Bicycle Mechanics - Help with Guerciotti bottom bracket

Hi all. I recently bought a new Guerciotti frameset (cast lugged etc.) with an Italian threaded bottom bracket (http://www.planet-x-bikes.co.uk/i/q/FRGURE/guerciotti-record-frameset) and Italian-threaded Shimano Ultegra bottom bracket cups to go with it.

Annoyingly, the Shimano bb cups don’t seem to fit – they’re very tight and won’t screw in past half a turn. I’m guessing that the thread on the bottom bracket shell on the frame needs to be re-done. But it also crossed my mind that maybe the Shimano bb cups and Guerciotti frame aren’t compatible, despite the fact they are both Italian threaded???

Any ideas? Thanks!

Have the threads in the bottom bracket shell "chased" with the proper tap. I expect the threads were slightly warped when the frame was brazed and the maker didn't clean them up afterward or there is some overspray from the paint that wasn't removed.

The probability that the Shimano bb threads are defective is remote and Shimano italian threads will be compatible with any properly threaded Italian bb shell.

Thanks Hillrider - very helpful! You are quite right - there is some paint overspray in the bottom bracket shell - I'll take it to my LBS for a clean-out.

Am I right in thinking that all Italian threads are the same? Is it 24 threads per inch?

Thanks again!

Stuart.

Thanks Hillrider - very helpful! You are quite right - there is some paint overspray in the bottom bracket shell - I'll take it to my LBS for a clean-out.

Am I right in thinking that all Italian threads are the same? Is it 24 threads per inch?

Thanks again!

Stuart.
Yes , all Italian threads are the same at 36 mm x 24tpi, an interesting mix of metric and SAE dimensions. BTW, both cups are right-hand threaded so be sure the fixed (drive side) cup is installed to the proper torque (i.e. VERY tight) so it doesn't loosen in use.