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Had one in front of me. Pried it off a bit and you can see it butts up against the machined section on the spindle, it's made of steel and too far away from the bearings to be of any use as a seal or dust cover. They're there to stop you installing the crank too far on, you may not need it if you reach installation torque before they contact but if you don't it'll stop them going on too far. Most BBs don't have them and I've always thought they're a bit weird.
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snottyotter
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Always thought it was (combined with those ears that stick out of the inside of chainset) to stop the chain jamming in the gap between BB spindle and inner ring.
You can pull them off if its causing problems, just a press fit.Edit, talking of shimano UN300, had a weird one of the other day, screw threads on the drive side stopped about 5 full threads before the collar with a solid chunk of alloy (full height of the thread inside of the valley height), ended up taking a burr grinder to it. BB axle clamped in vice, then just let it spin like a kind of master bodge lathe setup. BB has plenty of threads on it, and they usually aren't threaded all the way upto the collar. Was a 122/123 x 73mm, never seen that before.
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Hmm ok Yes, its a weird component on a bb
Majority of bbs I have serviced have the plastic ring instead of metal and it travels right to the bearing. The cheaper bbs have them but its not the best seal, you still end up with grit etc.
I suppose the plastic ones are used for a tighter seal to "try" keep the weather out and the metal ones could be used for the crank arm installation.
It wouldn't be ideal for a plastic ring to be pressed against a machined edge with a crank arm tightened on. The ring would fail or your crank arm would get loose.I have not seen one before that stops at the machined section but I know for sure, that little ring does not keep the spindle in place. The bearings contacting the spindle inside the shell keep it in place.
but yes weird haha
BrickMan
gubhi
you dont need them on technically, but they are just dust covers. They dont hold the spindle in place