Helicoil - stronger than tapping/rethreading?

[hsunchen]

I'm having a repeated problem with blowing out the bolts in the front engine/tranny mount to front firewall in my MR2.

The bolts thread right into the tranny, and they've stripped out before. I fixed it previously by tapping it to a larger bolt size, but it's happening again.

Will installing Helicoils add strength, especially in this case b/c they would be threaded into the relatively weak aluminum tranny case?

I will also rig up some strengthening braces to spread the load onto other attachment points, but I don't want to have to deal with again.

It only took about 10 passes at Bandimere (street tires - no drag radials or slicks) and a few weeks of street tuning before the holes stripped again.

[Dustin]

Helicoils will not be stronger than actual threads. Sounds like it would pull the helicoils right out when you launch.

[hsunchen]

hmm - not good.

What's the point of Helicoiling then? Why do they even sell them if they have the same effect and rethreading it and putting a larger fastener in it?

[Dustin]

Helicoils work great if you strip a manifold, carb, etc. You are just fastening things together. If you have enough force to rip threads out then a helicoil doesn't stand a chance.

strengthen the existing points re-tap and see what happens

[SleeperZ]

Helicoils are stronger than the original threads in aluminum. Think about it...you are moving the stress to a much bigger hole, and you are not weakening it by repeatedly threading and torqueing fasteners into it.

[Weston-work]

My first car, a 1973 VW Bug, had a problem with the #3 spark plug shooting out of the engine... I had it fixed with helicoil and it lasted another 9 months or so, then it was so messed up that it needed to have that #3 head replaced. Man, that car was nothing but problems.

[hsunchen]

I ended up just tapping bigger and ditched the Helicoil idea. The $40-60 cost of the Helicoil with tap helped convince me. I looked at using something called a EZ Loc, similar but much cheaper since it uses standard taps, but it requires a MUCH bigger hole to install it.

This time I went with a coarse threaded bolt rather than what I'd been told actually holds more torque (fine thread). It went ok aside from putting a bunch of aluminum shavings into my eye - took me all weekend get them out.

I think maybe with the soft material of the aluminum tranny case, maybe the taller threads will actually give more strength since the matching "peaks" will be less prone to be ripped out by the bolt.

What is the reason that fine pitch threads supposedly hold better - more surface area?

[hotrod]

Bolt strength vs thread strength --

The fine threads have a larger minimum diameter ( the minor diameter of the thread) so the BOLT will be stronger size for size. The threads the bolt screws into are limited in strength more by the depth of the threaded hole. If you put threads as fine as frogs hair ( highly technical machinist talk) in a very deep hole it would hold a great deal of force. A very heavy course thread in a shallow hole will pull out under the same load. The strength of the hole threads is the determined by the total area of the base of the threads that needs to shear to pull the thread out.

If you are pulling out the threads in the transmission one of several things is wrong.

If you tap the hole with the wrong size starting hole you can greatly weaken the threads. It is very important that the hole is round. If your doing this in the car you may be accidently ruining the thread before you start by having a poorly drilled hole.

A thread with 70% thread depth is pretty much the industry standard. It is nearly as strong as a full depth thread but much much easier to thread because the tap has less material to remove.

The other possiblity is something else is broken and transfering extra force to the bolt/hole that is stripping. Take a good look at your tranny and engine mounts and all the other things that should be carrying part of the load. I suspect one of them is bad.

Take a good look at the mating surfaces the bolt is holding together, if you have a burr, or a dowel pin that is not mating properly you could also be putting an un-natural amount of load on the fastener.

If the threaded hole is a through hole ( ie goes all the way through a flange) and you can screw a nut on the back you can double nut the bolt and about double its holding strength.

Use of a thread locking compound like locktight will also help a bit as it makes sure there is complete engagement of the threads.

Take a good look at the bolt and make sure it is not bent, or has crapped up threads. Be sure you torque it to the design torque.

That's all I can think of off the top of my head.

And yes a properly installed helicoil is "supposed" to be stronger than the original hole.


Larry

[Erron S.]

Just as an FYI.

Nascar, NHRA, F1, F3000, and just about every other major engine builder will take a brand new head and helicoil it. Even some of the "roundy-roundy" dirt guys locally do this as well.

Erron S.

[hsunchen]

Originally posted by Erron S.
Just as an FYI.

Nascar, NHRA, F1, F3000, and just about every other major engine builder will take a brand new head and helicoil it. Even some of the "roundy-roundy" dirt guys locally do this as well.

Erron S.

Thanks for the info Erron. I'm suspecting this is more to prevent deterioration of threads and stripping them from repeatedly removing and reinstalling fasteners rather than to increase maximum load capacity right?

[Erron S.]

Not sure about the remove/installing longevity. It would make sense though. All the motor builders I deal with do it for strength.

Erron S.