Does my BB need facing (picture)

scary_jeff

I am replacing a HollowTech II BB on a StumpJumper FSR. When putting the non-drive side part on, I noticed that it does not sit perfectly onto the BB. The picture below is representative of how it looks to the eye. By measuring the pixels I have (I think!) worked out that the BB face is about 0.19 degrees out of true. Does this matter? I can't find any reference to how perfect the face has to be in order to not be a problem.
The Park Tools site says that the face of the BB is used for alignment of the bearing, but I can't really see how this is the case considering how much and how wide a thread there is on the bearing cup - there's no way the cup is going to twist relative to the BB shell... is there?



Hopefully in this image it should be clear that there is a bigger gap at the top than the bottom. In the original image this is 4 pixels wider out of 1260 pixels for the 41.6 mm outer diameter of the BB.
I did a forum search for this but nothing came up.

Cheers.

nicklouse

no idea take the cups out and have a look. but it is possible.

wordnumb

scary_jeff wrote:

By measuring the pixels I have (I think!) worked out that the BB face is about 0.19 degrees out of true.

This isn't an accurate way to measure anything, no lens is perfect and without distortions, measure the real thing.

scary_jeff

Hi wordnumb

Thanks for the reply. I appreciate that this is not the best way to measure the error, but hopefuly the picture at least shows that something is wrong, and gives some idea of how wrong. It was difficult to get a vernier in there, and I don't have a feeler guage.

A colleague at work has pointed out that it could in fact be the thread in the BB shell that isn't true, I can't see what I would be able to do about that.

Even if my angle of error is wrong, the question remains as to whether this kind of small error makes any real world difference.

Cheers

- Jeff

rhialto

I don't know the answer to your question. On the other hand, you have 2 options: get it faced or don't.

If you get it faced, it may correct the apparent issue but it takes time and money, although probably not much.

If you don't, there may be no impact at all but if there is it might result in the premature wearing of your bearings. As these are normally replaced periodically, it may not make any real difference.

I suppose that the mis-alignment of the cups (if it exists) may result in more than worn bearings but I'm not sure: wearing of the spindle, increased resistance to pedaling, cracked bearing shims?

cooldad

How long did the original BB last?

nicklouse

scary_jeff wrote:

A colleague at work has pointed out that it could in fact be the thread in the BB shell that isn't true, I can't see what I would be able to do about that.

and that is why you get the BB faced so that each side is parallel to each other and Perpendicular to the axle axis.

see how it is done

http://www.bikeradar.com/forum/viewtopic.php?t=12654663

omegas

looks like its been cross threaded to me

The Rookie

Not enough error for a cross thread (I thread pitch, about 1mm from memory), my LBS charge £5 for BB facing per side.

Simon

scary_jeff

Hi All.

Thanks for all the advice. The error is certainly much less than one thread width, so hopefully it's not cross threaded. The cup also went in easily by hand, and did not produce swarf (the grey bit in the picture is anti seize compound that has been pushed out of the cup thread).

The thread does not go all the way through the BB shell, so it still seems possible that the two separately tapped threads might be slightly out of line with each other, but there is no obvious way to measure this. The old bearings have lasted about a year, but I don't use the bike all that regularly. It is also possible that the 'end cap' (not sure the right name for it, the one that is tightened only by hand with a little plastic tool) was over tightened at some point, which apparently can cause bearing wear.

I didn't realise that refacing could be so cheap, so it seems sensible to see if one of my LBSs can fit the work in. They would probably know if they can face it to any higher degree of accuracy than mine appears to be out by, I suppose.

Thanks again for all the suggestions.

- Jeff