Bicycle Mechanics - Bottom Bracket Question

Is it possible to install a BB spindle that is appropriate for 73mm BB shell in a bike that has a 68mm BB shell ?
What affect will it have on the drive train ?

There may be some type of adapter out there, but what's the point? Just so you can use a BB you have laying around? They ain't all that expensive, so pony up for the right one. Most likely if there was some type of adapter it would cost more than getting the right BB. As far as it affecting the chain line, so long as this theoretical adapter centered the spindle it shouldn't matter. Don't McGuyver it, use the proper part.

I think the spindles are the same; the only difference is the bearing cups are diff sizes. But I may be wrong; I've only worked with English-threaded (68mm) traditional-style bottom brackets.

The threading is identical but if you install the drive side (usually the cartridge) all the way in, the non-drive side cup won't screw all the way in as it will bottom on the cartridge before it seats against the bb shell.

I supose you could add 5 mm of spacers under the nds cup. Some Shimano bb's are made to fit either width so the spacers must be available.

The threading is identical but if you install the drive side (usually the cartridge) all the way in, the non-drive side cup won't screw all the way in as it will bottom on the cartridge before it seats against the bb shell.

And that would screw up the Q-factor, right? if so, that should be reason enough not to do it.

And that would screw up the Q-factor, right? if so, that should be reason enough not to do it.
No, it wouldn't screw up the Q-factor but it would offset the spindle a bit toward the NDS side and maybe bother the symmetry.

To maintain a good chainline, you may want to pull the fixed cup and slip on a 2.5mm spacer, which you might be able to scrounge from a freewheel application. Similarly, one can accommodate an Italian spindle in an ISO/English BB by using a 1mm spacer under the fixed cup's flange.

You can stick a 2mm spacer under each cup and come pretty close. If you have to buy the spacers the right bottom bracket will be easier to find and will probably only cost about $15.00 or $20.00 more. You never have to apologize for doing the job right.