mainly because the diameter is smaller and therefore pressure across the surface (psi) higher, alloing for a reasonable seal.
- Beyond that the 3.2 is 9.2:1 or so whereas the 3.6 compression ratio of 11.5:1 and the larger dome area create much higher total force to lift the head off the cylinder. More below.
Here's a picture of the SC setup: which had the raised ring and a gasket.
You can see that the cylinder is similar to the EARLY 964 with the groove cut in it. Notice that a narrow (6mm or so with the gasket groove) ring sticks up from the cylinder. This fits into a machined ring in the head to locate the cylinder to the head. You can see from the high performance 3.2 below that no such groove exists
So the Carreras seemed to be OK and Porsche said let's go with that so they made the 3.6s from 1989 through mid 1991 in this manner with no head gasket. Sometime in there, however they started to leak. Now the flat area on the 3.6 is fairly narrow and they didn't want to machine in a groove so they redesigned the entire cylinder to have a wide flat area, and they machined a much larger locating area the head to accommodate it. This is actually the key. According to several sources, because the head studs are now so far apart, and the SUPPORTED surface so narrow, the early heads actually start to "bow" so that the the pressure in the sealing area near the studs is very high, and between them low. This is borne out by the top photo, where you can see leakage is at the bottom and top of the cylinders at the center.
(The above are from a 993 Varioram, machining 964 cylinders has the same result.)
One of two things happened to the early engines:
- They leaked, but not enough to be a problem (even the engine shown above wasn't leaking to Porsche's definition, wet to the touch) and they are still leaking.
- This originally was really not a problem in general because they only tend to leak when they are cold.
- Problem is that as they continue to leak, they chafe the cylinders and eventually start leaking a lot and making a mess.
- Realize that this is combustion product and/or a very small amount of oil leakage, but it's stil pretty ugly and needs to be addressed.
- Porsche fixed them under warranty, or they have been subsequently rebuilt.
Here are 1989 heads before and after machining. The dimensions are critical as the notches on the four corners interlock with the cylinders. We usually do a batch of 3-4 sets at a time.
What does it take to fix the problem?
- Buy new P&Cs. About $3800 or so, maybe as low as $1500 for a nice used set.
- 993 P&Cs won't work because the small end of the rod is narrower, but if the 964 pistons are in good shape you can re-ring them and use 993 cylinders.
- You can also machine the small end of the rod to work with the 993 pistons. That is really not recommended.
- Do a top end (you'll have the heads to have them machined, so a valve job is in order, about 25 hours labor and $2000 in parts, plus machine work for a valve job).
- Have the heads machined by someone who knows what they are doing. We charge $600 or so for the machine work, plus bead blasting the heads, plus a valve job, plus guides...etc.
- Now that these motors are a quarter of a century old, our recommendation is a full top end with head studs, since they also seem to be going. Count on something in the $10,000 ballpark with R&R and case though bolts seal replacement...Yeah, that's happening now.
The bright side is that after spending the money top end you'll have a nice motor. We have been seeing 964 head studs fail lately as well, so think about having them replaced while you are at it. Did I mention the benefits of ARP rod bolts?