Stripped threads in mag case

[BK911]

Hello!

While attaching a 915 tranny to a 72 mag case, I was unable to tighten one of the nuts. The stud kept spinning. It's the stud on the bottom of the engine on the same side as the starter. I was hoping for recommendations for a fix. The easiest way would be to tap to the next larger size stud and install a bolt. Any problems with this?

Thanks!
BK

[Early_S_Man]

Plenty of future problems if you put in a bolt ... as nobody in their RIGHT MIND puts together Magnesium castings with bolts threaded into the casting!!! Look everywhere ... air-cooled VW engines and transaxles, 356's, 912's, and finally, 911's ... not anything but plugs and hydraulic fittings are used threaded into the case ... given the inherent weakness of Magnesium!

The proper repair is a Time-sert, installed with red Loctite, period. If you decide to forgo reason, and do the bolt thing ... don't be suprised if the case strips out again!

[BK911]

Thanks for the info! Looks like this in another item to put on the "I'm glad I asked" list!

But now I am a little confused. I thought the studs used for joining the cases and connecting the tranny tapped directly into the magnesium?

Looks like I will be doing some time-sert shopping tonight instead of an engine install!

[Jim Sims]

Yes, many of the studs screw directly into the magnesium (and later aluminum) but this is typically only done once (during initial assembly) or maybe a second time if the stud is replaced because it is damaged or needs to be changed for some reason. However most of the time the soft threads in the light alloy are not "worked" during subsequent disassembly operations. Instead, there is a steel nut going up and down a steel stud which is a more "rugged" arrangement. I do believe that in the OEM engine to transmission connection one of the four fasteners is a hex head cap screw that is inserted from the transmission side. I'll have to check, but I seem to recall that the engine case "tapped hole" into which this screw goes contains a factory helicoil or other thread insert. I don't know why this was done (too hard to line up four studs at once?) but I know that some people install a stud in place of this cap screw. Warren is right though, fix your stripped hole with a "timesert" and then use a stud. Cheers, Jim

[marcesq]

BK:

You are right the studs are set in magnesium, however, the stud is stationary while torque is being applied. A bolt on the other hand is being turned against the treads while torque is applied. The latter weakens the treads causing them to strip out.

My 1 cent.

[Wayne 962]

I was up at Competition Engineering, where Walt decidedly recommends NOT using time-certs because they have a tendency to stretch and crack the magnesium when you expand them into the case.

I'm not a fan of time-certs either, as I have seen them pull out of cases when they are not properly installed.

-Wayne

[5axis]

It is my understanding that timecerts use form taps. If you are having problems with stretching or cracking, I would suggest going up a few thou in drill size. That should leave less material for the tap to extrude and therefore less for the insert itself. Maybe one of you engineering types could look up the diameter needed for the timecert to get a minimal yet safe interference fit.
On second thought, make a call to the manufactuer and have one their application weenies figure it out.

david 89 turbo cab

[BK911]

Gentlemen!

A lot of good info here. I ended up using a heli-coil mainly due to availability. So, the engine is in the car! I'll probably get sick right after lunch today so I can go home and make all the final connections and adjustments. Then take the top off and enjoy the weather!

Cheers!
BK

[Jim Sims]

It seems the factory used Helicoils in the magnesium engine cases and transmission housings for certain tapped holes that could see repeated fastener removal and reinstallation during the life of the car. Based on service and installation experience for our designs when we install any of the "solid body" thread inserts (Keenserts, Timeserts, etc.) in softer materials (aluminum, copper, NEMA G-10 epoxy fiberglass composite, etc.) we usually go up to the next drill size for the tap drill to relieve the thread fit to ease installation. When a solid body thread insert is installed properly, a SAE Grade 8 or DIN 12.9 screw installed into the insert will fail before the insert/parent material in most structural materials. The need for thread fit relief doesn't apply to Helicoils which due to their form have an inherent "give" but this give also permits them to be "pulled out of the hole in some cases. Advantages and disadvantages with both types of thread inserts. I think I still prefer the Timesert for engine use. Jim

[rcwaldo]

I dont care for timecerts as well. They tend to crack. In racecar land, when we stud something in alu or mag, we always try to use "keenserts." The downside to these is they require a larger hole size because the insert is thicker.

In the case of head studs, unfortunately we dont have that much material to work with, so we are stuck with timecerts.

Chris

[biggtbiggt]

I had 3 stripped studs on the outside of the case. I tapped them for bolts. After this I am wondering if this is a bad thing. I used coarse thread bolts as to keep as much material in the threads as possible. this is on a 1975 2.7. I was also wondering if it would not be possible to drill out he hole and put the bolt right through with a nut on the other end??

Thanks...Tracy

[dtw]

Three of my transmission mounting stud holes were helicoiled. One stud just spun when tightened. All three helicoils eventually pulled out of the case and dropped the engine off the stand and very nearly onto my sandaled foot.

I don't care one way or the other, but my feet have made it clear that I am not to use helicoils Ever Again.

I've got another engine rebuild in the pipe- to give Wayne and his case-saver lobby a chance, I'm going to give case-savers a shot. My current engine project has around 30 time-serts in it, give or take.

Not really fair though- the time-sert engine will be pushing 200hp out of a heavily modified 2.4, while the case-saver engine will likely be a bone-stock 2.4T motor.