I'm fairly certain that the OD of 450/500T is 1-3/4"........not sure what the ID is, never checked it).1 3/8" is available. I went 1 1/2" electrical conduit bends once and it they were too big for a street 450. It was a noticeable low and mid range loss in power. Some research will confirm it. Cb500? I'd guess would be the same. You'll see 1 1/2" common on race bikes and sometimes bigger on full bore engines, but that's high rpm. If it were me for a stocker, I'd go 1 3/8". I think that's actually a little bigger than stock, but 1 1/4" is a little too small.