I bought a Walmart-like K2 MTB for my 11 year old son and am converting it to a pseudo CX/roadie. It's a heavy steel frame, but it's the right size and I have lots of decent parts laying around to make a fun little bike.

I'm pulling the crank and in the process I want to replace the cheapie BB with a 6500 Ultegra cartridge. The threads appear to be the same but the cups only go on so far - 4 or 5 turns - before they get tight and I'm wondering if it's due to wrong size threading? Or should I just try to crank it down? If it is the wrong size threads, can I get it chased to fit or am I smoking a big one?

55/Rad