73mm bottom bracket in a 68mm shell
 

Okay, I have a Giant Rincon AL-framed bike that I want to set up with a Shimano FC-550 crank. The bottom bracket I have on hand to make it work is 73mm shell-width, but the bike has a 68mm shell. It seems I can use the 73mm bottom bracket if I put a spacer on the drive side, and let the adjustable cup (this one's steel) sit outside the shell on the non-drive side after it engages the bottom bracket body. We're only talking 2.5mm difference on each side.

Is there a problem with doing this? I searched the forum, and can't find any reference to this question being asked in the past. I'm tempted to cinch it down and ride on...

You should be fine in using that, but make sure to use a spacer to fill up the gap because exposed threads can allow dirt in there, which will make it hard for future removal.

Some bbs actually come with a spacer so it can be used for both 73 and 68 widths.

Make sure there is not bearing play though. If there is some, than there is no question you need a new bb.

Putting a spacer on the drive side may put your chainline out a little farther than spec. You may want to trial install it first and measure the chainline before torqueing everything down.