[bad mechanic]

I purchased a XT HollowtechII crankset used, and it's the first one I've worked with up close so I don't have a good reference. I need to replace a couple parts, and after talking with a couple people I think something weird is going on. I can see the cartridge bearings pressed into the cups, but apparently there's supposed to be another seal on top of them which seats into the cups. However, that seals not there, and I don't see any groove on the inside edge of the cups for it to seat into anyhow. Could someone please explain what's going on here?

 

[sanjuro]

There is suppose to be a seal outside. I don't where it disappeared to, but you probably want to replace the bearings.

I recommend Phil Wood bearings, although finding a shop with the right tools is hard.

 

[DownCycles_Stef]

Sanjuro has it, it's not really a seal so much as a dust cover. The piece your looking for has a plastic cover with the same OD as your bearings, and it is connected to a metal ring that slides over your spindle.

 

[w00dy]

Those bearings have their own seals. Just run it raw.

 

[HAB]

I've never really paid that much attention to mine, but it looks (and by looks I mean in my memory bank somewhere I have some image of the cups) like there's a plastic shim that sits between the spindle and the bearings. I could be completly full of ****, but that's the way I remember things. If there's slop between the spindle and the bearings, I'm right. If it's tight, you're fine without anything. The bearings are sealed well enough.

 

[zebrahum]

I think it's a little top hat looking plastic cover. Like HAB said, if you have any play in the cranks when you install them, you'll need new bearings. If not, run it.

 

[Shimano Support]

I would be concerned about running those without the dust covers in place. Besides the lowered amount of protection from the elements, I think there might be an issue with adjusting the bearing preload. When that left arm is tightened onto the bottom bracket spindle it is designed to press up against that plastic dust cover which in turn presses on the inner portion of the catridge bearing. Without that dust cover in place the arm will not be pressing against the correct part of the bearing. It may even press against the entire bearing causing excess friction. Of course, I've never tried installing cups without dust covers so I don't know exactly where the crank arm will contact the bearing.

The dust covers are not available seperately but the cup units are. If you don't want to buy a whole new bottom bracket (or if the shop doesn't stock them) your local shop can order individual cup units from Shimano or a parts distributor.

 

[loco-gringo]

I would be concerned about running those without the dust covers in place. Besides the lowered amount of protection from the elements, I think there might be an issue with adjusting the bearing preload. When that left arm is tightened onto the bottom bracket spindle it is designed to press up against that plastic dust cover which in turn presses on the inner portion of the catridge bearing. Without that dust cover in place the arm will not be pressing against the correct part of the bearing. It may even press against the entire bearing causing excess friction. Of course, I've never tried installing cups without dust covers so I don't know exactly where the crank arm will contact the bearing.

The dust covers are not available seperately but the cup units are. If you don't want to buy a whole new bottom bracket (or if the shop doesn't stock them) your local shop can order individual cup units from Shimano or a parts distributor.

I really doubt that something that gets the torque band that a BB does would ever use a plastic piece to work as a bearing preload. I'm just guessing though.

 

[Willy Vanilly]

This may be a silly idea but is the dust cap by chance stuck to the backside of the crank arm? Maybe it came off when the crank when the spindle was pulled through?

 

[Shimano Support]

I grabbed an old bottom bracket cup and crank arm when I got to the office today and I did some exploring. The first thing I realized when I got the dust cover off is that you do not have stock bearings. Here is a picture of a stock BB with the seal pried off.



The cartridge bearing seal can also be easily pried off.



What you've got is clearly a different bearing in there, perhaps it is designed to be run with no dust cover. Another thing that became clear is that since the dust cover is a top hat shape and extends inside below the edge of the bearing and reduces it's inside diameter you would have considerable up and down slop if you installed the cranks without the dust cover.

If anyone is curious, my suspicion about the crank arm not pressing on the right part of the stock bearing without the dust cover in place was correct. In fact, without the dust cover in there the arm presses straight up against the BB cup and the cranks won't turn at all.



We get away with using a plastic piece to transfer this preload pressure because the torque value on that left arm cap is very low. 0.7-1.5 Nm Also, although there is lateral load applied to the bearings when you are out of the saddle cranking away it is applied directly to the inside races of the bearings, not the plastic piece.

If you want to read the service instructions they are here:

http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Deore-XT/FrontDriveSystem/SI_6M8FA_En_v1_m56577569830604958.pdf

So in the end it sounds likely that whoever sold you the cranks did not exclude any parts, he just neglected to tell you about the non-stock bearings he installed. Unfortunately, since this is not the stock Shimano set up I can't tell you for sure if there is supposed to be a dust cover there or not.

Hope that helps.