The 17-year-old mystery of the exploding 915

[djb25]

Hi everyone,

Ok, so this happened to my car about 17 years ago. I repaired it and never really had any additional trouble, but I've been wondering about how or why this happened ever since.

Here's what happened. Just put new Pirellis on the '77 911s I had bought about a year earlier. Coming around a slow corner in first gear. Punched it, and BOOM! loud noise from behind me, gear shifter starts bouncing all over the place. I shove the clutch in. Something bad clearly happened... Tow the car back to my dad's shop, pull the engine and tranny (for the first time ever). Chunks of metal throughout, and the pinion shaft moves fore and aft... ? I open it up, and find that the lip on the four-point bearing (number 16 in the diagram) was torn right off.

I was in my early 20s at the time and had absolutely no idea what I was doing, but I disassembled everything and replaced the bearing. I also replaced the sensor wheel, because several of the magnets were missing (and were shredded and deposited throughout the transmission).

As far as I could tell, the transmission looked great otherwise (it had around 55k miles on it at the time).

At the time, no one seemed to have any idea what happened. I always suspected that the sensor wheel lost a few magnets and one of them got crunched in the pinion/ring gear, which pushed the pinion shaft forward, tearing the lip off the bearing.

Looking back, I think I got very, very lucky. If I hadn't been moving so slowly at the time, the transmission probably would have been scrap. I still can't believe the transmission worked after I put it back together, or that it has lasted this long (although it's probably only had about 25k miles put on it).

Anyway, the transmission is sitting on my garage floor right now and I started thinking about this, and wondered if anyone had any thoughts or if anyone had ever heard of something like this happening.

[Jack Stands]

Your theory of a magnet coming loose and jamming into the ring and pinion seems reasonable as it would push the shaft back and then perhaps destroy the bearing. Either way, I give you a lot of credit for tearing into the transmission, repairing, and reassembling it. The fact that it's still running shows you did a good job on the repair.

[Joe Bob]

Either that or HP+grip and BOOM. I've blown plenty of rear ends but mostly on down shifts to prepare for a turn.

But good on ya for the repair.