The 964 Engine Guide
The 964's M64 3.6 is where the air-cooled story gets modern: twin plugs, a dual-mass flywheel, and a few known issues that every buyer should have priced before the auction ends, not after.
Head sealing, flywheels, and belts
Early 964s (1989–91) have no conventional head gasket — the cylinder-to-head seal is metal-to-metal, and leakers got updated parts. A documented reseal settles the question. The dual-mass flywheel wears and rattles; replacement (or a documented conversion) is a known cost, not a surprise.
The twin-distributor drive belt can fail in ozone-cracked silence; the vent kit is the cheap prevention. A PPI checks whether it's there.
What actually matters
Compression and leakdown numbers, the oil leak map, and the service record. The M64 is a strong engine with expensive deferred maintenance, which is why the market pays for documentation. The Turbo's 3.3 and 3.6 are different animals again — single turbo, lower compression, and the 3.6 is a one-year engine in the US (1993–94, plus the Turbo S specials).