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jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

I was actually going to give some attention to the '82. I'd just picked up a new battery and was stoked to spend a little time getting the CIS dialed in for some nice Summer mountain cruises. The car had been running pretty well, firing right up & idling nicely, but part throttle performance was a little sluggish. I started by pulling the plugs and all looked good, except for a slight oily residue around the thread body of #1. I decided to run a quick compression test and that's when things took a turn. Results:

cyl #1 - 120psi
cyl #2 - 134psi
cyl #3 - 136psi
cyl #4 - 140psi

I'm not sure what's considered good compression for an old 931 but cyl #1 is outside of the 10% variance window. So I pulled the head and BUMMER, head gasket carnage between cyl's #1 & #2. I think the block and cylinder bores look good, though.

Pics (click to enlarge):

-------H.G. Cyl 1&2-----------H.G. Cyl 1&2 flip side----H.G. (top) 4,3,2,1 left to right

Block: 4,3,2,1 left to right ------------Cyl #4---------------------Cyl #3--------------------------Cyl #2---------------------Cyl#1

Head: 4,3,2,1 left to right ------------Cyl #4---------------------Cyl #3--------------------------Cyl #2---------------------Cyl#1

Where to go from here? I'll freshen the head and get some good head studs, but to me it doesn't look like valve sealing is the main issue and there weren't any other symptoms of a failing H.G. No leaks, no water/condensation in the oil, or excess pressure in the cooling system. Would the H.G. damage be the only reason for the low compression? When I pulled the head, I expected to find broken piston land or ring bits, but no. I guess the only way to confirm piston & ring integrity is to pull the block, remove the pistons, and see what falls out?

What do you think? Observations? -Cheers
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'81/'81/'82

Fifty50Plus · Posted: Sat Jun 27, 2020 4:08 am Post subject:

I'd do a light skim on the head, valve job, new Cometic head gasket and bolt it back on.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car

Fasteddie313 · Joined: 29 Sep 2013 Posts: 2596 Location: MI

I'd probably just put it back together with a new hg and see what it does..

I wouldn't be pulling the pistons because the new rod bolts needed for that job are big bucks..
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80 Turbo - Slightly Modified

winddude · Joined: 15 Jan 2020 Posts: 61 Location: Vancouver Island

not an expert by any means, but it looks like the intake valve on #1 isn't seated as much as the others, guess it depends where in the stroke it is or maybe carbon build up. I'd inspect further and maybe lap the valves, and check lash.

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

Thanks for the replies.

924RACR · Posted: Sat Jun 27, 2020 10:44 pm Post subject:

Full clean and valve lapping, for sure. I'd not skim the head unless measurements show any lack of flatness; just be sure (learn from my mistake) to check flatness of the top of the block too!
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

emoore924 · Joined: 13 Apr 2004 Posts: 2815

I dunno. I see some funkiness right between 8:30 and 9:00 on that #1 piston. The head looks pretty oily for #1. That burn-through in the head gasket is ugly. There's no real explanation for the oiliness other than a broken land or failed ring. The block/cylinder looks good, which is good news but I'd be worried about the side of the piston and/or some failure with the rings.

All that said, put it back together and see what happens. But you might have to do it again (and in a turbo, that's a pain...).

Mike9311 · Joined: 14 Dec 2004 Posts: 1678 Location: Chicago-ish

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

safe · Joined: 18 Mar 2017 Posts: 588 Location: Sweden

I would skim the head if fitting an MLS gasket, they like a really clean, flat and smooth surface. Stock gaskets are more forgiving.
Pour some ethanol into the head combustion space to see if the valves leak or not. But you might just lap the valves and replace the valve stem seals if you have the head off.

Raize · Joined: 18 Sep 2013 Posts: 184 Location: Scotland

The head gasket is damaged but not to the extent where pressure could escape - the #2 fire ring is intact.

I think therefore that the only explanation for a lower compression on that cylinder is indeed a damaged piston ring.

Pity you didn't do a leakdown test as well before pulling the head. It could tell you more. I bet you could make some kind of adapter plate which bolts over a cylinder using four head bolts, a big O-ring to seal it, and a threaded hole in the middle for the leakdown tester to screw into. That would give you a direct comparison of ring leakage in each cylinder since the valves would be out of the equation.