The taper is not the same. You can pretty easily mess up a crank as the contact area wont be proper - but in some cases it just does not matter.

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What if the Hokey Pokey is what it is all about?

Taper is the same (2 degrees) but where the taper starts and how long differ, so a Campagnolo crank on a Shimano BB will sit further outboard than it should, and a Shimano crank on a Campagnolo BB will sit too far inboard. Since there are a lot of different spindle lengths, you might find a combination that gives the correct chain line, but I think there is also the possibility to bottom a Shimano crank on a Campagnolo BB since the Campagnolo is effectively a little skinnier.

Edit: I possibly have the above reversed, perhaps it's the Shimano that is a little skinnier in which case the chainline comments are also reversed. I no longer have any Shimano around to double check

Second edit: Really old Campagnolo (pre-Ergopower?) is a different taper, but they switched to 2 degrees somewhere in the early 90's

Brian

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Brian

Gonna buy a fast car, put on my lead boots, take a long, long drive
I may end up spending all my money, but I'll still be alive

Sheldon Brown did the best explanation of this I've seen: edit: Campy is ISO, Shimano is JIS

ISO/J.I.S. Interchangeability

If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length.
Confersely, if you install a J.I.S. crank on an ISO spindle, it will wind up about 4.5 mm farther in than it would on a J.I.S spindle of the same length.
Theoretically, ISO cranks should only be used on ISO spindles, and J.I.S. cranks only on J.I.S. spindles.
In practice, you can very often get away with mixing these sizes, as long as you select a spindle length that gives the desired chainline.
Taper matching was fairly important back in the day of loose-ball cup-and-cone bottom brackets, because these required regular maintenance/overhauls, and this required removal of the cranks. Every time you remove and re-install a square taper crank, the hole in the crank is liable to get very slightly larger.
This was particularly an issue when using J.I.S. cranks on ISO spindles, because over time, as the crank went on farther and farther, you could run out of taper, and the square end of the spindle would become flush with the surface the crank fixing bolt/washer pushed against. At that point, further tightening of the bolt won't make the crank any tighter, since the bolt is bumping onto the end of the spindle.
These days, however, most folks are using sealed cartridge bearing bottom brackets. With these, there is no routine maintenance required, so typically the crank will be installed once, and will stay in place unless/until the bottom bracket needs to be replaced. This greatly reduces problems of wear to the interface.
I generally avoid mixing sizes on customers' bikes, but I have a lot of experience mixing ISO/J.I.S. in both directions on my own personal bikes, and it as never given me a lick of trouble.

http://www.sheldonbrown.com/bbtaper.html

   Just to confuse matters a bit more - Sugino 75 track cranks use the Campy taper/BB and the road cranks use JIS so check which one is needed for the crank model you are using.

What model of crankset and BB are you specifically trying (not) to destroy?

-SD

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