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69tr
09-01-2006, 06:32 AM
I am replacing the inner axle on my 69 TR. I got almost everything put back together. When I started to install the outer housing-hub-bearing to the trailing arm I tightened the nuts with a short 1/4 drive ratchet. Some of the stud started pulling out with only a little pressure on the ratchet. The trailing arm is brass, which is my problem. Can I use a thread repair kit (heli coil) in this application? We used them in the mining industry for years with good results. Thanks for any help, pete

TR6oldtimer
09-01-2006, 09:23 AM
A common problem. I had a couple of studs on the trailing arm pull out as well as tapped holes on a gearbox. Why the Brits use NF studs in aluminum is still a mystery to me. Perhaps it is because they had lots of the laying around. Anyway, the threaded holes can easily be repaired with a heli coil. I also cross tighten the nuts across the axle, slowly pulling in the backing plate until all nuts torque to 10 lbs.

ALLAN
09-01-2006, 12:14 PM
I have an old article out of a 6PACK magazine that goes through the complete procedure using a NAPA Helicoil kit with stainless steel inserts, I can fax it to you if you want it.

Alan_Myers
09-01-2006, 02:38 PM
Hi Pete,

Yes, this problem is common and, as the other responses point out, NF or fine threads were simply a bad idea in the aluminum trailing arms. They're almost guaranteed to strip out eventually.

Helicoil (https://www.emhart.com/products/helicoil.asp) (or other brands that are similar design) will work, but are not what I'd use here, personally.

Instead I'd use Time-Serts (https://www.timesert.com/) or similar (https://www.fulltorque.com/) . These are superior to Helicoils, especially for use in aluminum. (Many motorcycle mechanics swear by them and won't use Helicoils in aluminum engine cases, for example.)

The difference is that Helicoils are just what the name implies, a "coil" of threads. They are sort of like little, brittle springs that can be expanded and contracted. To install them, you drill the stripped hole out oversize and tap in new threads, then turn the Helicoil into that. Most (all?) use matching inside/outside threads: i.e. coarse outside/coarse inside or fine outside/fine inside. Also, when installing them the coils can skip out of a thread to get twisted, out of alignment, bent, broken, etc., inside the hole (this is not quite as bad with a larger size, as it is with smaller).

Helicoils can only take so much torque, after which they can and will also strip out the threads in aluminum. This depends in large part on the quality of the new threads you cut when tapping the oversize hole. Also, it can be difficult to position the coil's depth within the ovesize hole. I.e., there is nothing to stop the coil from threading in too far unless it's in a blind hole and bottomed out, or little to prevent it from backing out if a stud (in this application... possibly bolts in other applications) ever needs to be removed.

The Time-Sert-style insert (there are other manufacturers, bsides the two linked above) is a solid piece or "bushing" with internal and external threads. In other words, it's not a "coil" of threads. These are installed in the same way (drill hole oversize and tap, then thread in the insert) but because they are solid, the threads cannot become misaligned, can take higher torque and the external and internal threads can be different. This is ideal in this particular application and many others. Use an insert with a coarse outside thread to properly anchor the insert into the aluminum trailing arm, and a fine inside thread to best install the steel stud into the insert.

Because Time-Sert style are solid, you can use some Locktite on the exterior threads of the insert, if you wish, as an added precaution against loosening. There is a "tensioner" in the design that tighens the insert when it's installed and help keep it from ever backing out. So using some Locktite is an extra margin of safety.

The only hitches are that Time-Serts are more expensive and harder to find. Still, it's well worth the extra effort and will make a much stronger and better repair for a critical application like this. You might check with motorcycle shops in your area to see if they have the tools and inserts, or can recommend a source.

If you are forced to use Helicoils because you simply can't find a Time-Sert or similar style of insert, I'd suggest using coarse thread with all new studs that have coarse threads on the end that goes into the insert, and fine threads on the opposite end. That way, coarse threads are used in the aluminum, which is best.

69tr
09-02-2006, 06:16 AM
Thanks. I found a thread repair kit at an auto supply store I use a lot. I also talked to a motorcycle mechanic about the repair kits. He said he has used them on his Harley and other bikes. The kit that I used has a "prewinder" to help keep the coil intact and tight during installation.
Alan, thanks for the tip on the Time-Serts system I will check it out