kd460

OK, this is for the guys familiar with metal working.

A brief description of problem: My wife drive a 2002 jetta tdi (diesel). Great little car, 45mpg's, but has it's own little quirks or flaws that need to be overcome (much like the dodge).

It has an engine mount called a "pendulum mount". This mount needs to be removed everytime you replace the timing belt (yes belt, gears would have been much nicer). The way it is designed, the whole right hand side of the engines weight rests on these two bolts, and if the bolts strip out or break, THE RIGHT SIDE OF THE ENGINE FALLS OUT. Great design! Nothing supports the engine but theese two bolts. Remove those and it freefalls.

Anyways, this mount is cast aluminum, and steel metric bolts thread into it. They are metric, not sure of the size yet, but are just under half inch diameter. When changing the belt, the steel bolts get removed, and they are famous for taking the soft aluminum threads out with them.

Well, thats what happened to me. The threads got damaged while removing the steel bolt. It seems a little piece of aluminum stayed on the bolt, and as it was removed, it dulled or mushed the threads. The replacement bolt went in fine with some medium strength loc-tite, and I was able to torque it up to it's normal value.

Ohers have had this problem, and ended up having to re-tap the threads to the next largest size, which happens to be half inch.

I was thinking, in order to prevent this problem, I install a steel stud to eliminate the whole screwing in and out of aluminum thing completely.

So my thinking is to tap to 1/2"x13 or would it be better to tap to 1/2x20 tpi. I think I read some where that the 20tpi would be stronger, but then I read that only the the first 4 threads of a bolt do most of the holding load. (According to this link):

http://www.gizmology.net/nutsbolts.htm

After I tap, I would install a grade 8 steel stud (the metric bolt is grade 10.9), use high strength loctite, and next time I need to remove, then instead off removing a bolt from aluminum, I just remove a nut from the steel stud.

I think I can get in there to tap the threads without removing the mount (need to remove allot of junk and lift and lower the engine to get mount out), it is a blind hole.

Does anybody know about a form tap? Whats the advantage of a form tap as compared to a regular cutting tap? And once again, 13tpi of 20 tpi? Any other tips? I never had great results cutting threads in aluminum. I thought about a helicoil, but I think I would need to remove to much metal to get a helicoil insert in. So tapping to 1/2 inch would remove the least amount of material.

I'll see if I can locate the size of the metric bolt. Kevin

sherod

Best thing to think when tapping something, "I'm moving too fast". Use lots of lube, and back the tap out every time it feels the slightest bit tight.

Currentlydrilling8brokenmanifoldboltsEd

Fronty Owner

I find that moving about 1/4 to 1/2 turn then backing up works the best with steel. Your not going to see alot of benefits between course and fine threads, but since your going into a soft material, I would go course.
FWIW, find a screw shop and talk to them, there is a tap design out there (but I dont remember what its called) that has the tip cut differently so it cuts faster and requires less backing up. Also, red locktite doesn't stand up to oil permanently, if you have room for a jam nut (or a thin nut) that would be a better route to go.

wgingg

or find a metric stud of the same threads. no need to tap

pogorules

If you have room, drill out and tap to 7/8" left hand thread. Cut a 7/8" bolt to the same length, drill and tap center of bolt to fit what you have now. Install with permanent loctite and never worry about it again.

Shovelhead

Have you considered Helicoils?

Iron Mike

I'd try too go the helicoil route if at all possible. We have 5 Aeroderivitive Turbines at work GE LM2500+ and on aircraft engines where you are bolting into Aluminum. A Helicoil is almost always used. When you damage a thread you can almost always just install a new helicoil and it spreads the torque out over the length of the heli-coil. Just my .03.

torquefan

I agree with Helicoils. Used them in aluminum many, many times. You will never have another moment's problem with those threads.

kd460

OK, thanks, I'm surpised at the amount of responses, and thanks again.

A little more info: The stock bolt that threads into aluminum (and designed to be removed with every timing belt change), is 12 mm. I am currently down here is Florida visiting my parents, it was a nerve reacking drive from Michigan, worrying about those threads giving away. I have been checking the mount as I traveled, no separation. The tourque value is 44 ft lbs, then an additional 90 degree turn. It took the torque and the turn without letting go, so the threads must not be that bad.

Anyways, down the street from my parents is a retired machinist. We did the math, and a 1/2 inch bolt is cutting it to close to the original 12mm size. Not enough. So he suggested I go with 9/16 by 18 tpi. He said more threads per inch hold better.

The hole is a blind pocket hole, so no way to get a nut from the bottom. Ther is some the aluminum pocket is ok, but not real thick, so I think a helicoil would require me to take out to much metal? Besides, the machinist seemed to think that the helicoil would not do so well in the aluminum? Not so sure about that. Need to do my homework on that. I thought helicoil would be the way to go also, but whatever I do, I still want to go with the stud, so I no longer have to remove it, just a nut on the top.

OK, metric stud won't work with current size (12mm) those are the threads that are damaged, so need to go bigger.

The 7/8" is to large and sorry, I'm not that good .

I am liking the helicoil deal, or tap to 9/16x18tpi, either way, install the stud, and use permanent loc-tite and be done with it.

Time to do my homework on the heli-coils to see how much larger I need to make that hole to take the helicoil.

Anybody know anything about a form tap? (I forgot to ask the machinist). From what I have heard, it forms the aluminum instead of cutting the threads?

Thanks for the info on cutting the threads. Whats a good cutting oil? Kevin

bmoeller

http://www.toolsource.com/ost/heli_c...pdie&brand_id=

Haven't heard of the form tap, and haven't found anything on them, yet.

Shovelhead

My buddy who ran a Top Fuel Harley uses aluminum cases that have helicoils for the head studs.
They come that way from the builders.
The guy who built my Shovelhead motor had engines in the top bikes in 3 different classes in AHDRA a few years ago.
Yup,..... Helicoils in all those cases too.

cruznsr

Form tap sometimes called a roll tap takes the material and forms the thread reather then cutting, since you alread have threads cut the form tap won't work unless you go to a much larger size. Its works realy good in very small holes as the chips won't brake the tap. Rule of thumb for depth is 2Xdia I add + 1/4. Helicoil is the way to go as it is much stronger then even the orginal threads were but make sure you use the right size drill if the kits dosen't come with one.

ptm600

If you haven't already fixed it. I am a machinist and have had some experience with threads in cast aluminium. I would recomend using a helicoil. You will eliminate future problems by strengthening the threads, as the helicoil will not have an electrolisis reaction with the bolt, and it also spreads the load over a larger area. If you can get away with going to a 9/16 bolt you should be able to helicoil it. Good-luck!

kd460

OK, I'm sold on the helicoil and I've been looking on line, no it's not fixed yet.

The problem I am having now is locating a helicoil in the length I need. I need a 12mm size not sure of thread pitch yet (1.25, 1.5, or 1.75) not sure how to figure tht out without getting a nut of known size and threading it on. Anyway to count the threads like sae threads?

The length of the helicoil (or the length of the threaded portion of the bolt) is 30mm long (or 1 and 3/16) long. The helicoils that I am finding are a max of 20mm.

Any suggestions? Thanks again, great ifo, Kevin

ptm600

I believe a 20mm helicoil should be long enough to serve your purpose, as the minimum recomended thread engagement is 1.5 x diameter. You also don't have all 30mm going into the threaded hole as it has to pass though the mating part first. You could also helicoil it to a thread size that you know you can get replacement bolts for. It doesn't have to be the same size or pitch that was originally used. You could go with an english thread.