Air-cooled 911 has no computer that tells you its past. No over-rev report like on water-cooled cars. Engine tells you through leak-down, leaks, sounds and what is under the valve covers. Body tells you the rest.
Leak-down I do hot, with the rings oiled. Cold numbers on air-cooled engine can be about two times the hot ones, and car that sat for weeks seals its rings with oil, so good cold number means less than it looks. Under about 10% hot is healthy. In 20 to 30% range we talk about rebuild.
Head studs are the big one, for sure. On the 2.7 from 1974 to 1977 the magnesium case lets the studs pull their threads out of the case. On SC and 3.2 the lower row, exhaust side, breaks instead, usually about two inches below the nut. Before the covers come off I go around every head to cylinder joint with a flashlight. Black crusty layer of burned oil there is a sign. Also fast pop pop pop under acceleration, sounds like detonation. Then lower valve covers come off to see the studs. Three of four studs still good on a cylinder, repair can be planned. Two or more broken on one cylinder, car should not be driven. The 964 breaks studs too, and not only lower row. The 993 rarely.
Chain tensioners. On SC the question is, was the pressure fed tensioner upgrade done, and with it the wider idler arms with two bushings. Arms matter same as tensioners. Part numbers are the only sure way to know. When tensioner fails, chain can jump and valves hit pistons.
The 964 has its own list. Early engines were built without head gasket and seep oil at head to cylinder joint, mostly when cold. Porsche fixed leaking ones under warranty, and heads can be machined to take a gasket. Early dual mass flywheel was the weak one, later ones are better. Bad one shows as vibration, hard shifting or rattle. And twin plug distributor on 964 and 993 has small belt inside. When it breaks, second rotor can stop right on one cylinder and fire that plug all the time. This cylinder runs lean and burns. Engine still runs on the other plugs, so the belt has to be checked, you can not guess it from the idle.
On the 993 exhaust valve guides wear first, and worn guides do not show on compression or leak-down. Secondary air system codes come from carbon in the head ports, from oil getting past the guides. Code cleared just before the sale hides it, because system needs drive cycles to report again. Tick that stays when engine is hot is lifters or worn guides.
Oil leaks are normal on old 911, question is from where. Oil on spark plug side of the valve cover points at rocker shafts.
CIS and Motronic. On CIS car with hot start trouble, I check the fuel pump check valve and the accumulator before I blame injectors. On 3.2 that will not start hot, DME relay is first suspect. I tell owners straight, EFI conversion makes better car, but if they want original CIS, I rebuild it and dial it in right.
Gearbox. 915 that is hard to get into second or third needs rebuild soon, and first and second synchros wear first. But shift linkage gets sorted first, bushing in the tunnel and the coupler, before I judge synchros. G50 from 1987 shifts much better. Rubber center clutch disc used from 1978 can come apart without warning. Clunk when you go on and off the throttle is the sign.
Body. On impact bumper cars it is unusual to find kidney bowls and sills without repairs. Front tank support, inner wings along the wing bolts and Targa areas get looked at, and on 964 and 993 the windscreen surrounds. Fresh undercoating on old car, I look even harder.
Numbers. Porsche Production Specification, paper that replaced the old Certificate of Authenticity, does not show engine and transmission numbers anymore. Only Porsche Classic Technical Certificate, done at Porsche Classic Partner, checks installed engine and gearbox against factory records. And stamp sets in factory font are sold openly, so correct looking number alone proves nothing. I look at the stamp pad itself, welding, filler, fresh machining, uneven spacing.
Air-cooled inspection is few hours, car hot and valve covers off. Better you find broken stud before you buy the car, not after.
Also: Porsche service · Porsche PPI · Over-rev report