915 Woes

[callard]

Last year I installed a used 915 tranny in my '72 911 SCCA racer. Before installation, I opened up the gearbox for a looksee and all appeared fine. I installed a new input shaft seal (inside version since it was an early '72 trans) and buttoned it up. I ran a day on the dyno and all was well. I went to the track and the car started smoking in the first practice session of 6 laps. There was synthetic fluid on the top of the engine so I thought I had an engine oil leak. I tightened up the fittings that fed the MFI and went out for a 10 lap race. The smoke was still there for a couple laps but diminished as the race went on. However the 3-4 shifts became tighter. I withdrew for the rest of the weekend and went home to investigate.
Over the winter, I pulled the tranny and had a friend investigate. He found that the input shaft seal had slipped back onto the input shaft near the ring gear, there was no fluid in the tranny, gears three and four were bright blue and most of the rest of the moving parts were toast. Dummy me. We sourced another early 915 box and did a full internal bebuild using the second case and being careful to properly install the input shaft seal and new vent along with Swepco inside.
I then headed back to the track a couple weeks ago and ran 5 laps trouble free in practice. On the next lap I started smoking again and shut it down. Green trans fluid on the top of my engine said the weekend was done. Another input shaft seal failure.
I'm searching for answers as to why this has happened twice to two different gearboxes. Two possibilities suggested include vacuum inside the box sucking the seals inside and poor seals from supplier. Two suggested solutions are to have the early box machined to accept an internal O-ring to capture the seal and to have the box machined to accept an externally installed seal.
Didn't the factory race these boxes in the early '70s? Are there any old time racers on this site who might know the cause/fix - Grady are you there? Pete Z?
What am I missing?
Thanks,
Chuck

[Peter Zimmermann]

What a weird problem! I have never experienced, other than high-mileage failure, a M/S failure on a '72 915. I've done a number of those transmissions, so I must ask, (1) are you sure the new seal was perpendicular to the shaft, (2) are you sure that you used the correct seal (nice, tight fit during install, and correct ID for the M/S seal surface), and (3) do you have a pilot bearing in the flywheel? I will wait for your answers, and we can proceed from here.

You don't say if the second seal failed, or crawled out of its opening. If it failed without moving, did you wrap the M/S clutch splines with electrical tape during gear stack installation? This is crucial to prevent the splines from cutting the sealing lip of the new seal as you push the M/S through it!

[Menmojo]

My 73 911 track car had the same issue right before I bought it.
The PO had the case machined and installed a WEVO part the accepted the O ring to be externally installed and sealed.
I've run the car about 10 times At the track since with no issues.
If I can find the part details I'll post.
My recommendation would be to contact WEVO with the issue. They have a fix.
I'm not sure what caused the problem. My car has a 73 2.4 with all solid mounts.

[callard]

Peter,
The seal was properly installed with a factory tool and it did back out completely. There is a pilot bushing in the flywheel. The seal was an Elring 23X40X10/14. My friend said that he was able to recreate the problem this morning by heating the bell housing and the seal walked back out of the case. (!!!!)
Unfortunately in SCCA Improved Touring racing, doing the WEVO sleeve replacement on the gearbox is not considered within the rules. I can machine the existing parts and seals are free.
I'm still wondering what the factory did during their period of using the early 915 case. Was that the reason to go with the external throw out bearing sleeve and seal?
Chuck