running 42mm bearings in a 40mm frame-for MUNI

there are lots of threads about shimming up 40mm bearing to fit 42mm frames, but what about the reverse…

will running 42mm bearing in a 40mm frame for MUNI cause undo stress on the bearings? or is it fine?

If your frame has machined bearing holders it wont fit.

I used to do it on an old nimbus with pressed holders and it was fine. With the more modern machined holders it won’t work. Tell us your frame if you’re not sure what type it’s got.

it “fits” with enough room to bolt down the bearing holders but the contact is in essentially four areas between the housing and the bearing instead of being distributed across the bearing

yes the bearings won’t in theory last as long as they might in proper housings, but experience has shown this really isn’t a huge porblem, I muni’d hard enough to break stuff for several years on mine with no problems.

I think it will put too much pressure on certain parts of the bearing and could damage them. Ten your wheel wont spin properly. I doubt it will work personally.

so kington youi are running 42mm in 40mm housings and had no issues?

Your problem is already solved

Kris wrote recently, I think in the new 08 stuff thread, that he is ordering up a batch of 22x40 mm bearings, so he can sell moment hubs to riders with 40 mm frames. I am sure that if you email KH, he would sell you some, if he has them yet.

I found it

Post 38, he doesn’t have them yet, but is getting some I think.

I’m sure it’s something anyone can do. Pick up the phone and tell the bearing Co to whip you up a batch of some odd sized thing.

I am curious what the start up cost on something like that would be. I mean, maybe to get them for 5 $ each, you might have to buy 10, 000 of them, because if you ordered only 1000, they might charge 50 $ each. Someone has to pay for the custom made dies for the bearing shells.

KH is the perfect candidate to perform this public service. I think there is a good chance KH will lose money on the bearings (never sell enough to recoup production start up costs ). But hopefully sell enough extra hubs for it to be profitable. :slight_smile:

i sent an email… and my fingers are crossed… and thye probably would recoup the cost in the hubs and cranks

not any more, but for three years I ran a KH hub with 42mm bearings in a nimbus frame with 40mm pressed bearing housings and had not one spot of trouble with the setup. Others can guess what they like, but I did it and it works, and I’m not the only one. Even if it junks one set of bearings a year that’s hardly an serious outlay.

I’m running 42mm bearings (Nimbus ISIS hub) in an old 40mm frame on my 36er - not a muni, but I do ride mostly cross-country rather than nice smooth roads. Seems to be fine, but I’ve only done a few hundred miles on this setup so far. The pressed bearing holders aren’t really a very good fit even with 40mm bearings anyway, so it’s not really making things much worse - just don’t overtighten them.

For muni, the more extreme the terrain the more weight you tend to put on your feet anyway (so not on the bearings).

If KH gets some 40mm bearings for ISIS hubs though that would be very useful.


Unless it’s a fancy frame like a hunter, or a 36, you could just buy one of the new nimbus isis frames - they’re pretty cheap, and have 42mm machined bearing holders now.


I don’t like the idea. if you’re on a really tight budget you could take the risk but if you’re a heavy or not particularly gentle rider I definitely wouldn’t do it.

chatted with kris, most likely 2009… maybe later for the 22mmID 40mmOD bearings…

I am going to run with them in my 40mm frame. I see the stress going on the bearings and partly the frame… If the bearings go… then a new set is not all that pricey… and if the frame does not hold up a new frame is also not that pricey(nimbus) Ill run it as an experiment…

My MUni has an old Yuni frame with the pressed bearing holders. I noticed with 40mm bearings that the bearing housings only contacted the bearings in two places, never had bearing problems but went through 5 sets of square taper cranks. I replaced my wheel about a year ago to one built up around a NImbus ISIS hub and find that the bottom brackets seem to fit better on the 42mm bearings and the top half still had only two contact points.

Had my wheel off the other day and the bearings are still perfect.

FWIW, I “machined” out the 40mm bearing holders on my nimbus 36 to 42mm. It destroys bearings. I don’t recommend doing it.