Advice from the experienced porsche engine builders please

christian977 · 05:15 AM

Hi Guys,

I purchased a "80 911SC which had been sitting for 15years with the engine out and in pieces. I was told through another person that it had an issue with head studs, that was the orignial reason it was dismantled. It never got re assembled till now when i am doing it. I got the engine in a bunch of boxes.
It has plenty of documentation supporting the 160000km on the clock. Engine is a 930/09 and i have been told it is the original "matching" engine from the car.

I am a Heavy Vehicle mechanic by trade, and rebuilt plenty of engines, and i have done a bit of VW stuff years ago, including a performance build, so i decided to go about this myself.

Clean up of all the parts i received showed a few things out of the ordinary:
-Alusil liners
-Pistons had valve kisses on them? only slight but small "C" marks none the less
-Valves were straight (taking into account the above)
-Valve guides were worn, (so i though maybe that was the reason for the marks on the pistions. i have seen valves clip liners in truck engines with worn guides. and been running perfectly fine on dissassembly...)
-Marks on the cylinderheads matting surfaces & combustion chambers from poor storage and transport

Taking the above into account, and the fact the engine was completely dismantled, case split etc i have done the following:
MEASURED AND CLEANED EVERYTHING. Plugs removed and Oil passages in the block and camboxes rodded through.
All new bearings, new little end bearings (machine shop),
new valve guides, valve seats re cut, new valves, (margin was too thin once old valves were ground) heads were re surfaced by a machine shop.
New 993 turbo head studs
New Mahle nikasil 95mm liners without the fire ring groove (later 3.2l liners)
New Pistons from CPS italy CPS Special Pistons - Cars
Cams have been reground locally to similar specs to 20/21 cams which is just a bit bigger than 964 cams. rockers rebushed & refaced, (machine shop) New rocker shafts.
Heaps and heaps of NEW parts... I really want to do this one time only...

I am in the re assembly process and have issues with the squish/deckheight/clearance at the outer perimeter of the piston and the cylinder head to liner mating surface...the shape of the perimeter of the piston makes measureing the deckheight very difficult now... (see photo), but using some clay type material i measure it to be 0.2mm at the tightest point... I have just ordered 0.5mm base gaskets so it will end up at -.45mm. CR will come down a bit, but thats OK. The other thing is when the head mating surface was machined i assume that makes it thinker and closer to this area of the piston.

Does anyone think that clearance acceptable??

Thanks
Christian
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christian977 · 03-09-2022, 05:12 AM

christian977 · 03-09-2022, 07:06 AM

Hi Again,
I have re measured again, this time with aluminium foil, made a round slug and then refitted heads and rolled the engine over. it is 0.2 - 0.25 at the edge at cyl wall then tapers out as it goes higher up the dome of the piston.
This is ideal in my mind, no pockets. Just worried about the small clearance at the start of the taper...
As a note, this is a street car, i want it to sound angry and run well.
Thanks

dannobee · 03-09-2022, 07:35 AM

Is the 0.2mm clearance with or without the base gasket?

christian977 · 03-09-2022, 08:49 AM

with .25 base gasket

Dpmulvan · 03-09-2022, 11:05 AM

You want clearance closer to 1mm

reclino · reclino's Garage

1.0mm is considered minimum clearance for a race engine. Most stock engines have 1.25-1.50mm clearance, the rods stretch a bit at rpm. You engine is going to need thicker base shims.

stownsen914 · 03-09-2022, 04:32 PM

You probably know this, but make sure to measure the clearance at a spot in line with the piston pin (or at opposite ends of the piston to account for rock).

christian977 · 03-09-2022, 08:05 PM

Thanks, so i understand 1mm is minimum period...

christian977 · 03-10-2022, 01:01 AM

Dpmulvan, yes, i think you are correct. but my stack height will also increase... I am worried about that. I think i will need to shim the timing chain covers?

mikedsilva · 03-10-2022, 01:37 AM

Quote:

Originally Posted by christian977

Dpmulvan, yes, i think you are correct. but my stack height will also increase... I am worried about that. I think i will need to shim the timing chain covers?

I think there are a couple of options here. The gasket kits from Wrightwood racing come with 2 sets of differing thickness gaskets for the chain boxes.

Or you might be able to use the gaskets from a 964.

christian977 · 03-10-2022, 04:21 AM

Thanks Mike, really good info!!! Does anyone know the thickness of the 964 gaskets??

christian977 · 03-10-2022, 04:53 AM

Spoke with the company who made the pistons they are willing to adjust the pistons to make the height correct.

prschmn · 03-11-2022, 05:32 AM

If you have a negative deck height you're really going to need thicker base gaskets.
Unless the pistons were made with an incorrect compression height.
The height of the chain cases is not the issue.

christian977 · 03-11-2022, 11:17 AM

HI Mark,
Yes, infact the pistons were machined with an incorrect compression height. The company is correcting all the pistons for me.
Thanks

prschmn · 03-11-2022, 11:36 AM

Aha! Good news then. Thank you for letting me know.

Ollies930 · 03-11-2022, 08:37 PM

Before you send the pistons back, you might want to check the piston to valve clearances first, just in case they need to make the valve pockets deeper as well.

RobFrost · 03-12-2022, 12:03 AM

At 160k miles these engines suffer head studs and valve guides. Burning from head stud leaks, leads to getting the faces machined. If the pitting was bad, a lot may have been taken off, leading to your problem.

You will need to carefully measure compression ratio with a butette to make sure it's not too high. You can end up needing twin spark, racing petrol or worse.

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