lunarmike

I have recently purchased two Shimano UN55 bottom brackets for our new tandem frame. On close inspection I have noticed that the UN55 adapter cups which slide onto the bb main body are a very sloppy fit. Space measured as 0.8mm! They certainly do not engage with grey mastic band inside the adapter.

I have an old, unused UN72 BB and the adapter cup is quite a tight fit on the body.

Both the UN55’s and the older UN72 adapter cups have a grey mastic band inside the loose adapter cup to grab the bottom bracket body when fully tightened. My old, unused UN72 is a tight fit in its’ adapter. The new UN55's are very loose in the adapter cups. The 0.8mm space on the UN55’s do not allow the adapter mastic to engage with the body at all.

Surely two adapters can’t be wrong. Am I missing something?

Comments on this problem would be appreciated.

rogerstg

Maybe you're trying to use a 68mm bottom bracket in a 73mm shell???

onespeedbiker

Shimano has gone through several designs for their left side "cups". Some appear to be a close fit uninstalled while some are loose. The only important thing is they are tight after being installed. Your UN55 is loose fitting until installed so torque it on and try not to worry. If it's the one I'm thinking of, there is usually some gray material at the bottom of the cup, but the cartridge does not appear to install that far down. When you install the cup however, the torque will press the end of the cartridge (like a pressed in bearing) all the way in.

gyozadude

Are you specifying radial slop? Or axial slop? Don't worry about the latter. As onespeedbiker notes, proper torquing, provided you have the right length BB shell (hasn't been machined too narrow) will put proper axial pressure on the bearing races and hold them in place. If you have 0.8mm radial slop, that's sounds like a LOT of slop and it shouldn't be that loose even with plastic cups. Check to see if the radial slop disappears because the plastic is slightly conical and will self-center when compressed.

lunarmike

Thanks for your thoughts. Yes, I have observed the grey matter on the inside of the aluminum adapter. On my old UN72 BB that's where it grips the barrel. The UN55 BB body pushes right into that point without contacting the grey matter. That is where the excessive clearance is. All it will contact on tightening is the stepped circular rim in the adapter at that point. I wonder if the bevelled end on the BB body pushes into that stepped rim and deforms the aluminum stepped rim to hold the body end in place?

FBinNY 

I suspect that Shimano redesigned the system to get around the problems that the old tight fitting sleeve had with corrosion. If water got into the BB the tight fitting overlap area would corrode, and the BB would be nearly impossible to remove. Reducing the contact area reduces the chances of the BB freezing together, but as noted by others, once it's in all the way, the ring makes a tight proper fitting contact with the BB unit.