Do freehubs come in standard sizes, ie if I buy a Shimano MTB freehub? would this fit and be a suitable replacement?
not a cat in hell's chance that a different type of freehub body will fit.....
Yes there are standard sizes but there are
lots of standard sizes....
You might get an idea of what you are looking for here;
http://www.bikeplus.co.uk/c/641/Freehub-BodiesBTW I would (slightly controversially) suggest that the freewheel bearings (eg in screw-on freewheels and bolt-on freehub bodies) are unlike every other bearing on a bike and therefore can be treated differently.
Freewheel bearings only turn when they see
no load and only see load when they are
not turning.
Thus (to my mind) a lack of free play in the bearings is a priority (else the freehub body will tend to move around under load which accelerates wear the bearings and wrecks the seals) even if it is at the expense of exactly how smoothly the freewheel body rotates when freewheeling. IME if a 'bad' freehub body is flushed with oil and spun round a few hundred times even slightly rough bearings will usually start to turn freely enough to allow further use. Provided there isn't too much free play (which can be adjusted using shims) and there are not (say) bits of broken pawl floating around inside, the freehub body is probably still OK to use, provided it is kept well lubricated (with oil or a SFG).
IME if the bearings are rough freewheeling can make a bit of a rumble, but that is usually nothing to worry about; they can go on like that for years.
For example I reshimmed a (very rough) 'Formula' MTB freehub body in about 2001. This was on my most-used MTB. After nearly fifteen years of use (goodness knows how many cassettes and chains) it was still going strong, but still a bit rumbly when freewheeling. If anything it had become a little quieter over time. However I decided to have another go (whilst the hub was being overhauled anyway) and this time I lightly reground the bearing races; the result is quieter freewheeling, but it has made no difference to how well the freewheel engages etc.; it was perfectly reliable either way.
cheers
