Hi Brian,
Because the external BB cups can extend beyond the eccentric, they can make the lot too large to insert in the frame if pre-assembled. For this reason I assemble the cups into the eccentric in the frame, making sure to first coat the inside of the frame's steel hanger with grease or anti-seize to prevent future galling against the aluminum eccentric.
In the case of a new external BB and eccentric, to prevent putting unneeded dents in the eccentric I'd first put the eccentric into the greased frame, then hand-tighten the cups, spacers, and sleeve into the eccentric and then slide the crank and spindle in from the right (no left crank arm installation needed at this point),mount the chain and turn the eccentric to set proper chain tension, then set and secure the grub screws so the eccentric is held firmly in place at the point where chain tension is proper.
I'd then slide the right arm and spindle out, properly torque the hand-tightened cups into the pre-secured eccentric insert, and then reassemble the crank and properly torque the left arm in place, keeping in mind the need for the left arm key, bearing preload and proper torque of the bolts.
The advantage of this is you only have one set of initial dents made by the grub screws in your new eccentric. For final cup torque, the eccentric already will be secured properly in the frame so you can secure the cups as intended. The chain can then be installed knowing the tension will be proper. The new eccentric will only have one set of dents in it, leaving the rest pristine for future adjustments.
Hope this helps.
Best,
Dan.
