Originally Posted by
SoSmellyAir
Cannondale supposedly made the switch from a 1" to a 1-1/8" head tube and fork steerer in 2001. Mine must have been an earlier example because I got the former, which requires a shim to use a modern stem, so I agree with
cyclezen that it would be preferable to get a 1-1/8" one, but many sellers do not have that information.
For what it's worth, I've had a 1999 R4000 and a 2000 R300 and both were 1 inch threaded.
Additionally, don't be afraid of 1 inch steerers. 1" threadless forks can still be readily found online if you want to convert from threaded and same with stems and headsets. FSA still produces a brand new 1" threadless headset for about 40 bucks. As for stems, I haven't exactly looked for brand new so I don't know if new is still an option but used certainly is, primarily on ebay.