I’ve run into a pretty significant issue with my 36" unicycle and wanted to get some advice on the best way to handle it.
I discovered damaged threading on the frame, specifically at the disc-side dropout mount where the bearing holder bolts on.
I’ve attached a photo showing the damage. The bolt is an M6, but I can slip an M5 hex wrench into the threaded hole pretty easily, which makes me think the threads are definitely compromised. Oddly, the M6 bolt still threads in without much resistance, but I’m concerned it may not hold torque properly or could loosen over time.
Right now I’m trying to figure out the best fix to make sure the bolts stay secure. Has anyone dealt with something similar? Would you recommend a thread repair like a Helicoil/Timesert, re-tapping, threadlocker as a temporary fix, or something else entirely?
Helicoil would be my go to fix. It is surprisingly easy and the result is stronger than the original thread. You can even get Helicoil with a self locking feature so the risk of loosing a bolt is near zero.
Be patient and drill straight.
There really isn’t enough wall thickness on the bearing side of the hole for a helicoil. it would breakout and and interfere with the bearing. Also drilling out a hole by hand, that will breakout on 1 edge, is not recommended.
Yes, the wall is quite thin. And there is no way you can drill from the other side due to the brake fitting. Is there room to machine a flat surface on the other side without compromising the strength of the frame and use a nut?
I would take my chances with the hole. It will probably not look pretty but if there is enough depth to the hole the Helicoil could sit 6 to 8 mm into the hole where the distance to the bearing surface is larger…
You could use a Dremel to (start to) enlarge the hole off-center. Problem is to do this round enough so that the hole can then be properly finished with a drill.
The ideal way to enlarge the whole would probably be to use a mill with a mill bit of the right size. That could be used as a drill which is immune to wanting to follow the existing hole.
What if you put the helicoil in, and then used a dremel tool to grind off the part that stuck out into the bearing space? Sort of a pain in the rear, but you’d have some better threads to pull on.
Or, you could just bed the helicoil in deeper, so that the part where there would be interference with the bearing would be pushed deeper into the assembly?
I’m assuming these holes are blind, so instead of having a blind hole you could drill a through hole and tap M6 all the way through and use a longer M6 socket screw to go all the way through. The tap will clean up the damaged threads and you’ll hopefully get a decent hold with the newly threaded part. You might even get away with just drilling the blind hole a bit deeper
Alternatively, if you think you are getting a decent hold with the threads that are there and are concerned about it slackening off, consider putting some Loctite on it and keep an eye on it.
Safest bet: its likely that below the worn section of threads, there are some threads that are still in good shape, so if you clean out the hole really well, and get a threaded rod that you can thread all the way to the bottom with a little red lock-tite and trim it at the correct length to put the lower clamp on and then use a nut.
Agreed, there is not a lot of tension on those screws, their purpose is to hold the cap in place. Too tight and the bearing is crushed and can’t spin freely. If @BigRobPage 's idea doesn’t work, I would drill thru and put a long screw with a bolt on the other side.
The problem with putting a nut on (either at the top or the bottom) is that the bearing-holder/end-cap is round on Mad4One frames. You’d have to either file/machine a flat on it (significant amount of metal to remove) or make a custom spacer to put on to give you a flat surface for a nut to sit on.
Personally I would just use a long bolt to reach (hopefully) less damaged parts of the thread and hope for the best. Keep an eye on it, but wouldn’t expect too much difficulty.
For a real fix, M7 is not on the list of preferred metric sizes, but readily available and should work. (Drill to 6 mm and tap with M7). Looks like that extra 0.5 mm of space is just aboutavailable there, although you should probably measure and check.
Added annoyance of having one bolt that doesn’t match the others, but I think I could live with that.
Helicoil looks a bit tight and may require some grinding to not interfere with the bearing, attaching a nut somehow is possible but not very elegant.
