Will it work?

[PhilMag]

OK, I've got an 11 year old Trek 4500 which was recently stolen, but recovered but in the 5 days the little scrotes had it they've pretty much knackered the drive train. Having recently bought a new road bike the missus WILL kill me if I buy a new MTB, so repairs are the only option (and yes I've tried the cost comparison excuses, but they just won't wash). So in order to keep costs down I'm going to attempt it myself. The bottom bracket sounds ok, but given the age could do with being replaced. What do I need to be looking for and if I want to fit these what do I need to do. At the moment the cranks are unspecified Shimano.

 

[simon.r]

There are three things that need to be right when fitting a new BB / Cranks:

• BB shell width to suit the frame. Almost certainly 68mm or 73mm (measure the frame before removing the BB).
• Axle type (square taper on the cranks you've linked to).
• Axle length (normally specified by the crank manufacturer, but not obvious on your link).

(Actually 4 - thread type - but the vast majority of modern bikes use the same thread which is pretty much standard).

You probably have a Shimano type suare taper axle BB in the Trek, so you'll need a BB removal tool (and a big spanner!) and a crank puller suitable for that type of BB (different axle types may require different pullers). The removal tool will also work to fit the new BB and you'll most probably need just an allen key to fit the cranks. Coppa-slip on the BB threads will make it much easier to remove next time! (Links are just examples of the type of tools).

I'd be tempted to buy a new BB and cranks as a 'set' to ensure compatibility - you'll then only need to ensure the shell width is right.

 

[PhilMag]

There are three things that need to be right when fitting a new BB / Cranks:

• BB shell width to suit the frame. Almost certainly 68mm or 73mm (measure the frame before removing the BB).
• Axle type (square taper on the cranks you've linked to).
• Axle length (normally specified by the crank manufacturer, but not obvious on your link).

(Actually 4 - thread type - but the vast majority of modern bikes use the same thread which is pretty much standard).


You probably have a Shimano type suare taper axle BB in the Trek, so you'll need a BB removal tool (and a big spanner!) and a crank puller suitable for that type of BB (different axle types may require different pullers). The removal tool will also work to fit the new BB and you'll most probably need just an allen key to fit the cranks. Coppa-slip on the BB threads will make it much easier to remove next time! (Links are just examples of the type of tools).

I'd be tempted to buy a new BB and cranks as a 'set' to ensure compatibility - you'll then only need to ensure the shell width is right.

Yeah, see what you mean about the axle length. when measuring, is it the outer ends that are used?

 

[simon.r]

Yeah, see what you mean about the axle length. when measuring, is it the outer ends that are used?

Yes

 

[PhilMag]

Thank you.

 

[Monsieur Remings]

One other thing if it's square tapered...

Once you've taken the cranks off with, first, the allen key and then the crank puller you will be able to put the teeth of the BB Removal tool onto the BB. Even with a large adjustable wrench, which is what I'd recommend (don't bother with any socket wrench, you won't get enough torque on it) it may not be enough torque without the teeth of the removal tool slipping from the BB - there are only a few mm, if that, of contact between the two and all the torque is sideways!

To remedy this if it's a problem, and it only works if you are going to replace the BB altogether, find a nut and a bolt! The bolt slips through the square hole at the back of the BB removal tool to hold the BB removal tool in place. Take one of the crank bolts to a DIY shop and find a bolt that is the same size.

When you start, make sure the nut is the head side of the bolt, or the nearside nearest to you and that it's is big enough to not go through the square hole of the BB removal tool. Tighten the bolt as far as you can into the actual thread of the BB where the crank bolts would have been. It doesn't matter if the thread is slightly wrong as long as you can screw it in a way which you'll be able to do. Then once it is in as far as it can go, move the nut up the bolt until it is faced up and tight to the removal tool. Your removal tool shouldn't move now...