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I have a bolt hole in a head that's stripped. The hole takes a M7 bolt, should be tightened to 17NM, and is used to fasten a water pipe onto the head. The bolt just spins when I try to tighten to torque. It's in a hard to reach and see spot, so even if I was able to get a drill in there, I don't trust myself that I could drill straight. I'd rather not take the head off and have to order new gaskets and head bolts, so I'd like to try other options first.

I've ordered Loctite Form-A-Thread. Will this be able to take the torque and heat it will be subjected to? Is there anything else I can do while I'm waiting, or if the Loctite doesn't work?

Could I tap the hole to take a M8 bolt without drilling?

Would using some kind of high strength, high temperature epoxy (JB Weld, Loctite, etc) to bond a stud into the hole have a better chance of success?

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  • Using some strong loctite to hold a stud in sounds like a good start if the hole is inaccessible. Leave the loctite to set for a day before tightening the nut.
    – HandyHowie
    Sep 29, 2016 at 10:43
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    Run a m7 tap through it and see if that helps. You might get lucky... Otherwise, find the next closest size up, even a standard size if necessary, and try to tap it. It's sorta hard to tell if you'd need to drill any material out first though. It's more of a trial and error process.
    – cory
    Sep 29, 2016 at 12:28

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If the hole is truly stripped, then yes that would actually work pretty well. The major diameter of an M7 thread is, well, 7mm. I'm much more comfortable in inches, because that's what my shop works in, so translate that and it's about .275 inches. Lets look at what that is appropriate for on a tap drill chart:

Tap drill chart

If you look at the two M8 options, you'll see that the tap drill size is remarkably close to 7mm, which means that your hole, having its threads ripped out, is properly sized for an 8mm tap. Please note: I would still recommend chasing with the appropriate drill size before running the tap.

If you can't drill it straight enough when only removing trace amounts of material, the tapping is not going to go well either. You also may need a tap extension, and some good cutting oil. Also just be careful not to break the tap. That's just awful.

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  • Looked on the Loctite Thread Repair Kit package, and it's rated at a pretty low torque, so didn't even try it. Tried bonding a long M7 bolt with the head cut off in the hole with JB Weld, and that didn't hold. So, I guess I'm going to try tapping the hole to an M8. Will need a ~5 inch extension to use a tap wrench (and damn are they expensive).
    – user43704
    Oct 7, 2016 at 8:16
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M7 sounds like an original 1/4 UNC opened out, however... there is a great amount of information regarding the repair of stripped threads, a German company called "Böllhoff Group" invented a stainless steel wire insert called "helicoil". As you were prepared to tap a thread, you are in your element, as the kit gives you a tap to cut an initial thread into the workpiece. From here, you wind in the insert and break off the "tang" using the supplied tool. You now have a repair that is stronger than NASCAR could care about. Your main issue is determining the original thread, purchasing a helicoil kit to suit, and carrying out a professional repair... not hard, but like everything, pride of workmanship and attitude will determine the outcome.

Cheers Hutch

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  • Helicoils are really cool stuff, but you need the insert and the very special tab (it has a lower pitch, different diameter and tread shape than regular tab). And M7 is really uncommon to unknown. I'd recommend to just use a wider screw whenever possible.
    – sweber
    Sep 29, 2016 at 17:15

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