Thun Thompson BB
 

Hello .. so i've got this late 70' bike which has a thun thompson BB .. is there a way to convert it into a square taper one ??

Possibly might be able to use a 3-piece adapter such as might be used on a Schwinn Ashtabula BB shell to allow a square-taper BB and crank. Depends on the dimensions/diameter of the shells. I've never done it but somebody in the mechanics forum might know

Like this?

http://en.hollandbikeshop.com/images/RB0150.jpg

Home / Bicycle Parts City Bike / Bottom Bracket / Bottom Bracket Complete / Thun Bottom Bracket Cotterless 127/40mm Thompson Press-Fit

Better yet^ and seems to be made for the job!

the fact is that i have a similar BB shell to this picture , so i don't know if the one JohnDThompson suggested will work (not my BB shell by the way ) .http://bikeforums.net/attachment.php...hmentid=446620

The ID of your shell (not something that looks like it) is what you need to know. AFAIK Thun Thompson shells are 40mm, and Schwinn (probably most other cruisers and BMX shells) are 2.02" so about 51mm. If your shell ID measures something between those (guessing 46mm if it's exactly like the pic shown and the shell wall is about 1mm thick) then you might be out of luck. If it's closer to 51mm you MIGHT get a machinist to turn down the adapter "cups" of a Schwinn-type conversion kit but that's a big "MAYBE" and would cost something additional to the $20 or so those kits usually run.
Good Luck...and try the Mechanics forum (but measure the shell first).