UKH

Despite all the helpful answers on my recent thread about a new mtb, I still need to keep the old one going for a while…

I’m replacing the bottom bracket on my mtb. It’s a SRAM Powerspline. Do people actually use a torque wrench (don’t have one any more, broke it on a Capri) to get the right torque when putting the new one on, or just use experience? It needs 25 to 30 ft lb.

I surprised myself by getting the old one off without any drama. 

I did use one on a threaded bb that I fitted into a steel frame last year, but it probably wasn't necessary, and if I didn't already own an appropriate torque wrench then I wouldn't have bothered. The torque setting is basically "good an' tight", but not eyes bulging hernia inducing tight. 

Much more important is to make sure all the threads are clean and greased, and that you don't cross thread it when putting it together. And of course remembering one of the threads goes backwards! 

Never used one on a bike. If you've got a bit of mechanical nous I think you'll get it right. Clean the threads with brake cleaner or similar and use copper slip so you can undo it next time.

Thanks both. I’ll use my judgement as suggested. Might calibrate my arm beforehand with an appropriate kettlebell. 

God I feel sorry for your bike, and really hope it’s not carbon…

How narrow minded of you, that side isn't "backwards", it just identifies as left handed.

Clearly what I should have done is to service the bike from one side only, that way both the cups seem to screw in the same way. 

I think you'll be fine for a bb - you'd have to be a bit of a beast or be really trying hard to knacker it. 

If you get a nice new bike in the near future then the investment in a torque wrench might be worth it. I don't use it for everything, but for some things I think it is worth it. For example, the nut to squash the olive into the hydraulic line for the combined brake/ shifters on my road and gravel bikes. It's a pretty lightweight piece but too little torque and it's not sealed, too much... and it's also not sealed, but in a way that costs you 200 notes for a new combined brake and shifter. And then once you have it, you can use it for all sorts of jobs and borderline cases where it's probably fine, but just to be sure. Another tip - my torque wrench was pretty cheap, but now feels like it's top of the range since I picked up 3 Wera hex sockets in the sizes I use most. No more faffing with the soft, always deforming crap that it came with. 

I’m tempted but the bottom bracket needs around 40 Nm whereas I imagine most other things such as the olive compression nut you mentioned are way below that. Not sure if there’s a budget torque wrench which will cover the whole range with any accuracy?

If it’s a metal frame then no, for Carbon Fibre you’d want a torque wrench 

It’s an Aluminium frame. 

Don’t worry about torque wrench for BB then. Just don’t cross thread the threads.  To start, turn by hand opposite way then turn right way to tighten.  Don’t use your BB wrench till necessary.

Oh yeah, bb is one of the things I'd be least concerned about - don't get a torque wrench just for that. I really think cross threading us the biggest danger for threads like that (large diameter, fine pitch). My bike torque wrench is something like 2-24 Nm (from memory) but I doubt I've used it beyond 12 Nm. I used the one for my car wheel nuts for the bb!

This reminds me of the general mirth when news reached the evening league that the local torque wrench fanatic and part time ham fisted bungler had ruined a set of forks and integrated stem by using his torque wrench but neglecting to reset it after some high torque application.

I'm not anti torque wrench (I've got several) but gaining a feeling for what you're doing is so much more valuable.

I've only once used a torque wrench for installing a Campag UltraTorque BB into a carbon frame - and even then, mainly for the joining bolt.

Never bothered using one on any of my other bikes, all of which are either steel frame, alu frame or alu BB shell in a carbon frame (not counting the press fit one).

Out of curiosity I just tried picking up an appropriate sized kettlebell to gauge the correct force I’d need to apply to a ten inch spanner on my bottom bracket. I was unsurprised to find that I’ve probably been something of an over-tightener. I remember decades ago getting through cotter pins by stripping the threads in a Hulk type ignorant rage induced by creaking cranks. 

For the bottom bracket itself it is not so important because  the LH and RH threads mean its self tightening. However I would  always use a torque wrench to fit the cranks to the bottom bracket axle, if they come loose it knackers the interface. 

And if you over tighten square taper cranks it will mess things up. 

On more modern hollowtech 2 cranks it's easy to strip the threads of the left arm pinch bolts.

Really, what most people here prove by their anecdotes is that it's more important to know when and where you need to use a torque wrench than anything else.

My anecdote about bb torque is that you should make sure your torque wrench measures in both directions. I found out that mine doesn't while massively over tightening the bb on my old bike, waiting for the click that never came. Luckily it didn't do any real harm.

That’s a good point. At the moment I only have an Allen key to use for tightening the 8mm crank bolt, which needs 42Nm. My 3/8 socket set doesn’t have the right size hex bit. Hmm. There’s an 8 to 105 Nm Silverline torque wrench with hex bits online for £40 which should cover most of my uses.

Edit: although the Silverline one has a reversible ratchet it’s not clear if it actually clicks in both directions. 

Unlikely it does, left hand clicky torque wrenches are priced £££. The only torque wrenches that do left hand threads in the lower price bracket are beam style ones.

Thanks. I was slowly realising that through much Googling. Best I could find is a Cromwell Tools own brand one, with the push-through drive to reverse it, for £70. Not sure I’d get much accuracy using a beam style one, especially for the relatively low torques needed for bikes. I was though inspired by a YouTube video of someone holding a dumbbell on the end of a ratchet drive to get the right torque…

Just in case anyone else has the same question. In the end I bought a Kennedy (Cromwell’s own brand) 14 to 68 Nm torque wrench. More than I’d wanted to spend at £70 but about half the price of the exact same model branded as Laser / Draper / Britool. Has a test certificate, signed by “Ray”. Seems a really nice well-made tool which works for left and right hand thread. Doesn’t have a ratchet but that doesn’t bother me at all, possibly a reliability advantage.

Having just replaced my bottom bracket I think that without the torque wrench I probably would have over-tightened the bottom bracket and under-tightened the crank bolts. Of course now that I have more of a feel for torque I might not need the wrench.

Thanks all. 

You don't and you will. 

30plus years in engineering and I still use calibrated torque wrenches. 

Generally people get M8 AND M10 about right by feel. Smaller stuff usually overtightened and bigger under tightened. 

As long as you say ‘click’ out loud when it’s tight, it’ll be just right