I have a kc reg dog, does he have to be reg as a stud dog with the kc before  he can sire a litter, or is his kc papers enough to reg the litter when they r born?

Any KC registered dog/bitch (unless registration documents endorsed progeny not eligible for registration) can be used for breeding, though there are some restrictions for bitches (no bitch can be under 1 year old at time of mating, produce more than 6 litters, and be 8 years or over at time of whelping (except in special pre agreed circumstances a bitch over 8 will be allowed to have a registered litter).

In reputable/responsible circles a dog may become a stud dog by a bitch owner recognising his good points and asking to use him, not because they simply want their bitch to reproduce. 

This Will be because he has done well at shows/trials/canine activities and has acceptable results in  canine health screening schemes (Hip scored, eye tested and any other breed specific testing).

Before allowing a dog to be used at stud a stud dog owner would b e wise to be very knowledgeable about their breed and the lines within the breed to know whether their dog will suit any bitches brought to him, or be sufficiently sure that the bitch owner has this far reaching knowledge, as the production of puppies will effect not just the bitch owner, their puppies new owners, but for go or ill the breed itself.

very well put brainless :)