Doh moment! Shimano UN54 bottom bracket

[Chuck Glider]

What is the crank bolt thread size in a UN54?

Foolishly, I used the wrong type of crank puller - one with a domed piston. The domed end ruined the thread entrance circumference. I should have used a flat end piston type, not the old tool which is for nutted square taper axles. Doh! Anyway, I was wondering if I could re-tap the first few turns of the axle thread. Not sure it will work, but it's worth a try or else I end up throwing away an otherwise fine bb.

[james01]

Don't know the thread size, but I've done exactly the same. The damage was confined to the entry only, and by filing and general brutality I got the bolt back in. I reasoned than this area is over-engineered and the loss of say 1 mm of thread in a thread of around 15mm was insignificant. I've had no problem since. Incidentally, when removing cranks I now insert a sacrificial undersized bolt into the hole as a load bearing surface for the tool.

[Mick F]

I've used a domed end one in all sorts of BB and never ruined the internal threads. Ever.
I never even thought that I would.

Now I have a Campag Chorus BB, my old tool won't quite reach, so I too use a small bolt. This is only to extend the tool as I was running out of tool thread before the cranks would come off. The tool was always (just) ok with other bolted Campag BBs before.

[rootes]

i pop a m6 cap screw in the end of the axle and let the domed bit push on that. As for damage does depend on the puller you have -

[reohn2]

I'd try the crank bolt in,providing you can get it in square it will retap the thread as it goes,use plenty of oil on the threads turning 1/2 to 1 turn then back off 1/4 turn then oil some more, a bit at a time until it goes easier.

[theking54]

Friend of mine did the same thing,thread size m8 x 1 pitch.

[goatwarden]

I'd try the crank bolt in,providing you can get it in square it will retap the thread as it goes,use plenty of oil on the threads turning 1/2 to 1 turn then back off 1/4 turn then oil some more, a bit at a time until it goes easier.

I'd be inclined to use a tap rather than the bolt; the axle is quite har steel and could easily bruise a male thread. If you do do it with a bolt, chuch it away afterwards and assemble with a new bolt.

[reohn2]

I'd try the crank bolt in,providing you can get it in square it will retap the thread as it goes,use plenty of oil on the threads turning 1/2 to 1 turn then back off 1/4 turn then oil some more, a bit at a time until it goes easier.

I'd be inclined to use a tap rather than the bolt; the axle is quite har steel and could easily bruise a male thread. If you do do it with a bolt, chuch it away afterwards and assemble with a new bolt.

Yeah fair doo's,I was thinking if only the first two or three threads were damaged the bolt would easily clean them up but if the damage is further then yes you're right.

[Chuck Glider]

Thanks all. 8x1.0 mm taps bought off ebay for a few quid. Will have a go at it later. Should I use a taper or a second?

[rootes]

really if you are not cutting a new thread you should use a thread chaser... but second tap will be fine and rubbish quality ones are cheap as chips on fleabay

also you can make a basic limited use chaser from an old bolt, just cut some slots in one say 3 or 4 length ways, trim with a small triangular file

example here for a car use i wrote some time ago:

http://www.theimpclub.co.uk/viewtopic.php?f=5&t=10475

gives you the idea

[Chuck Glider]

All fixed.

Well, it was pretty easy. Taper tap, held the axle in a workmate clamp, tried to keep the tap coaxial. I used really light pressure and didn't cut more than a turn or so of the tap handle. Then used old crank bolts to "smooth" the thread out. Had to work them in an out a few times, but I could tell it was cleaning up the thread, not crossing it! After that, fresh new crank bolts went in without a problem.

Total cost: £7.50 (£4 for the taps, £2.50 for new bolts). But now I've saved the BB and have taps in case I need them in future (but sod's law says I never will!)

Mick F, the domed puller I used was v old and the dome radius was small, like a bullet tip. It was obviously made for the 80s cotterless cranks with crank nuts, not crank bolts. Since then, I bought a flat face puller, and have not had any issues at all.

[mattheus]

Rather pleased to see that someone has had the same problem as me. It might be from 7 years ago, but it's still improved my morale slightly

So anyway ... as someone new to thread tapping/chasing/repairing/bodging ... what do I need for this job? I have a Screwfix local to me, and they sell several tap/die kits (it would be nice to sort this out tomorrow while the weather keeps me off the bikes!). Will one of their kits do the job? Reviews are mixed for this one, but I'm only cleaning up a thread, not cuttting a new one. Or should I go to eBay for the correct dedicated thread chaser?

https://www.screwfix.com/p/carbon-steel ... _container

(Their dearer Hilka brand kits get even worse reviews :-/ )

[Brucey]

the thread is M8x1 on most crank bolts; you won't find that size in a cheap kit.

A cheap (but slightly bodgy) solution is to dress the first (damaged) thread in the spindle using a dremel tool, and then to use an old bolt, well greased, as a chaser. You can lose one thread and it won't make any difference.

Note that the spindle is hard steel, and is difficult to cut even with the correct taps.

cheers

[mattheus]

I found that kit by searching on M8x1.0 !!!

I don't have a Dremel - in fact, assume that I only have a cutlery draw and an adjustable spanner But I was worried about the hardness of the spindle.

I did much less damage to another BB recently (before I twigged the tool problem :facepalm: ), and fixed that by chasing with a spare bolt. The difference there was that I could actually get the thread started. Not possible with this one (so far)

[Brucey]

that particular set does have an M8x1 tap in it (it says) but £25 is an expensive fix for that problem. You could buy another bottom bracket for less.

Maybe you could borrow a dremel tool?

cheers