Picked up 2 UN55 bottom brackets for our Ventana tandem. The stoker BB is a 73/113 and the captain BB is a 68/113.

I noticed immediately the loose fit of the left side cup onto the BB shell.

My experience with BB installations has always had the left side cup as a good near zero clearance fit onto the BB shell. As described by others, there is a definite loose clearance.

I agree the design of, or manufacture of the left side cup is not correct. The reason I say this is that the entire inside surface of the left side cup is coated with a teflon like material. The purpose of the coating is to prevent metal to metal contact of the steel BB shell and aluminum cup. Also, the area where this coating is applied, matches the needed area of support for the left end of the BB shell.

The idea of the thin lip of the BB cup, pressing onto the end of the BB shell for full support does not even make sense when you see these parts in hand. I would throwback also that the replies that contend that there is a lack of understanding or an OCD should see the concern first hand. Yes it can be made to work, even if the left side cup is wrong. Seems kind of a hack install when it is obvious the coated inside surface of the cup was meant to handle the loads.

PK