DrummerDad

Okay, settle an argument for me guys.....

I have a friend who swears by helicoils. Even in new parts (machined aluminum parts, made from a billet of 2024-T3 aluminum, and machined to aviation tolerances, with aviation hardware) he insists on over boring the hole, and installing helicoils for the thread pitch he requires.

Ive always been taught helicoils are for repairs. Tap the unit, then if something happens, do the helicoil. He insists on taking parts we have machined to a bigger size, and inserting helicoils straight out of the box.

The guy who machines the parts agrees with me. He makes several suspension brackets for race cars, and none of them have helicoils. They bolt straight through into the aluminum.

Shift06

I work at a Machining / Fabricating shop and we build stuff for the Military, and we use helicoils right brand new (in aluminum parts at least).

DrummerDad

Well we do aviation parts, and all AN or MS specs on everything. Do you know of any FARs or regulations requiring helicoils? I cant find anything.

AlkyIROC

Depends on a lot of factors. If the fasteners are subjected to a lot of stress then a helicoil is less likely to fail. If the fasteners are regularly removed then a helicoil should be used to keep the aluminum threads from stripping or galling by repeated tightening. Using a helicoil isn't the proper way to reinforce a threaded hole. A proper insert should be used. The most common ones are called Keensert (use google). They're threaded into an oversized hole then pins around the outside are tapped in which locks the threads. They are far superior to helicoil which as mentioned above, are typically just used to repair a hole.

For the majority of fasteners into aluminum, the threads in the aluminum are fine. My heads don't have any helicoils and I have my headers off every now and then. I take my rocker covers off more than that and there's no helicoils for them either.

Environment is another factor. A steel bolt in aluminum will get a lot of corrosion from dissimilar metals. Using a helicoil allows the steel bolt to contact steel threads. The helicoil thread in the aluminum won't react the same way as a bolt will. A Keensert would be a better solution.

DrummerDad

I agree with that. But we use stainless helicoils, in aluminum, with steel screws, talk about disimilar metal corrosion. And Im sure you guys know what a PITA removing a helicoil is.

And if its a part with a high removal rate, I would agree it should be re-enforced. But we tap our aluminum (or did) to a 75% thread. That meets the class 2 to 3 thread fit required by aviation. All close tolerance bolts too.

I see alot of high stress (of course in double sheer, never tension) aluminum parts that see high loads, without catastrophic failure. I could understand maybe if we did install these parts in tension. Then I could understand the helicoil being stronger. But in a sheer, or double sheer application, it cant help.

To me, by installing the helicoil first, you remove a future repair oppurtunity. If the threads strip, helicoil it, then if the helicoil strips, replace it. By helicoiling first, you not only lose the first set of threads from tapping and the material, but also risk damage by coil replacement.