Author Topic: Damage from square taper Campag cranks on JIS bottom bracket?  (Read 1068 times)

CountrySickness

  • mostly puzzled
Damage from square taper Campag cranks on JIS bottom bracket?
« on: 02 August, 2017, 06:48:45 pm »
I got sick of rubbish Campag square taper bottom brackets and decided to run my lovely Record cranks on a JIS taper (Stronglight) instead (seemed a good idea ayt the time ::-)). It's now getting creaky and I think it's the interface between crank and taper.  After 2000k is it too late to switch back to ISO or will the crank taper be screwed?

Re: Damage from square taper Campag cranks on JIS bottom bracket?
« Reply #1 on: 03 August, 2017, 01:13:13 pm »
Regardless of maker, there's no short answer to this ... and it's endlessly debated.

My experience is, from 35+ years in the trade, is that it entirely depends on what crank you have and why it is creaking.

As a generality - JIS does not normally engage far enough into the square taper, plus you are losing the engagement at the "fat" end of the taper which means that proportionately you are losing a lot of the support for the load that you are putting on the crank. The taper angle of the tapers is the same (2 degrees), so it is mostly just a matter of load bearing area.

The nett effect *can* be a tiny amount of movement - hence the creak - and if that movement persists, you will get distortion of the square taper in the crank that can't be rectified by any economic means (machining out a bigger taper and making an axle to suit is scarcely economic).

If you are really unlucky, you can get a crack propogating from the corner of one of the tapers in the crank allowing more movement. This scraps the crank.

Specifically - Older Campag cranks and the current Pista crank, because they use(d) a proprietary taper and may, if older, also have repeat-fitting wear and tear, can fit further up a JIS taper but then a secondary problem can kick in, which is that the taper may not be long enough and the crank comes up against the end of the taper where is "scallops" into the cylindrical shape of the BB axle. In this case, too, a crack can propogate from the corner of the crank taper as this point is over-loaded by the curvature at the end of the taper. This likewise scraps the crank.

Older and the current Pista Campag cranks need to sit on their own proprietary taper to avoid these sorts of problems.

More current (since the end of C-Record) cranks work best on a standard ISO taper.

If you are lucky, you'll have had just a bit of squirm, light damage and re-mounting on a Campag BB should be fine.
Look carefully for any evidence of cracking or distortion in the corners, especially, of the taper in the crank, though. Distortion (burring) is possible but you might get away with it.

Use a Campag BB. They know what their tolerances are and make accordingly. ISO defines the taper angle and starting / finishing square dims. It does not necessarily define the interpretation of those numbers for every manufacturer ... a tolerance is included but some makers will work to a tighter tolerance as a matter of course - if you get (say) a crank maker applying a "tight" interpretation of the tolerance and a BB maker employing a "loose" interpretation and each is at the limit of the tolerance band, it's not impossible that a problem will occur.

Fit the cranks dry and tighten to the required (40-45 nm) torque on the end-bolts.
If you need to exceed the 40-45 nm to prevent noise, or if you are frequently having to re-tighten the end bolts, or both ... you probably have damage that can't be rectified.


Re: Damage from square taper Campag cranks on JIS bottom bracket?
« Reply #2 on: 03 August, 2017, 03:03:36 pm »
one of the differences that can become problematic is that the shape of the flats is different, which makes the fit in the corners of the square taper vary from one type of BB to another.

Specifically it matters (a lot) if the flats are shaped (NB I will refer to milling here but some spindles are forged to shape) as if they were milled onto a tapered blank or not, and if they are, what shape the taper was exactly.  With most ST spindles, the corner lands are left with a radiused edge to them. If this is the same width up the length of the flats, the blank was tapered before the flats were milled, and if the corner lands vary in width, the taper on the blank didn't match perfectly. If the flats are milled onto a parallel sided spindle, the corner lands are widest at the base of the taper. Each type of spindle will fit quite differently, even if the taper angle and nose dimension are basically similar.

Anyway, with a new crank you can interchange spindles a fair amount. However with a used crank there are two main types of mismatch (apart from the obvious one that the taper is the wrong length inside the crank)

1) that the taper angle is wrong (very many spindles are not really quite two degrees....) and
2) the spindle doesn't fit at the corners, or fits differently to the previous one.

If in doubt about the quality of the fit, clean everything until it is bone dry and spotless, then cover the flats on the spindle (and the female socket in the crank if you want) with marker pen markings, and assemble the joint. You can tighten it to about half the usual torque setting. Then disassemble the joint and examine the marker pen markings. They should be scuffed away all over the flats. If they are not, you have a bad fit and you maybe need to do something different.

If you use a crank that is moving on the taper at all, you will usually develop ridges inside the socket; these ridges may inhibit further tightening of the joint, even with the correct spindle fitted.

So with used cranks it is impossible to be prescriptive about what BB will work with a given crank, because the cranks will almost certainly have been 'sized' by any previous BB spindles that may have been fitted and used.

cheers