Thanks Brucey
Re point a. I'll insert the lh cup first to see if it fits before backing it out and inserting the BB.
the fit problem is worst between the cup and the cartridge unit. Often there is a 0.5mm clearance between the parts before they are fitted. The LH cup usually 'crushes down' on the cartridge unit but not always that much.
I'm a bit lost on point b. Re how I can damage the bearings by overtightening the BB. The shell is self contained . I'll have a think and see if I can understand.
BB threads are usually not perfect and not perfectly aligned either (check the cartridge for concentricity before the LH cup is installed). The LH cup bears against the cartridge unit with a mighty force; (with low friction in a 1mm pitch thread (eg an M6), a modest torque will easily generate an axial load of about one tonne; the installation loads on a BB unit are usually much more than this).
The result of all this is often (usually) that there are considerable (installation generated) loads on the bearings. Often you can feel that the spindle is mysteriously slightly bindy when the cartridge is installed, in contrast to how it felt beforehand. It isn't unusual for a unit that has noticeable free play to manifest none once it is installed.
The shimano design is considerably better than many other cartridge units in terms of isolating the bearings from installation-related loads but it is not immune by any means.
Point c . I'm not skilled enough to try this.!
But great info as always, thanks.
I tried first on a well-used unit. It was nowhere near as difficult as I had first imagined. If you don't fancy this route you can pick the LH seal out of the end of the unit instead, but this risks seal damage.
Having sleept on this problem.. I now wonder if the lack of a front mudguard is causing a fine mist around the BB and early failure.
(Mudguard no longer has clearance since I switched from 13mm to 17mm rims with 25c tyres)
I have had some rainy Audax this season.
no contact seal can be expected to work without a film of lubricant between the seal lip and the shaft. If you look inside a new unit it is pretty obvious that they have been so parsimonious with the grease that it won't take much road spray to overwhelm the grease. What is less obvious is that the grease they use is 'not at all resistant' to salt water. This means that winter use without mudguards will indeed be likely to cause premature failure due to water ingress.
So there are three main reasons why a unit might start making noises early on;
1) that the parts never fitted properly and the cartridge is moving about (which usually risks that the BB threads will be damaged)
2) that the (installation generated) preload on the bearings is excessive and the bearings are starting to fail
3) that the seals have failed and water has entered the unit.
Looking at the SKF unit, this is manufactured with seals that are not
that much better than the shimano unit, and the thing is filled brim full with lubricant (a gear oil I think) so that it can have a long service life. I don't know whether the design confers an immunity from installation-generated bearing preload, but I doubt it.
The seals don't have to be that great to retain a goodly fill of SFG, (although there may be some weeping in service) and SFG is mobile enough to re-wet the seals etc, so that is what I suggest you use inside units of this type. The SFG that is used in land-rover front swivels has loads of EP additives, anti-corrosion additives etc so that is what I suggest you use.
It may seem like a trivial point, but if you fill a BB unit with lubricant, you will have x10 or x20 the amount of lubricant present. If the lubricant is also mobile (oil or SFG) this means that any wear debris (or contaminant) is diluted by x10 or x20 as much. By contrast with a small amount only of grease present, and/or sticky (non-mobile) grease, the lubricant in the bearings is soon loaded up with wear debris and contaminant, so that wear rates are much higher than you might expect; once wear starts it is like an avalanche towards failure; debris only ever begats more debris, and you are soon on a slippery slope.
Occasionally UN series BB units seem to lead a charmed life; it is my view that this occurs when the unit has somehow managed to avoid large installation-related loads, and something (cleaning regime or use of mudguards, say) has managed to keep the seal lips wetted, and has deterred the ingress of crud past the seals.
cheers