I'm pretty certain I have it figured out. Here's what happened (and what you all need to check to make certain it doesn't happen to you!). There were two contributing factors:
1. I had the wrong pressure plate. It should have been a Porsche pressure plate, but for some reason I had a KEP pressure plate. The KEP plates have the big balancing pins, the Porsche plates do not. The pins aren't the problem though. The problem is that this pressure plate requires more force to be applied to the throw-out bearing to release the clutch than would be required with a conventional Porsche pressure plate. When you push the clutch pedal, the force on the throw-out bearing pulls it toward the rear of the car to release the clutch. This pull is transmitted directly to the crankshaft, and the crankshaft thrust bearing counteracts this force to keep the crank in place. The bearing faces of the thrust bearing are rated for a only few hundred pounds of force. Bottom line, the thrust bearing cannot withstand much axial load - and the clutch system is capable of overloading the thrust bearing, especially if you use the wrong pressure plate!
2. I was an idiot an never put in a clutch stop. I was under the impression you needed a clutch stop to limit travel of the clutch fork so it didn't hit anything. When I installed my clutch I measured all clearances and had no issues, so I never installed the clutch stop. BUT THAT IS NOT WHY YOU INSTALL A CLUTCH STOP! You install a clutch stop to limit the force applied to the crankshaft thrust bearing! You want to adjust the clutch stop so the pedal travels 1/4"-1/2" past full clutch release. Any more motion than that only serves to put added stress on the crankshaft thrust bearing.
Taking these two things together leads to a failed thrust bearing at only 3500 miles. I measured my crank play and have nearly 0.065" - spec is less than 0.005". And when my clutch is fully depressed, it can move the crankshaft enough that the crank position sensor no longer lines up with the reluctor wheel and so I lose the signal and the engine shuts down. I believe I started hearing the tic-tic-tic when the thrust bearing failed, allowing the crankshaft/flywheel/pressure plate to be pulled backwards into to transmission housing, and the balancing pins started to hit the boss inside the housing.
So I spent this weekend preparing to remove my engine, so that I can replace my thrust bearing. Hopefully the crankshaft thrust surfaces are not damaged, but I won't know until I get in there and look. I've removed the rear hatch, cold air intake, exhaust, headers, wiring harness, intake manifold, and valley cover, and replaced the valley cover with an engine hoist plate. Also drained the oil and coolant, and removed the idler pulley assembly so I can lift the engine up and out without removing the rear wall. And removed the flywheel and KEP adapter plate. The engine looks downright naked!
The only thing remaining attached to the engine are the AC compressor and AC hoses. I'd like to remove the compressor/hoses together from the engine so that I don't have to recharge the AC system, but it looks very challenging to remove the compressor in the engine bay. If I do have to take the hoses off, do I just remove the aluminum hose manifold from the rear of the compressor and let the refrigerant escape into my shop? Not really certain what to do here.
As soon as I figure out how to handle the AC I'll pull the engine. Maybe tomorrow or Monday.
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