Tightening up the crankset onto square taper BB.

[Tom B]

Hi again

I have replaced my BB and have reinstalled the Crankset, but I am not too sure how tight I should tighten the crank bolts. I am using a Tee shaped allen key and have so far gone as tight as I can using my wrist action/arm action. When I rode the bike noticed the sprokets didn't run true which caused the chain to run on the front mech.

When I got home took of the crankset off and reseated it using a smear of copper grease on the square taper and I made sure both the crank and the BB was clean and free of burrs. I again tightened it up using the same tee key. When I took the bolt out, it was looser than I expected. It is now much better, but still there is a slight out of trueness. The crankarms are not loose.

My Tee keys only have Hex's on the long bottom bit, not on the top. Should I be using something with more leverage?

 

[Drago]

What NM figure does the crank manufacturer recommend?

 

[S-Express]

T-bar is very unlikely to be able to provide anything like the minimum torque required, IMO.

 

[Yellow Saddle]

Cranks should be torqued to 40 NM. It is impossible to describe how much that is but unless you are Tarzan, you can't do 40NM with your wrist on a T-wrench. You will need something longer.

You don't make it clear whether the sprockets rubbed because the crank was loose. If the crank came loose and you rode it like that it will be ruined, but it seems you say they were still tight after removing the crank bolt. That means you had to use a crank extractor to get it off, yes?

Crank bolts appear to become looser with use. There's good reason for this. It is not that they unscrew, but rather that the crank squirms further up the taper with use, hence the use of "dust caps" on cranks, to prevent bolts from becoming lost.

40NM is pretty tight. If the bolt is greased, like it should, then it is an uuuuuurgh tightness with a 200mm long allen key. More uuuuuurghs required as the key you use is shorter.

 

[Yellow Saddle]

The good news is that you cannot ruin a crank by tightening it too much. The bolt will break off before the crank cracks. However, you can crack the crank by tightening the bolt at intervals between riding. As I said, the crank squirms upwards so if you follow up by tightening the bolt, eventually the crank's taper head cracks.

So, make it very tight and don't fiddle again unless you then remove it and use a torque wrench when it is convenient, to get it perfect.

 

[raleighnut]

If the crank runs 'out of true' try rotating it one flat (1/4 turn)on the shaft (obviously you need to do both sides)

 

[Tim Hall]

Hi again

I have replaced my BB and have reinstalled the Crankset, but I am not too sure how tight I should tighten the crank bolts. I am using a Tee shaped allen key and have so far gone as tight as I can using my wrist action/arm action. When I rode the bike noticed the sprokets didn't run true which caused the chain to run on the front mech.

A likely cause of the chain rubbing on the front mech after reinstalling the crank set is not enough torque, as you're already suspecting, on the fastening bolt. This causes the chain rings to sit further outboard (or to the right as you sit on the bike), and therefore further away from the front mech.

 

[Yellow Saddle]

[QUOTE 4748058, member: 9609"]providing you remove the crank before tightening it on to 40nm, is it a job you could do indefinitely ? I am always a bit wary of this type of fitting that eventually it will fail, I therefore only takes the cranks off if absolutely necessary.[/QUOTE]

No, they can handle a lot of fitting and refitting. I had a test frame with crank in my mechanic training school which each second week got taken off and refitted to torque perhaps 12 times. That crank remained in service for about seven years. I also used to break off a bolt in the class to show that you cannot break the crank. It was an old Shimano 600 crank. However, just cycle 10 kms with one that's loose and it is ruined.

 

[Tom B]

The good news is that you cannot ruin a crank by tightening it too much. The bolt will break off before the crank cracks. However, you can crack the crank by tightening the bolt at intervals between riding. As I said, the crank squirms upwards so if you follow up by tightening the bolt, eventually the crank's taper head cracks.

So, make it very tight and don't fiddle again unless you then remove it and use a torque wrench when it is convenient, to get it perfect.

Thanyou everyone who has replied. When I noticed the chain rubbing on the front mech I took off the crankset, with the extractor tool and reinstalled it. I was concerned there was dirt between the BB and the crankset. I had to use the tool, but no where near as much uuummph as I did the first time around. The crank was not sloppy or moveable by fingers on the square tapered boss of the bb.

A torque wrench is on my wants list, just finding one that does everything, or as near to everything at a sensible price is confusing. In the mean time I am going to get out the 10" or so ratchet handle and buy a suitable hex key socket tomorrow.

 

[raleighnut]

Thanyou everyone who has replied. When I noticed the chain rubbing on the front mech I took off the crankset, with the extractor tool and reinstalled it. I was concerned there was dirt between the BB and the crankset. I had to use the tool, but no where near as much uuummph as I did the first time around. The crank was not sloppy or moveable by fingers on the square tapered boss of the bb.

A torque wrench is on my wants list, just finding one that does everything, or as near to everything at a sensible price is confusing. In the mean time I am going to get out the 10" or so ratchet handle and buy a suitable hex key socket tomorrow.

Take your time with a big ratchet, 40Nm isn't hanging off one of those.

 

[Twilkes]

If the crank runs 'out of true' try rotating it one flat (1/4 turn)on the shaft (obviously you need to do both sides)

An oldie, but what is the thinking behind this 1/4 repositioning? Is something worn on the crank/spindle?

 

[raleighnut]

An oldie, but what is the thinking behind this 1/4 repositioning? Is something worn on the crank/spindle?

It doesn't take much at the crank-spindle interface to throw alignment out by the time it has got to where the rings are, not helped by the fact that square taper BB's nowadays are seen as 'el cheapo' kit and are manufactured accordingly using cheap metal, dodgy case hardening and poor/inaccurate machining. As for these 'sealed unit' cartidge jobbies that only last a year or two words fail me, I have 3 bikes running square taper cranks and the spindles are 25+ years old and still perfect but then I do strip and service them every couple of years and replace the caged bearings.

 

[Twilkes]

It doesn't take much at the crank-spindle interface to throw alignment out by the time it has got to where the rings are, not helped by the fact that square taper BB's nowadays are seen as 'el cheapo' kit and are manufactured accordingly using cheap metal, dodgy case hardening and poor/inaccurate machining. As for these 'sealed unit' cartidge jobbies that only last a year or two words fail me, I have 3 bikes running square taper cranks and the spindles are 25+ years old and still perfect but then I do strip and service them every couple of years and replace the caged bearings.

So would removing and replacing the crank in the same position potentially fix misaligned cranks/chainrings? Or is that position knackered and it has to be rotated 90° to get a good alignment on the spindle again?

 

[raleighnut]

So would removing and replacing the crank in the same position potentially fix misaligned cranks/chainrings? Or is that position knackered and it has to be rotated 90° to get a good alignment on the spindle again?

That depends on whether the cranks are wobbly or not, probably why I fitted HT11 or Ultratorque cranksets to the bikes I've built up recently well apart from the Raleigh 'City' 3 speed, that got a Stronglight Square Taper which was an upgrade from the Cottered original (Cottered crank on a mid 80's bike )

 

[Twilkes]

I asked on bicycles.stackexchange, got a great answer about manufacturing tolerances: https://bicycles.stackexchange.com/a/82894/46010