Hey all. I had a post elsewhere, which initially had a different purpose and I'm getting no traction there. As such, hoping to make a specific post here for some input. I had 2 stripped sets of threading on the front engine mount bracket, as indicated. I retapped with helicoil, and the fit is great but they're only torquing to 30 foot pounds instead of 40. I have to imagine on such an important structural piece, that's not good enough, but I've honestly no idea what to do about it. The process went smoothly enough for drilling, tapping, and rethreading, and the only thing that comes to mind is that while I blew out the shavings and used shop towell to get the tapping oil out, I didn't use any cleaner to make sure I had all the oil residue out. I havent been able to find any cases online of similar issues with helicoil retapping, so I'm coming to the brain trust for some input. What should I be doing here?




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Are the helicoils the staked type, where you gently hammer in two thin vertical bars to lock them into the material? Those can hold massive torque.

Another point: I recall that you had a journey of discovery with drill bits. With inserts, it's vital that you drill the exact correct size hole with zero taper from wobbling drill. If the hole is large and tapered, the threads you tap to receive the insert won't be strong enough to hold it.

If you've gone this far, you may have to do something drastic - one solution may be to drill a larger clearance hole, then fabricate a plate to bolt over it with a tapped hole in it. This would be machine shop work, but not impossible and cheaper than wholesale replacement.

 

It was a process of drilling, tapping, and then screwing in the threading and punching out the prong used to screw it in and done. The drill bits was a misunderstanding of a chart and how they size them (#10 instead of an actual measurement based designation, for example) to let me get a drill bit with a smaller drive head to use my existing equipment. Once that misunderstanding was cleared up I got necessary adapters and used the drill that came with the helicoil kit, fit to the bolt and threading size, so the fit should be perfect and it does feel perfect screwing in the bolt. It's nice and tight, until I pull out the torque wrench and get it to 30lbs and then it spins.

Any machining processes would be a greater problem as I definitely have no equipment to do so, nor know anyone who does.

 

The issue is why is it spinning at 30 ft/lbs. Are the threads on the outside of the helicoil slipping or is the helicoil being driven in further by the bolt/screw? OR are the threads on the inside of the helicoil stripping? In any scenario you have to figure that out to make sure torque from the engine doesn't rip them out.

If it's the helicoil slipping you may either have the hole for the insert too large or have it stripping the threads. You'll need a keyed insert that will resist spinning. If its the threads on the inside of the helicoil, your kit may be too soft to use. If the insert is being driven futher you'll have to see if part of the tang is causing the issue.

 

As above, try key locking inserts.