Is ISIS compatible with BSA? I want to get a Cyclone motor to install into my bike. My bike has a 120mm BSA bottom bracket, and the motor is available in Square Tape and ISIS varieties. Are these compatible?
BSA refers to the diameter and thread pitch of the threaded bottom bracket shell in the frame. Square Taper and ISIS refer to the crank interfaces on the ends of the axle of cartridge bottom brackets (that fit in BSA threader shells).

See this Sheldon Brown page for info on cartridge bottom brackets (pictures near the bottom). 

A quick look at the Cyclone page shows they make kits compatible with 120mm MTB BSA BB shells. If you are using your existing cranks you'll need the kit with the axle that matches your cranks.

Note there are 5 crank-axle interface standards: JIS an ISO square taper, 2 versions of Octalink and ISIS, so make sure you know what your cranks are if you plan to re-use them.