I re-posted this here because it has some good info. In my experience the engine does not need to be dropped for thermostat replacement:
I just purchased my first 911 SC and it also had a known oil overheating issue when driven hard. The car was 95% restored so I bought it anyway and decided to tackle the project. Under normal conditions the oil temp would remain at about 210, but when driven hard the oil pressure gauge would begin to twitch and act erratically (due to cavitation) and the oil temp would rise to nearly 300!
Here's what I did to troubleshoot (in order) with the problem persisting until the final step:
1) I grabbed the front trombone cooler to check for heat. It was getting hot so the external oil thermostat was OK.
2) I removed the fan to check that the engine oil cooler ramp was free of blockages. It was partially clogged with sludge, so I blasted two cans of degreaser on it and on the cooler itself (from the the top down). During reassembly make sure you properly orient the shroud behind the alternator. Also clean your fan while it's out (sand blast, then powder coat and polish). Unfortunately all of this didn't prevent the overheating (still saw 280 on the oil), but it did improve the looks of my engine bay. Photos below.
3) Assuming it to be the engine oil thermostat, I removed it. On this car there is no need to drop the engine. I did it in 2 hours with an endoscope, a mirror, and a few extensions. First pull the AAR vacuum pipe, the intake air plenum and disconnect the breather hoses from the oil tank to help gain access from the lower right side of the CIS. Pulling the thermostat out of the hole was the hardest part, but since the thermostat is lipped you can use a $10 O-ring gasket removal hook to pop it out. I checked the thermostat using a stove (see photo below) and it was functioning perfectly. Also, replace your gasket with the green one pictured.
4) At this point, with everything properly sorted I was almost resigned to the fact that this was a design flaw and I would never be able to track the car... until I read the TSB mentioned in this thread. The 78 and early 79 models shipped with an improper oil pressure relief valve and a bad sump pickup which caused the sump to flood, improperly scavenge, and cavitate. I replaced the rubbish with these parts, solving the issue:
999-064-026-01-M260 Oil Pressure Relief Cap
930-107-533-01-M100 Oil Pressure Relief Sleeve
930-107-531-01-M260 Oil Pressure Relief Spring (Vertical)
930-107-314-00-M253 Engine Oil Sump Screen
PEL-RW-SP Vintage Aluminum Sump Plate
I can't believe it took me almost a week to figure that out. This is my very first Porsche, and although I have lots of experience building small block Fords this is definitely not the same. I'm glad I have experience, because this thing is an incredible mess of vacuum and oil lines! Elegant engineering though.
Oil cooler air ramp on right
Alternator wiring in case you forgot what goes where
Man, that is NASTY!
Now we're cooking with gas!
The engine oil thermostat is somewhat difficult to remove, but with skill it can be done with no drop, partial or otherwise.
Here it is functioning to spec. Fully open at about 186.
This is what makes all that hard work worthwhile. Very rewarding!
