Middleburn Crank not fitting onto Bottom bracket
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2manyoranges
Old School Grand Master
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Tiny tiny differences in tolerance make a big difference re how far a crank goes up the taper. If the Middleburns have been really torqued down in the past then they will ride up the taper more. They are milled from billet and not forged so are quite soft. What’s the chainstay clearance like and is it even both sides? A test is to take the non drive side arm off and put it on the drive side and see if both arms have the same degree of taper. If the one rides up higher on the taper then it has been tightened more and deformed. Not a game ender but good to establish. Indeed easy remedy - longer BB or BB which you can align from side to side - eg Hope. Far better a longer BB than be wary of tightening crank and leaving it too loose, thus wrecking the taper in the crank.
I think you’ve hit the nail on the head here. Measuring from the end of the crank arm to the chainstay there’s a 5mm difference in clearance between drive side and NDS.
Will this degree of variation be an issue? Obviously if I put a longer bb on it the DS will fit appropriately, the NDS will be kicked out a further 5mm. Would this be an issue?
bikeworkshop
Senior Retro Guru
Longer bb axles don't mean both sides are increased in length.
A lot of shimano bb axles the next step up is longer on one side only.
Who knows why?
Certainly looks like this chainset has been overtightened. Let's hope the axle doesn't reach the end of the taper next time it's done up.
I'd suggest using a Torque wrench, (40Nm?) then remove the bolt and check the axle hasn't pulled all the way to the end.
If they have, then they are scrap.
A lot of shimano bb axles the next step up is longer on one side only.
Who knows why?
Certainly looks like this chainset has been overtightened. Let's hope the axle doesn't reach the end of the taper next time it's done up.
I'd suggest using a Torque wrench, (40Nm?) then remove the bolt and check the axle hasn't pulled all the way to the end.
If they have, then they are scrap.
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I’m tempted to see if I can track down another set of cranks tbh. For a bike that’s going to see a lot of use I don’t really want to risk it
If it’s going to see a lot of use. I’d fit a XT. Middleburn albeit good cranks , do seem more likely to fail than Shimano.
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Longer BB will not fix the fact your taper is buggered and have no room to actually 'clamp' it down at the axle end.
You can double check by putting the drive side on the other side and vice versa.
You could look for a longer taper version but really you're just making it worse.
Or be looking on a fitting.
The ring on Shimano BB probably is for the same reason, (I believe, been a long time), they have them on ISIS versions.
It is the chainline setting designed for new cranks and you just torque them down to that spot. Makes it quick and easy and you don't bugger the tapers.
Not so good for old worn buggered tapers though.
You can double check by putting the drive side on the other side and vice versa.
You could look for a longer taper version but really you're just making it worse.
Or be looking on a fitting.
The ring on Shimano BB probably is for the same reason, (I believe, been a long time), they have them on ISIS versions.
It is the chainline setting designed for new cranks and you just torque them down to that spot. Makes it quick and easy and you don't bugger the tapers.
Not so good for old worn buggered tapers though.
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FYI, The RS7s were the first forged middleburns.
I had a set of RS7 like this (also bought 2nd hand) - although the taper looked fine, it was not and once ridden they continually came loose. Sadly, they are now scrap.
bikeworkshop
Senior Retro Guru
Vintage parts like these are taken off a bike back then because they're worn out.
They've sat in a box for 20 years, then get sold on, for the new owner to discover why they came off in the first place
They've sat in a box for 20 years, then get sold on, for the new owner to discover why they came off in the first place
2manyoranges
Old School Grand Master
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They are lovely cranks but not without issues re age related wear.
A ‘sleeper’ quality historical crank which is really light, forged and tough is the Sugino Fuse. Used those on bombproof lightweight builds in the past and available for peanuts.
A ‘sleeper’ quality historical crank which is really light, forged and tough is the Sugino Fuse. Used those on bombproof lightweight builds in the past and available for peanuts.
