Transmission mounting stud repair options

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panama86
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Transmission mounting stud repair options

#1

Post by panama86 »

Hey all,
I'm starting to run through what needs to be done to the cases before building up the transmission for my pan project. The motor is a 51FL but the box of parts contained this 48 box (I'm assuming its August 48, H8 date stamp and 121-35 casting number).

This case is a shambles, someone has tapped and installed oversized studs in 3 of the 4 mounting stud holes. Whoever installed the oversized studs didn't use the extra oversized colony plugs as the thread is not 1/2" UNC but appears to be 9/16" UNF? All of the stud holes were drilled crooked and the 4th hole has been heli-coiled. The drain plug has also been repaired at some point. Among other things.

I'm unsure of the best way to approach repairing the stud holes, I want to do it right this time so I don't have to revisit the issue down the track.

If the larger holes weren't so clapped out I would go to the heli-coil/t-sert route however there is not much material left on the boss for it to be drilled straight as the holes are currently crooked.

Another option is to drill out each stud and use hts-2000, durafix, alumniweld etc aluminum brazing rods to fill the holes then drill and tap but i have heard mixed reviews of those products and I'm not sure if it's the best way to go.

Ideally I would like to tig the stud holes and drain plug hole up and drill and tap for a new thread however i don't have the skill to do the job right and am concerned about distorting the case in the process. There is a mob here in Sydney that specialize in cast alloy repairs and they assure me they can do the job without distortion. I know it can be done but I don't know if i want to take the risk as I cannot afford to buy a replacement.

How should I best approach this?

Cheers!


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Bigincher
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Re: Transmission mounting stud repair options

#2

Post by Bigincher »

My transmission case was buggered pretty badly, too. Aside from the bad holes, one entire corner was blown out as well. The repairs to my case involved a lot of Tig welding, but also some machine work after the welding. The bottom was re-decked so it mated perfectly with the mount plate. The holes aren't merely 'drilled and tapped', but threaded on a mill or some such machine.
Whoever does your repair welding must also be a competent machinist.
nmaineron
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Re: Transmission mounting stud repair options

#3

Post by nmaineron »

You got some big holes there :D I think I would clean the holes up and put a good thread in them.Make some aluminum bolts to screw into the holes,put the green loctite to them and then weld the top.Drill and tap as needed.Some of the threads will poke through to the nside of the case ,if not all of them.I would weld them inside as well.

That would have been a good place for the MOCO to put the inserts like on the fifth hole in those studs.
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Re: Transmission mounting stud repair options

#4

Post by Andygears »

I agree with nmaineron ! On a mill, open the holes and screw an aliminum plug flush or low to the outside and get a TIG guy to weld in. Nowhere near as much heat as trying to fill the holes and nice bar stock metal to machine the new threads into. We would do that type thread repair on rotating machinery at my former job and it always works well.

Andygears
panama86
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Re: Transmission mounting stud repair options

#5

Post by panama86 »

Thanks for the replies. That's an option I hadn't considered.

Andygears, when you say on the mill open up the holes and screw plug in do you mean drill out the current threads, tap new threads then screw the plug into that? If so, I think I might be out of room on the outer studs as there is not much meat left on the boss. I might be able to run a tap through the existing threads to clean them up a little, problem is the person who did the last repair didn't thread or even drill all the way through the old threads so I only have about 1/2" of thread I can use. Will also make it tricky to weld up the other side as the tungsten will need to get pretty deep down in the hole to fill in the top.

Lucky I just bought a lathe so I can likely make the plugs myself and save some $$$ there.
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Re: Transmission mounting stud repair options

#6

Post by Andygears »

Ok if the old thread is 9/16 fine, try to clean that up. The idea is to get the holes big enough that the minor dia. On the welded in insert is somewhat larger than the major (outside) dia. of the small thread. That gives you pull out strength. If you can get the thread decent, make a threaded plug that will fill the hole. If it's uneven at the bottom, try to match each plug to each hole, hacksaw a slot in to screw it in and out. The slot gets filled with weld along with a bevel on top of your plug.

Then, get it on a mill to locate the new holes and fresh tap for standard studs. Loctite in the new studs. And if you wish, coat the inside of trans. stud hole with sealer.

Andygears
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Re: Transmission mounting stud repair options

#7

Post by Dave_R »

I had similar issues.... back in early 80's I had pulled the studs out while tightening the transmission down. I could not get the transmission tight enough to keep from moving, so I did something similar to what nmaineron mentioned. The difference is that I used a chunk of 9/16 threaded steel rod. Since you already have 9/16 holes, you may need to go a little bigger to get a good clean thread cut.
With this, there was no welding involved.
I cut the rod into wafers (plugs) about the thickness of the case.
Using a drillpress:
• Bored out the mounting holes and threaded them for 9/16"
• Inserted the threaded rod plugs flush (with Loctite).
• Drilled a small 1/8" hole on side of the threaded rod plugs (half of the hole in the steel, half on the case), threaded, put setscrew in.
• Peened them over. (This keeps them from walking in or out).
Using a mill (or carefully using a large flat file):
• Decked everything smooth and square.
Back to the drillpress:
• Marked new stud holes, drilled and tapped into the steel inserts.
• insert new studs with Loctite.
Good to go
The good thing is now I can really "reef” on the mounting nuts and I don't worry about threads pulling out. Now I just have to be cautious about breaking studs.
May not be like original, but I ride it hard and it has held up for 25 years.
Here is a picture after about 20 years of use:

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- Dave R.
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Re: Transmission mounting stud repair options

#8

Post by Andygears »

Dave, that's a great repair that stood the test of time. Probably better than my suggestion.

Andygears
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