Stripped Threads in Trans. Hsg.
 

Ugh! I just discovered that the mystery oil leak is coming from one of the shift rod detent bolts in the side of the transmission – seems the threads are stripped in the housing and the bolt can’t be tightened to seal.

Does anyone have any suggestions as to the best way to fix this – short of replacing the transmission housing? Maybe - Drill & tap oversize?, Inserts?, Adhesives?, Magic Spells?, ?? Please help.

Thanks,

By the way, this is my first post, so if I stray from the straight & narrow don’t hesitate to let me know.

Tryan,


Welcome to the Forum. Sorry your first post is dealing with a problem. There are fun projects to come.

The easy solution is a Helicoil insert. You will probably need to go to an industrial supplier to find that size.

What year transmission? Aluminum or magnesium? Exactly which detent?

BTE, where are you located? It is also fun to post pictures and fill out your profile.

Best,
Grady

What Grady said...Sound, spot-on, and least expensive fix. If you set a drill fixture so you drill straight, this fix can be done without ever leaving the comfort of your own garage.
Paul, ex Machinist

Tryan, or is it T. Ryan?
You've identified most of the solutions to the problem and Grady has rightly said that the coil insert is probably the best. The main problem with putting an insert into your housing is that the drilling could force swarf into the gearbox, which would be difficult to remove unless you had the gearbox off the car. How bad is the leak, can you live with it? It is possible that some thread tape would do the job and last long enough for you to wait until the 'box is off the car for other remedial work. Failing that I would try some high temperature 'glue', which should easily withstand the oil because the casing is not pressurized like an engine.
Good luck
Ian

the hole is stepped and the threads are shallow. the detent lives in a clean bore just beyond the threads. you have to use two taps to cut the threads. first a regular one, then a bottom tap, so the threads have minimal taper and you can make use of the majority of them, as there is not very many. the drilling and tapping NEEDS to be perfectly in line with the detent bore, or when you get done installing the helicoil or timesert, you may not be able to get the detent in. chips won't get inside because the hole is blocked by the shift rod. just blow them out. one tip might be to leave the detent in there so a somewhat misaligned thread repair won't keep you from inserting it. after all, it really doesn't need to be removed on overhauls, just the spring and bolt, so it's not pushing on the shift rod. i use timeserts and trim off the excess and carefully file the outer flange flat. remember, DO NOT tighten this particular bolt very much, as the threads are very easy to rip out, especially on mag cases.

Paul is right on. It is worth the effort to make a drill/tap fixture to insure a strait and consistent drilling, tapping and insert install.

Ian’s idea is good of just sealing and leaving it alone until you have some reason to disassemble the transmission. The repair can be done best with the casting properly set up on a mill. It also totally prevents any debris from getting in the detent bore.

JW points out an important fact. The threads aren’t very deep (not many turns.) You probably will need two or possibly three Helicoil taps and have one or two modified to be bottoming taps. A standard Helicoil tap assumes some clearance in the hole.

A good solution for preventing debris from getting in the bore is to tap some waxed cotton cloth in the bore. After the machining, a sheet metal screw type puller will remove it properly.

Doing this properly in situ will require some planning, a custom fixture, and some specialized/modified tools. I think the critical component is fabricating a single drill fixture that can be properly aligned with the bore, accept the drills, taps, and the Helicoil install tool without being moved – just inserts. I’ll bet Paul can give some insight here. Depending on type and year clearance will be an issue.

Another issue is to inspect the flat sealing surface for damage or corrosion. It is possible you also need to spot-face that surface.

First make sure you can locate the proper Helicoil – length is critical here.

Of course you want to use a like new plug with perfect threads and sealing surface.

JW, Tell us more about your modified Timesert and pro/con compared to a Helicoil.

Best,
Grady

Hey, everyone, thanks for all the input. I thought it might be a while before I got any response, but you all have blown my doors off. Also, thanks for pointing out my missing profile “stuff”. It’s now updated.

The car is an ’86, 911 coupe, black all the way, and has about 95,000 miles. It’s a “street” car, but I run it in DE events and “track days”. The engine was repaired, with some minor upgrades, last summer. (due to my boneheaded shift from 3rd to 4th via 2nd at about 85 mph). (A WEVO shifter, or something like it, is next on my upgrade list.

The transmission is the stock 915, and it is the upper detent (3rd and 4th), that is stripped. The lower seems OK - took the spec. 12 lb. ft. torque w/no problem.

I like Ian’s thoughts on using a good high temp glue to hold it in place. I’ll probably wind up doing that.

But, all your comments about using some sort of drilling jig to be straight and clean with an insert triggered some possibilities, too. I will take a look at trying to come up with a fixture that will enable me to drill/tap/insert a straight/clean hole. That would be the solution that appeals to my “engineerness”., but given my limited resources, the adhesive idea may be more practical.

Thanks for the input, guys. I think I am going to like this forum stuff. If anyone thinks of anything else that I might consider, send it up. I’ll be here. Thanks, again.

Terry

something like JB weld should hold it until you get around to doing it right.