Spacer for 68mm Shimano BB to fit a 73mm crank set

I have a Shimano 68mm BB and replacing the crank set with one that fits a 73mm BB.

Can I use 2x2.5mm spacers with the current BB to acomodate the crank set?

Just thinkng about it now I realise it might be a problem as the plastic tube wont reach the cups.

In this case anyone in Bristol would have 2 spacers and a 73mm plastic tube available for collection please?

If it’s an MTB crankset that you’re trying to fit the MTB cups are 1mm each narrower so you only need 3mm (IIRC) of spacers.

The tube in the middle still won’t reach

The crank set I think I MTB. Is a chineseone as it was the only one fulfilling my requirements(40/26T). Shifting looks good and, as much as I measured, I need a 3mm spacer on non drive side and 2mm on the other.

If of any help the BB is RS500.

Just buy the correct bottom bracket

BB RS500 is a road bottom bracket. It won’t work with mtb cranks regardless of the shell width.

With Shimano mtb ht2 bottom brackets you run 3 spacers for a 68mm shell, 1 for a 73mm shell so the plastic spacer always matches up.

@M_V Thanks. Opted for a XT BB. However I have only installed 1 spacer on on each side. Took it for a test and shifting is ok. Noticed that the non drive side pedal seems a bit closer to the chainstay but the spindle(non brand) is covered fully. I consider installying a 2nd 2.5mm spacer on that side.

Regarding the BBRS500, the incomaptibility is just because there is not enough thread on it to accomodate the spacers?

In another note I have used the Tiagra 4700 front derailleur and STI shifters with 40/24 crank. Shifting is more than reasonable.

Should be 2 on the driveside, one on the nds for 68mm shell, one on the driveside for 73mm shell and subtract 1 driveside spacer if you are using a bb mounted front mech or chain guide.

Or at least that used to be how it was.

Think the road bbs are narrower cups to allow for smaller q factor. There’s no need to build in the 73mm option on them as that’s a mtb thing as are bb mounted chain guides and mechs so you could be right that the threading on the cups will be shallower.

I guess you could look at it like the road bbs are 68mm wide, the mtb ones are 75.5mm that you space down.

Should be 2 on the driveside, one on the nds for 68mm shell

If I do it this way the chain line won't allow me to use the Tiagra derailleur, in particular won't manage to shift on the larger chainring.

I guess I would keep as it is, as long everything works.

You are right about the spacers use, as it is according to the manual. At a quick look the MTB cups are narrower than the road ones, or at leas the XT compared to RS500.

By the way did anyone tried to use an MTB crank(40/24) with road derailleur?

If I do it this way the chain line won't allow me to use the Tiagra derailleur, in particular won't manage to shift on the larger chainring.

Yeah, I’ve run ht2 bottom brackets with the spacers in the wrong places but I’d just make sure you have the right number in there though.