Help Re-Threading Transmission Mount Hole
 

The Bad:

When I got my '72 911 there was a stud welded in the hole of the passenger side transmission mount.

I removed the stud and the threads are almost non-existent as I can simply slide the stud in and out of the hole.

Re-tapping it with the original 12 MM 1.5 tap I'm pretty sure will not work as I can slip that in the hole as well.

Where the hole is tapped into is very small, not very deep and not very wide. Look at the back of the bracket picture...

The Good:

I have the '72 911 on a rotisserie rotated at 90degrees and it is in bare body form so nothing is in my way. I could only imagine dealing with this when the transmission is mounted with the car on jack stands.

Question:

What should I do here?
Is there a replacement piece I can weld on?
Should I drill it out and use a bigger bolt for the transmission mount?
Should I use a helicoil?

Thanks,

Rodney

The top one with the bolt in it is fine, it is the bottom one.

http://forums.pelicanparts.com/uploa...1600620230.jpg

http://forums.pelicanparts.com/uploa...1600620230.jpg

Up close and personal...

http://forums.pelicanparts.com/uploa...1600620230.jpg

Backside of mounting hole. See the small center section... crazy.

http://forums.pelicanparts.com/uploa...1600620230.jpg

Current state of '72...

http://forums.pelicanparts.com/uploa...1600620230.jpg

What about a helicoil? I think that the threads are M12-1.5

Hi Harpo,

That is what I'm thinking. Shouldn't be a bid deal right? The tranny is held on by a multiple points.

I have not used helicoils in a long time and it was in an industrial mechaical setting. There seems to be tons of different off name brands. Have you used any of these? I wouldn't mind getting a set that way when the mechanical build starts if I run into any issues I will be ready to deal with them.

Thanks,

Rodney

How about you welding a m12X 1.5 nut to the back of that mounting flange since nothing is in the way.

Hi Yelcab1,

Here is the back of the hole. There is some bracketry back there see this pick. I would somehow have to cut that out.

Thanks,

Rodney

http://forums.pelicanparts.com/uploa...1600635580.jpg

Had the same problem, installed a heli coil and has worked great. Due to some transmission problems removed engine/trans several times with no problems. Needed to buy proper size drill bit. Which is likely to collect dust.

I used a time cert 14x1.5 and worked great.

I would use a Timesert in that position then. It has more integrity than a helicoil because it is one whole unit.

Timesert. use the transmission mount as a guide to make sure it is installed straight and at the right overall width distance.

And don't forget to loosen the transmission cross member bolts so that it can float just enough to keep alignment upon installation.

Thanks guys. Ok, I will have to look into timeserts I have never used one.

I repaired both of mine many years ago with kit called a "Pro Thread". It included the inserts, proper sized tap, and installation tools. The only other thing to buy was the proper sized drill bit, a 31/64. I think I found it on Amazon.

http://forums.pelicanparts.com/uploa...1600658705.jpg

http://forums.pelicanparts.com/uploa...1600658705.jpg

Keysert Another Option
 

Keyserts have pins which help hold the threaded insert into the hole. I have not tried these but I believe it secures the insert better.

http://forums.pelicanparts.com/uploa...1600660707.jpg

Keysert Insert

I became quite familiar with the use of Keyserts over the course of a 34 year career in tool engineering. They work great in softer materials like aluminum and composites in applications wherein fasteners are continually removed and reinstalled, like with removable components on tooling. Those pins are meant to keep them from eventually backing out under those conditions. They are not really intended to be a repair option for damaged threads in harder materials like steel - those pins can be damn near impossible to drive down into those harder materials. The various inserts without the pins are more suitable for use in steel, and are meant to be used in more or less "permanent" applications wherein the fastener is not being continually removed and reinstalled.

Interesting information Jeff. Thanks for the lesson in Keysert use.

Harold

Small variances from the factory will always be there.

When matching up bolt holes to existing attachment points, the other part should be mated in place to know exactly where they connect.
Put the parts together and match them exactly.

(Maybe use a dab or two of superglue to keep it aligned before welding? idk)

Another thought is the OEM tube itself.
It has brackets at the ends.....but not where the engine/trans pushes back and forth.
The tube looks like it is independent from the flat sheet metal at the center.

Some braces closer to the point-of-roll or whatever the engineering term is might reinforce the transfer of on/off to the chassis, but it will also add vibration.

The chassis is at bare metal stage and anything can be done at this point.
http://forums.pelicanparts.com/uploa...1600665191.jpg

Some 911 (dont remember year) came with the reinforcement that you mentioned but between the torsion tube and rear seat bottoms.

repair
 

Just get a Time-sert repair kit then sell the kit to the next Pelican who is in this situation.Kit comes with extra time-serts.It is a PROPER REPAIR.Real steel threads.Fred

I ran in this on my SC, except I had to do the repair with the car on jackstands.

I used a time-sert on the passenger side and it worked beautifully. However I tried to do the same on the driver side and I tilted the tap slightly due to lack of clearance with the clutch cable and other stuff in the way and I screwed up the repair. So I had to go back and re-drill it square and use a Big-sert (bigger time sert for when time sert threads are no good).

In your case the time serts would probably work since you have lots of room. You also have lots of room for someone to weld it shut, redrill, and tap new threads, which I would have preferred if I was able to do so.

If you choose a solution that involves drilling, I suggest caution. Go slow! I tried to do a thread insert in this area and bought the requisite drill bit. I went pretty slow, but not slow enough. The material hardened from heat.

So I decided on a different solution.