my 1981 xs400 special was 54 same with my buddies 79, They should be 54 but I would measure to make sure. My book said 48 and I know others' did too and they bought before measuring and they ended up needing 54mm instead of 48. So it's most likely 54 but measure to make sure. Also I hooked my pods directly to the carbs and haven't had any problems, been running like that for a couple months now. I obviously had to rejet, but that's easy. I was going to hook the pods up to the stock tubes but found it was nearly impossible to get a air tight fit because if you tighten the clamps too tight they just crush the tubes since they are such a thin plastic, not sure if anyone else has had this issue.