Today's self doubt!

[MBradbury]

Hi, Background... building a 3.2L short stroke from a '83 3.0L SC. Other mods include 964 profile cams reground from the original SC cams. The pistons and barrels were sourced from Andial who also machined the heads to accomodate the larger diameter 98mm pistons. Andial also supplied sheet steel head to cylinder seals (porsche part 930 104 381 01). All of these parts and work are laid out in the photo below. The problem... almost completed assembly to the point of bolting down the cam tower on the RHS, when it dawned on me that the head seals had a smaller inside diameter than the pistons. This has got me doubting I have the correct seals. I have drawn what I think is going on below. Surely, if this is the case, when I was busy measuring piston to head clearance, I was actually measuring piston to seal clearance. I know the seals are only 0.02", so I still gained 0.10" from the head machining, but why aren't the seal inside diameter bigger than the piston diameter? Is it because of the bastard piston diameter of 98mm? To get to the 0.050" (1.27mm) piston to head clearance I had to add 0.050" shims under the cylinders. If the seal had a larger inside diameter, I would have only needed 0.030" shims. The excessive shiming under the cylinders is due to case machining and to compensate for a difference of 0.015" in cylinder height less than standard SC barrels. The upside of this setup is I get approximately an additional 0.020" valve to piston clearance. Although, clearance on the valves didn't seem to be a problem. Has anybody out there built a 3.2L short stroke, and did they use similar head seals? Regards Mark Bradbury 1983 911 SC 1988 Carrera (long term '73 RS lookalike project)

[emcon5]

No seals at all on mine.

Tom

[davidl]

Hello there.

The way I would want to build this is with no seals.... I have had bad experiences with them...and as you say, it is an abomination in your engine..

The clearance should be over 1mm whith no seal, and this is actually a bit bigger than I would like.. about 0.9mm or 35 thou is widely favoured especially with high compression race engines..


This is the set up used in the 3.2 motors from the factory..

I daresay they cut the top of the new cylinder on a very small angle so as to make the compression exerted by the studs highest at the inner rim of the cylinder where it touches the head...about 1 or 2 degreees I believe.

If this is done, no lapping of the cylinder to head is needed...

Kind regards
David

[cstreit]

No seals on the 3.2L SS I am building either.

[350HP930]

No seals on turbos either . . .

[Bobboloo]

So as I understand the sketch your deck height has effectively stayed the same with the gasket but the combustion chamber has moved up and the seal extends past the cylinder wall and is exposed in the combustion chamber.

I'm no expert but couldn't this cause detonation issues? Couldn't that thin gasket start glowing sticking out into the combustion chamber like that?

[MBradbury]

Hi,

So the concensus is I shouldn't fit the gasket. Another reason has emerged why this might be good advice. After fitting the cam towers with the gasket and the sealant between the heads and the towers, the camshaft has moved out of center when lined up with the chain covers. The out of alignment is significant and is causing me to revisit the set-up.

I think what I will do is unbolt the heads while leaving them fastened to the cam towers (to avoid resealing), remove the gaskets and do some more deck clearance measurements. If I have the same clearance I will leave the gaskets off. The only problem I can see at this stage is whether the 1.2" relief in the head is deeper than the cylinder ridge. Can anyone who has responded so far comment on the dimensions of their setups, i.e. depth of head relief and height of cylinder ridge?

In answer to Bobby's point. Yes, I think it is protruding into the combustion chamber and yes, I also thought this might have some negative effects with combustion.

Regards
Mark Bradbury
1983 SC 3.0L to 3.2L short stroke conversion
1988 Carrera - 73 RS project

[MBradbury]

Hi,

I mean 0.12" not 1.2"

Mark

[911pcars]

So what does Andial say? You have asked them?

Sherwood
http://members.rennlist.org/911pcars

[MBradbury]

Hi again,

I will be asking Andial and Ollies Machining who did the head work this week. I will post their comments. I would still like to compare the actual dimensions with others who are building same though.

Mark

[Superman]

This is interesting. When I rebuilt my engine, made the mistake of installing the heads and cam housings before the air channelling sheet metal. So, I had to remove and reinstall. That cost me six new sealing rings (which are more expensive than you'd guess). Now what I think I'm hearing is that the sealing rings are not necessary. That was my impression, candidly, when I looked at them.

Is this true? Can you just delete the sealing rings?

[Doug Zielke]

Originally posted by Superman
Can you just delete the sealing rings?

This is an area for *expert* opinions.
SteveW, JW, Otto, Tyson....need only apply.

[MBradbury]

Hi again

I promised I would contact Andial and then post their response / advice on this matter, and here it is...

Firstly, I have attached an updated diagram showing more detail and extra measurements. One important additional measurement taken is that between the head and the barrel cooling fins. With the heads and cam towers in situ, I took a measurement of 0.33mm (0.013") using feeler gauges. This measurement is important, because it turns out to be less than the thickness of the sealing ring. If I were to rebuild without the sealing ring, then the head would bottom-out on the top of the cooling fins before making a seal on the proper mating surfaces. See diagram below.



Armed with the extra measurements, I contacted Peter at Andial and he explained that the set-up as machined would require the sealing ring. I also described an out-of-center cam position with respect to the chain box, and he advised there was something definitely wrong with my shimming. Referring to the diagram, Peter explained that I should be aiming for a meaurement of between 0.8mm and 1.2mm with 1.0mm being the optimum for DIM "Z". This would allow me to reduce the shimms underneath the barrels by a full 0.75mm from the current build state. This would result in the target dimensions listed in the critical dimensions table on the diagram. This would also help bring the cams back into the center of the chain box opening.

I also asked whether the steel sealing ring protuding into the combusion chamber would cause any interference or pre-detonation problems? The answer was no. Andial have built quite a few of these engines and they have never experienced any problems of that nature.

In one respect, Peter's answers give me some comfort. By applying these changes, I can redress the off-center cam issue. However, that target clearance of 0.5mm (0.02") for DIM "Y" looks pretty close. In the end I have to listen to Andial because they sourced the pistons and handled the machining of the heads, etc. Not to forget that Andial have an excellent reputation, their experience is race-bred and light-years ahead of mine.

The down side to this is that I now have to dissassemble some of my work, but there are some short cuts I can take like unbolting the heads and cam towers as one, and removing the cylinder barrels and pistons intact so I can fit the correct shims.

So there you have it. Thanks for listening.

Regards
Mark
1983 SC 3.0L to 3.2L short stroke conversion
1988 Carrera to '73 RS project

[350HP930]

Originally posted by MBradbury
I also asked whether the steel sealing ring protuding into the combusion chamber would cause any interference or pre-detonation problems? The answer was no. Andial have built quite a few of these engines and they have never experienced any problems of that nature.

Arg, thats not exactly the best argument against a very legitimate concern of hot spots.

I still have memories of filing and deburring the sharp edges off the spark plugs and combustion chambers of our race cars when I was a kid.

That excess material is a bad thing even if it is not a really bad thing but unless you got access to a good machine shop there isn't much you can do about it.