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Stripped thread in fork for disc brake mount?

Old 06-14-20, 03:18 PM
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Lost in NH
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Stripped thread in fork for disc brake mount?

I am sure everyone knows the definition of an expert. It appears I am becoming an expert on torque limits for disc brake mounting bolts. I just changed the pads for the first time on a new bike with Shimano BR-M447 disc brakes. I changed the pads with the wheels on the bike by removing the caliper mounting bolts. I had already finished the rear pads with no issues, but when I screwed in the top bolt on the front fork mount it kept turning when it should have become tight.

The top bolt is snug, not rotating freely, and the bottom bolt tightened properly. Together, the bolts are holding the caliper in place, with no Loctite. I just went for a seven mile ride with lots of hills and braking, and everything stayed in alignment. But I know I have a problem that needs to be addressed with more than just some blue Loctite.

The bolt appears to be fine, and the threads in the fork look good for as far as I can see. However, I twisted a Q-tip with acetone down the hole and it came out with one metal thread stuck to the bottom of the Q-tip. I am surprised if turns out I stripped the female threads rather than the bolt threads.

Comments? Has this ever happened to anyone else? I wanted to check this forum for thoughts and advice before I call my LBS, just so I'll be more prepared when I call them. Worst case, I'll take in my bike. Medium case, I'll ride with Loctite until fall and then take it in. I suspect my LBS will say bring it in now.
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Old 06-14-20, 03:35 PM
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Old 06-14-20, 05:13 PM
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I would try to re-thread the hole with threading tool.
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Old 06-14-20, 05:16 PM
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+100 This is absolutely the way to go. The repaired threads will be stronger than the original aluminum ones. Don't forget to use tapping fluid for aluminum when cutting the oversized threads. AlumTap Info

Edit: AMazon has it cheaper: https://smile.amazon.com/Heli-Coil-H.../dp/B0002KKPXK you can use the money saved to buy the tapping fluid: https://smile.amazon.com/Tap-Magic-2.../dp/B07CMNLYRL

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Old 06-14-20, 05:21 PM
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Would you not first try throwing a washer on the bolt and seeing if it doesn't tighten up? Just give it a bit more "margin of error" at the end... could save you a lot of hassle.

... and don't overtighten things
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Old 06-14-20, 06:14 PM
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A longer bolt may work, it will get past the stripped threads and hopefully into any good threads left. If that doesn't work as above helicoil it they are not hard to use. Check the bolt size if it is the threaded hole in the fork that is stripped I think it is an M5 can't recall the thread spacing.
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Old 03-09-21, 06:42 PM
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Threaded Fork Disc Mount

Originally Posted by wsteve464 View Post
A longer bolt may work, it will get past the stripped threads and hopefully into any good threads left. If that doesn't work as above helicoil it they are not hard to use. Check the bolt size if it is the threaded hole in the fork that is stripped I think it is an M5 can't recall the thread spacing.
I had this issue and fixed it by removing the bolt and cleaning the threaded aluminum off the bolt. I then removed 2 spring washers and reapplied the bolt. The extra thread (3 - 4 MM) was able to go past the threaded area and tighten in the fork.
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Old 03-09-21, 06:58 PM
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Originally Posted by Lost in NH View Post
I am sure everyone knows the definition of an expert. It appears I am becoming an expert on torque limits for disc brake mounting bolts. I just changed the pads for the first time on a new bike with Shimano BR-M447 disc brakes. I changed the pads with the wheels on the bike by removing the caliper mounting bolts. I had already finished the rear pads with no issues, but when I screwed in the top bolt on the front fork mount it kept turning when it should have become tight.

The top bolt is snug, not rotating freely, and the bottom bolt tightened properly. Together, the bolts are holding the caliper in place, with no Loctite. I just went for a seven mile ride with lots of hills and braking, and everything stayed in alignment. But I know I have a problem that needs to be addressed with more than just some blue Loctite.

The bolt appears to be fine, and the threads in the fork look good for as far as I can see. However, I twisted a Q-tip with acetone down the hole and it came out with one metal thread stuck to the bottom of the Q-tip. I am surprised if turns out I stripped the female threads rather than the bolt threads.

Comments? Has this ever happened to anyone else? I wanted to check this forum for thoughts and advice before I call my LBS, just so I'll be more prepared when I call them. Worst case, I'll take in my bike. Medium case, I'll ride with Loctite until fall and then take it in. I suspect my LBS will say bring it in now.
This comes down to material strengths. The bolt is most likely steel and the fork boss is Al. Besides overtightening starting a bolt crossed up is likely the next reason that threads get damaged. I agree with the longer bolt as the first, locktight as second and would only consider a non reversible method last (perhaps no mater how strong that might be) Andy
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Old 03-09-21, 09:08 PM
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Originally Posted by Andrew R Stewart View Post
This comes down to material strengths. The bolt is most likely steel and the fork boss is Al. Besides overtightening starting a bolt crossed up is likely the next reason that threads get damaged. I agree with the longer bolt as the first, locktight as second and would only consider a non reversible method last (perhaps no mater how strong that might be) Andy
Thanks Andy. I rode the bike all summer with no issues. I actually forgot about the caliper bolt problem in the fall, so I never took the bike to my LBS. My plan now is to replace the pads w/o removing the caliper the next time they need to be changed. Perhaps a bit more fiddly to do, but I want to leave the suspect caliper bolt alone as long as it seems happy.
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Old 03-10-21, 01:56 AM
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I'd recommend tapping the threads (should be M6) and using the longest bolt that fits. While aluminum/magnesium in fork lowers is relatively soft, it's not particularly easy to strip them out. I wouldn't be surprised if somehow you're using too short of a bolt in the first place.
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