Stripped transmission mount threads

[Jerome74911S]

In the last few days my engine was installed with the help of an "expert" with much 911 experience, according to him.

I am worried that he cross-threaded the transmission mount bolts that thread through the transmission carrier's rubber mounts and into the tub.

Those large bolts should be easy to turn into the threaded "socket" in the tub. I think I remember turning them in with my fingers with no problem.

Following an animated discussion, he wrenched them in with great resistance for the entire length of the threads. He said that the threaded portion in the tub is a tapered thread that gets tighter as the bolt goes in.

I'm afraid to remove the bolts to check them. What do I face here?

[KTL]

Your expert is wrong. Those threads are not tapered. The only thing special about that fastening assembly is that Porsche used bolts with tapered noses on them to help the threads get started cleaner. The threads themselves in the tub's socket "tube nut" are not tapered.

You likely now have cross-threaded threads in your tub. You're correct that you should carefully thread these in by hand as much as you can. I recall reinstalling several engine-trans assemblies where I have always made sure to get the threads started very cleanly with little resistance to ensure they're not cross-threading. Once the bolt is several turns in by hand, the bolt can be wrenched with a socket wrench very easily until the assembly cinches-up together. Often times it's easy to cross thread this bolt because the socket is lubed with anti seize, which makes it easy to start the threads crossed and get a few turns in before feeling the resistance of cross threading.

I specifically remember dealing with this on a friend's '87 when we were reinstalling his drivetrain and we had to take a break and return another day when we had a M12 x 1.5 tap on hand to clean the threads to alleviate the mild cross threading that was happening when starting one of the bolts.

[dshepp806]

Have you any present problems?

Were they torqued to spec?

Best!

Doyle

[dshepp806]

Good stuff, Kevin. Thanks.

So, what would be Jerome's fix?

Best!

Doyle

[DRACO A5OG]

I had a tough time installing my Cab Mounts. Finally used a Wood Clamp to help align the hole and it went in by hand, then I torqued to spec.

[Scott Douglas]

One of the things I learned while doing my Sportomatic fix was that you never use the motor mount bolts to pull the mounts up into position.
I had questionable trans mount threads when I started the job and ended up stripping one of those tub mount holes. Had to helicoil it while it was jacked up on stands, not a fun job but I did it.
I think I ordered the kit from McMaster-Karr.

Had to improvise a tap handle as you can see in the picture.

[1990C4S]

Remove the bolts individually and inspect them. There might be remnants of the body thread adhering to the bolt. Re-tap the holes, clean up the bolt thread and re-instal. Torque them properly. If they hold the specified torque they are fine.

And your wrench doesn't know what he is talking about. Mal al la tete?

[tobluforu]

Man, I just did this on my 72 with wevo mounts and it was a bear to get it to align just right so I could get both bolts in without cross threading them. I spent about an hour, gave up, came down the next day and with fresh state of mind was able to finesse them in. If you have no issues then I would let it be, but that is just me as I’m getting lazy with age. They should turn right in and you should not have to crank them hard at all until you torque them.

[Cloggie]

Did the same thing with the result that the threads were indeed stripped. Got a set of helicoils (M10 x 1.5 if memory serves) and did a proper repair. Only concern is to make sure that you drill and tap deep enough as the captive nut mount is closed at the top. I just barely did enough, but it is workable.

Remember that there are only 4 bolts holding that engine to the chassis, and it will definitely droop if one of them comes free.

D.

[Jerome74911S]

Originally Posted by 1990C4S

Mal al la tete?

You can add belly ache, too.

[Scott Douglas]

a properly helicoiled hole!

[Jerome74911S]

I guess I have no choice but to lift the car back onto jack stands, support the trans on a couple more jack stands, and pull those bolts to see what's going on. I'd rather go through that work than to drive the thing until it droops and causes a bigger problem.

Thanks, guys, I do have the correct size taps on hand, so hopefully it is just a clean-up and forget-it situation.

[Scott Douglas]

I would recommend you have on hand a helicoil set before you take the bolts out JUST in case clean up of the threads doesn't do the trick.

[Walt Fricke]

You will find a lot of these holes with Helicoil fixes in them. But it is a fix, not a band aid.

And can be done in situ. Not really harder that way than however else you might do it - short of putting the tub on a rotary stand.

[Cloggie]

Remember you can remove the rubber bushing assembly from the transmission mount which leaves a large hole in the crossmember that you can work through. Not that difficult actually. I left the engine hanging on the 3 mounts while I did the work.

D.

[Gunter]

Originally Posted by Jerome74911S

I guess I have no choice but to lift the car back onto jack stands, support the trans on a couple more jack stands, and pull those bolts to see what's going on. I'd rather go through that work than to drive the thing until it droops and causes a bigger problem.

Thanks, guys, I do have the correct size taps on hand, so hopefully it is just a clean-up and forget-it situation.

Installing motor/transmission is a lot easier if the bolts holding the cross member to the transmission and the motor mount bolts are left slightly loose. With all bolts loose, the whole load aligns itself.

I suggest you support motor/transmission, crack all carrier bolts lose and then remove the pointed bolts for the transmission and inspect the threads.

People do overtorque these bolts or cross-thread requiring a Time Cert (Or going to the next size SAE)

Also, make sure the cup washers are correctly positioned.

[r_towle]

using a proper thread chaser instead of a tap will be the right way to go.
A tap removes more material and will recut the existing threads which could make the bolt loose over time.

Rich

[Jerome74911S]

I discovered that both sides were cross-threaded, so I took off the transmission carrier (with a jack stand under the transmission) to get plenty working room. I'll have to drill and tap. Interesting that the bolts don't look too bad, but the threads in the socket in the tub are a mess - there is not enough solid material to clean up the existing threads, so new ones will have to be cut.

I'll get it back together by tomorrow - real life interferes.

[Nine9six]

Originally Posted by dshepp806

Have you any present problems?

Were they torqued to spec?

Best!

Doyle

Hmmm, torqued to spec, potentially cross threaded bolts? How does that occur
Come on Doyle, you know better than that...

[1990C4S]

You could bastardize it and go to 1/2"-20. I did. Once. A long time ago.