964 cylinder to head sealing rings: steel or graphite?

[ChrisBennet]

I was talking to a mechanic friend of mine and he recommended using the graphite head sealing rings instead of the current steel sealing rings (for 964 motors). I guess he's had the steel ones leak sometimes. Does anyone here have experience with this?

I'm leaning towards the graphite as I haven't prepped the tops of the cylinders and the bottom of the sealing ring groove has imperfections that I don't think the steel rings would comform to as well as the graphite.

-Chris

[ischmitz]

Chris, I went with Ni fire rings and have no leaks so far (6.000 miles) and 3 track events. Read here for more details. http://forums.rennlist.com/rennforums/showthread.php?t=111791

[Bill Verburg]

I vaguely remember that maybe the steel was better, but I can't remember the context of the discussion.

[ChrisBennet]

Thanks guys.

Bill: The factory TSB 9404 says that some of the old (composite) gaskets may be in inventory but to make sure to use the new (steel) gaskets.

ischmitz: I thought about doing something like that but I didn't want to take a chance with a customer's motor.

I guess I should describe the problem for the benefit of others even if it is a little embarassing. Hopefully I'll be able to share the solution in a few days as well.

The motor is a 1991 964 motor that had a bum cylinder when I got it. I'm [EDIT] not building this motor for myself so rather than take a chance with the '91 cylinders (which have been know to leak at the cylinder to head juncture) I bought some low mileage late style cylinders (with the sealing groove) and had the heads modified according to the factory directions. The modification involves machining the seating surface wider (out to 145mm).

This a picture of the cylinders with the steel sealing rings already placed on the groove. Note that the grooves and the sealing rings are nothing like those on an SC motor. The groove is .25mm deep and 3.9 (4.0?) mm wide. These cylinders came with a graphite seal/gasket filling the groove completely. I didn't measure them before I mounted the heads but after being partially compressed the steel gasket rings are still 1.4mm thick.



I installed the heads individually, put the cam carrier over the head studs to align the heads and snugged up the head bolts enough to keep the heads from moving when I removed the cam carrier. I removed the cam carrier and torqued up the heads. I know most of you don't do it this way but I like to because I can check that the heads are all squared up and the same height by putting a straight edge across the tops of the heads.

After I bolted both cam carriers on I noticed that the heads hadn't seated all the way. Aaargh! Below is a picture of where the head and cylinder meet. The gap is most visible on the right side of the picture.



I removed the heads and measured them and found that some had been machined a little under the required 145mm width. I didn't follow my own rule (Trust No One, Measure Everything Yourself) and I got bitten. Up to now, the guy who does my heads is the only one I've trusted. I guess he's human after all.
In his defence, this sealing width dimension wasn't critical on the earlier motors but it turns out that on the 964 the heads are a tight fit over the cylinders.

I measured the heads and removed a little material from the edges of the cylinders so they would clear. I put the heads back on the motor (without the rings) and they sat nice and flat.

I put one set of heads (w/sealing rings) back on and torqued them up. It's smaller but there is still a gap.

I'm wondering if the groove or ring dimensions changed at some point.

Per my mechanic friend's advice, I'm getting some of the "old" style gaskets from Wrightwood. I'll try again when those come in.
I'll let you know how that works out.

-Chris

[ChrisBennet]

Doh! The grooves were shallow because I hadn't removed all the gasket! The graphite gasket has a layer of aluminum layer that looks just like like the bottom of the groove.... Plenty of room once you remove the whole gasket...
-Chris

[1fastredsc]

Chris can you post another picture of those pistons, the dome shape on those 3.6 pistons look very interesting.

[ChrisBennet]

Originally posted by 1fastredsc
Chris can you post another picture of those pistons, the dome shape on those 3.6 pistons look very interesting.

I have some 3.6 pistons I can take some pictures of but I'm kinda busy right now so I'll have to take pictures later.
-Chris

[Bill Verburg]

stock 964 piston set


Stock 993 piston set


102mm 964 Cup/RS piston


102mm set

[Bill Verburg]

993RSR coated 102mm w/ tapered wrist pin

[ChrisBennet]

What can I say? In Bill we trust.
-Chris

[1fastredsc]

Thanks you bill. Does anyone know why those pistons have large bowls in the middle? Why not lower the total height of the dome and shape it like that rs/cup piston with a plateau on top?

[Sid]

Chris (or any else who might be able to tell me..)

I note that you measured the groove on the barrell before you realised that there was still some of the original aluminium gasket left (and hence the heads did not sit very well). Did you measure the depth of the groove when it was fully cleaned?
I ask because I am considering modifying my barrells to accommodate the new gasket (my car is '90C4 with no gaskets and a combustion leak and a v.tight budget...)

I know that the 'proper' way is to replace all the pistons and barrells and then have the heads skimmed (as per Adrian Streathers book etc). I have also read that a 'fire-ring' can be used as an alternative to the latest steel gasket - but I cant find any details of this fire-ring anywhere or the groove that should be machined into the barrell to accept it.

Help/info/comments gratefully received.

[AES]

Sid, look here for a description of the fire ring on an earlu 964, Ingo Schmitz repaired his that way.

[Sid]

Thanks AES,
I have seen this link and found it very useful - except that it doesnt say where to get these flame-rings or what the dimensions of the cut-out should be in the barrells/cylinders?
I wonder if they are made to order?

[ChrisBennet]

Originally posted by Sid
Chris (or any else who might be able to tell me..)

I note that you measured the groove on the barrell before you realised that there was still some of the original aluminium gasket left (and hence the heads did not sit very well). Did you measure the depth of the groove when it was fully cleaned?
I ask because I am considering modifying my barrells to accommodate the new gasket (my car is '90C4 with no gaskets and a combustion leak and a v.tight budget...)

I know that the 'proper' way is to replace all the pistons and barrells and then have the heads skimmed (as per Adrian Streathers book etc). I have also read that a 'fire-ring' can be used as an alternative to the latest steel gasket - but I cant find any details of this fire-ring anywhere or the groove that should be machined into the barrell to accept it.

Help/info/comments gratefully received.

Sorry Sid, I didn't measure the groove depth.

I've heard of people adding the groove to the earlier cylinders but they also added material on the sides of the cylinder. If you look at the picture of my cylinders you can see how close the groove comes to the edge. The early cylinder's mating surface isn't as wide so I think if you machined the late style groove it would either hang out the edge or come very close.

If you're on a budget, get some used late style cylinders and have the heads modified when you have the valves done. The 2 places I checked had them for $800 and $1000 a set. I realize none of this is cheap (wait till you see what 964 valves cost), only cheaper than brand new P&C's.

The late style heads do not have a groove in them, they just have a wider machined face to match the wider cylinder sealing surface.
-Chris

[john_colasante]

Bill, in the photo above labelled "102mm 964 Cup/RS piston", are you sure about that? If so, where is that information from? The reason I ask is because the 964 Cup/RS piston is supposed to be the same as the street car, except it is bumped to 11.3:1 and blue printed. Here you are showing a completely different piston.

[Bill Verburg]

John, those are the 102mm RS/RSR pisons. 3.8liter. I was looking at a set for my 964 engine before I decided to go w/ the 993 3.8

[jeffstev]

Anyone know where I can get 3 'used' Mahle RSR 3.8 P/C like Bill Verburg shows in his picture? I had a "slight" mishap with a 3.8L 993 engine that completely destroyed itself, save 3 pistons...

Also looking for a 3.6L case if anyone has one. I have THREE 993 longblocks worth of parts I'm also selling minus cases/cranks.

Jeff

[rdane]

I am looking for a 993 crank?