My 925 sporto blew up and and I can not figure out why

[caproader]

Guys,
First the problem:

Driving my 73 S 925 sportomatic at a constant speed on the Interstate, with no warning indications, there was a loud bang and the speedometer quit working. When I got back to the garage (slowly and 3 miles later), I noticed that there was a circular piece of metal about 2.5 inches in diameter missing from the center of the front cover plate (the one the shifter rod goes through). It appears that the speedometer drive gear retaining bolt had punctured the front case.

Now that the engine is out, the front cover is off, and I have inspected more closely, the female threads that capture the speedometer drive gear retaining bolt have stripped, which seems to have caused the retaining bolt to launch itself through the front nose cover piece.

So now the WTF questions:

1. Has anybody ever seen a sportomatic problem like this before?
2. Does anybody know what caused this to happen?
3. I have a new speedo drive gear and stretch bolt from Porsche. Can I just put a time cert in the female end and re-assemble or is more tear down and more parts required.

The good news is that I have a spare 925 transmission from a parts car and hope to be able to source any NLA stuff from the spare trans. But I would like to keep the sporto installed and running in this car.

Any thoughts, help and direction sincerely appreciated.

John
member #322

[blucille]

Where are you located? I know a guy in Phoenix who could help....bock? Buck??? Something like that....can’t remember.

[Jon B]

... the female threads that capture the speedometer drive gear retaining bolt have stripped, which seems to have caused the retaining bolt to launch itself through the front nose cover piece...

John, you're saying that #8 in the illustration blew through the front cover?

The female threads for that bolt are part of the pinion shaft- if they're stripped, I doubt they can be repaired effectively.
How do you know the female threads are stripped?



Jon B.
Vista, CA

[caproader]

Guys,
Thanks for the replies. I am in Cincinnati. Yes it appears that the bolt listed as # 8 in the picture is the culprit. When I removed bolt #8 from the trans, pieces of thread came out with it. Additional fact: roughly two or three threads on the # 8 bolt were stripped. The rest of the #8 bolt threads appear to be normal. But there appears to be more than 2 turns of threading that come out with the bolt.

Thanks again for the help.

John

[Jon B]

John,

The fastening torque for that bolt is 110-120 Nm, or 80-87 lbs-ft.
You could install the new parts and see if the replacement bolt threads in cleanly, with sufficient engagement, and if so then see if you can torque it to spec.
If the female threads are badly damaged or stripped, then the damaged pinion shaft should be replaced, meaning both the pinion and the ring gear.

Here's another possible issue...

That bolt holds the assembled pinion shaft together. Coming undone allowed the parts on the shaft to spread, and might have caused the pinion to push rearward into the ring gear.
The whole gear cluster should really be removed and inspected for any damage caused by the bolt failure.

Jon B.
Vista, CA

[JackMan]

in re Jon B's comments, ditto! Until you take apart the gearbox, you have no idea what lurks inside after an incident like the one you reported. Yes, you can just put in a new #8 bolt and if it torques down to spec you'll still be missing two rings of female threads for bite on the bolt. Take the time to inspect the gearbox. (IE, remove the gearstack not just for signs of failure but also to clean out all the little pieces of metal that are withouot doubt floating around in your gearbox.) . Our cars are not young anymore and quick fixes can lead to complete loss of other components. Plus, it sucks having to call for a flatbed when you are a few miles from home. I know that what Jon B and I are recommending is going to cost you some real money, but the alternative (total failure of the gearbox punching holes all over the case) would really suck, and even suck more if its a matching numbers case to your car.

[jim amato]

John, If you need expert service in Ohio contact John Truman enterprises, Pm me if you need his number, over 40 years experience

[caproader]

Jim,

PM sent to you for Truman Enterprises. Meantime, I agree with the comments that it is best to do it right and not try to slide by. This is a car with only 60k original miles and as I said, I want to keep it original, up to and including original numbers and all the pieces issued at the factory.

John

John

[jim amato]

PM replied to, You cant go wrong with John Truman, Porsche factory trained, Old school, Worked as a tech for Stoddard when our cars were new

[caproader]

Thanks to all for the help. Called around to get as many data points as possible. Looks like the majority says to put in a helicoil, clean it up and reassemble. As they used to say on the news: "film at 11".

John