I need to replace my Shimano BB-UN53 as the square tapper has rounded off the square tapered hole of my crank arm.

I have a Sctott YZ2, its brilliant, and I would for the most part just replace like for like. It seems as though after a bit of research I need to replace the BB with a UN55 (the new version which seems to have a metal cup instead of plastic). But looking at the square tapper cranks I am having some trouble.

The details on the BB are: BB-UN53 68 BC1.37 x 24 JAPAN VIA L < MM110 > R

So shale I just keep looking for a crank arm set that fit the sqaure tapper, or are the other better options I can upgrade to (for minimal cost difference).

Thanks in advance

If the square broach in the crank arm is damaged, you need to replace the crank arm, not the bottom bracket.

N.B. this type of damage happens when the crank arm is not securely fastened to the bottom bracket. The bottom bracket spindle is hardened steel, but the crank arm is soft aluminum, so if the arm is loose on the spindle, the hard steel eats away at the soft aluminum. You can prevent this from happening with yoyur new crank arm by making sure it is securely attached to the bottom bracket.

Hi John, thanks you very much for such a quick response. I was thinking that but was worried that it might hav been slightly worn down as well meaning the new crank arm would also end up rounding even if it was secured tightly.

But if you think it will be ok then that is a much easier option for replacement (and a lot cheaper) 👌

It's unlikely that the bottom bracket spindle has suffered any damage; it's far tougher than the crank arm. I'd just look for a replacement arm of the same type as your current crank. You may end up having to buy both arms, but orphan single arms do turn up on e.g. ebay.

More likely you ruined the crank arms,* aluminum, rather than the BB spindle, heat treated steel.

* Typically by riding with the fixing bolts loose..

As fietsbob noted, it's unlikely the hard steel bottom bracket spindle is damaged but is much more likely the soft aluminum crank arm is damaged and probably beyond further use. If you replace the crank arm(s) be sure to torque the fixing bolts properly. The recommended torque is in the range of 350 to 400 inch-pounds and that is FAR tighter than you would think with out a torque wrench to guide you.

On a square taper crank, make sure you check them for tightness a couple of times soon after initial installation. This will make sure you do not end up with the same issue on the the new parts.

Checking torque after 1st ride is quite important.

Good luck.

The crank arms are what's rounded off, correct? BB shouldn't be damaged.

Left, right or both? If the left arm only, that should be a cheap fix. Just buy the left crank arm that matches the right or doesn't match if you want to be Punky Brewster.

Hi everyone thank you very much for such detailed responses!

It's brilliant to find a place to come and ask questions about this kind of stuff, and to have so many people to offer advice.

I have found a nice replacement crank arm (I o it needed the left hand arm) it should arrive in the next few days. I will keep you updated as to how the installation goes.

Oh and I don't have a torque tench so I'll just do it up as tight as I can and then keep checking it as I ride, unless any one has any tricks to figure out how tight it should be?

Many thanks.

DON'T keep tightening the crank as you ride. That'll force the crank further and further up the BB spindle until it splits.
It's possible to do a kinda-sorta torque estimate by standing on a scale, doing some maths for length of wrench and how much your push reduces your weight.

Not enough is very bad. Too much is very bad.

It's easy to get enough torque without a torque wrench. Just use a cheater bar. But since you can't tell the difference between enough and too much, you risk damaging the arm.

Buy or borrow a torque wrench.

The typical torque spec for square taper crank bolts is 30 ft/lbs. That's quite a bit more than I would use if I were left to my own judgement using a 6 inch long Allen wrench. If you don't have access to a torque wrench, imagine a 30 lb. weight hanging off the end of a 12" long wrench.

Hey guys, I thought as a result of your answers that I should go buy a torque wrench, so I am now the proud owner of a new torque wrench and can tighten all my nuts and bolt up to the correct level of torque, thanks very much guys.

Oh and dacab, I did seriously think about the scales idea but realised I don't actually own a set of scales either lol.

An article by Jobst Brandt on installing cranks, https://www.sheldonbrown.com/brandt/...ng-cranks.html He also warns against repeatedly re-tightening the cranks, exactly what dabac said. Also explains why it is usually the left arm that fails.