I dunno.. but pipes (outer diameter) tend to be based on the olde style Imperial system;
3/16" (e.g. brake pipes) = 4.76mm
1/4" = 6.35mm
5/16" = 7.9mm
I'd hazard a guess that the pipe is 6.35mm.
What are you going to do - use something like "brake pipe" and unions, to make a hard connection, or use a bit of rubber injector hose and Jubilee clips to join the old and new sections?
Whichever route you go make sure you put a flare on the end of the old (steel) pipe, otherwise it's bound to leak.
Ralf S.
Ralf S.