Technical > Transmission
Thorn Mercury Mk3 bottom bracket click or knock - any solutions?
Moronic:
Had a an amiable chat with Robin Thorn. He apologised for what was likely not enough tightening by the assemblers. The fix is as above, although he suggested removing the drive side cup, cleaning it and the mating threads and applying some thread locker before reassembly. He observed that even if threads had been damaged through being ridden loose, on a Mercury the threads were in the eccentric rather than in the frame, and so could be wholly refreshed with just a new eccentric. Obvious but I hadn't thought of that.
Not sure what I think of the Loctite idea. That would mean tightening a big fine steel thread dry into aluminium, on a part of the bike that's exposed to splashes. Otoh lubing the threads invites a repeat loosening. I suppose if Shimano supplies its BB with threadlocker preapplied, thats the direction to take.
JohnR:
--- Quote from: Moronic on November 11, 2021, 10:57:38 pm ---Not sure what I think of the Loctite idea. That would mean tightening a big fine steel thread dry into aluminium, on a part of the bike that's exposed to splashes. Otoh lubing the threads invites a repeat loosening. I suppose if Shimano supplies its BB with threadlocker preapplied, thats the direction to take.
--- End quote ---
I use threadlocker as a matter of course on everything except the pedal spindles (which get grease) to reduce the risk of vibration causing things to work loose. It's possible that whoever assembled the bike was so used to using the Shimano UN55 with some pre-applied threadlocker that they omitted to apply some when using a different bottom bracket. There's a description of the different grades at https://www.henkel-adhesives.com/uk/en/insights/all-insights/blog/difference-between-threadlockers.html. Keep away from the red stuff and you'll have no problem with disassembly in the future.
mickeg:
I am not sure which country you are in, if you are in USA, this is what I bought last time I bought some thread locker.
https://www.truevalue.com/6-ml-removable-blue-thread-locker-1
There are lots of brands, I wanted a small bottle that was cheap and that was what I saw at my local hardware store.
Note that it says removable. I think blue is a common color code for removable, it says blue on the package even though the bottle is red.
Shake well before using. The tip on the bottle can clog, I keep a paper clip attached to the bottle with a rubber band to use to unclog the tip.
After several occasions on tours where I met people that lost rack bolts or some other important fastener, I have started to carry a small bottle of that thread locker on tours. But lately most of my tours were on my S&S coupled bike, so I was installing and removing racks at the start and end of the tour. Thus, I primarily carried it to use on my own bike.
I use that thread locker on all rack bolts, bolts on cantilever brake posts, kickstand bolts, recently added shoe cleat bolts to that list too. But use grease on water bottle cage bolts, seatpost bolts, stem bolts and just about everything else. I usually have used grease if it was a steel and aluminum interface, such as an aluminum bottom bracket thread on a steel frame.
Dave W, formerly of SJS had commented that dissimilar metal corrosion can be an issue with Rohloff drain screws, so I started adding thread locker on those. Apparently I use the wrong thread locker as the one I use he says is soluble in oil, but I continue to use that thread locker on my Rohloff drain screws, which i continuously re-use.
Some bike shops do not even have a bottle of thread locker in the shop, they just tighten things way too tight, some mechanics use a lot of grease and some use too little.
Grease is a very viscous fluid, so it can also function to reduce the chance of a loose bolt from vibrating loose. So, even though grease is a lubricant, it can help prevent loss of some fasteners.
Some times specifications say to use grease or threadlocker, but sometimes they say not too. But I think most people working on a bike never bother to read any such specifications.
I am not saying that what I do is best, just describing what I have settled on doing after decades of wrenching on bikes, mostly my own but I also worked in a bike shop before I went to college.
If you have an interest in learning more than you ever wanted to know about greasing threads, an interesting discussion in part of this post on that topic.
https://www.velonews.com/gear/tech-wearables/technical-faq-velon-transponders-greasing-fastener-threads-and-warm-shoes/
PH:
Glad you got a response from Thorn,shame if it was an assembly error, but on one's perfect.
Shimano have pretty much relegated ST to the cheap end, they don't make any higher end ST chainsets so it's unsurprising they downgraded the BB as well, add to that the less common width and there isn't a lot of choice. I've been swapping to high end IRD when mine need replacing, 10 year guarantee and the bearings are replaceable, I'm hoping they'll be the last BB's I need.
Park Tools recommend threadlock, or if you haven't got that, anti-seize paste, or if you haven't got that grease - Mainly they're telling you not put it in dry! it's a bit of an odd one, because they also tell you to tighten to the recommended torque, but that will vary depending on the thread treatment.
I use Copperslip and have never had a BB loosen or seize, but I also remove and re-apply on an annual strip down service. I save threadlock for those things I don't intend to disturb, racks, bottle cages, light fittings. I see no reason not to use threadlock on the BB, just for me it's easier to clean and re-apply Copperslip.
A BB in a EBB shell is better protected than one in a frame, it only has to deal with what's coming at it from the crank side not internally, I once removed one from another touring bike and a cup full of water poured out!
lewis noble:
Well done for getting it sorted, and Robin for giving clear advice.
Threadlock vs copperslick . . . . . I used to put threadlock on pretty well everything, cycling and non-cycling bolts - but found that some became too hard to undo if needed - too many rounded nuts and bolts. Probably I used the wrong grade of threadlock.
So now, on all of my bikes,family bikes etc, I use copper slick / copper grease on pretty well everything, rack, guards, stem bolts etc., and nothing has ever worked loose, I suspect because the threads 'seat' better and over the whole of their surface. No problem in undoing them however wet they get / exposed they are.
Not expensive, small tube in toolbox lasts for ages, usually starts leaking or gets squashed before it runs out. And if repairs need doing on tour, no problem and no need to take the tube, as enough grease stays on the threads for 2 - 3 dismantlings at least.
Lewis
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