Hi Bruce-- I think you may be unsuccessful on this one; you're asking it to wrap more chain than it was designed for. When you say the chainrings are the "usual 42/52", actually you are jumping into the NR era and your Gran Sport derailleur will be unhappy with that.
Try a 47 inner chainring, that should be more in line with what it was made to do. In the old days, about 4-6 teeth difference up front was about all those rear mechs could handle. The subsequent alloy NR derailleur was quite an improvement because it would do better with the 42-52 set-up. Also, the 28t in the rear might be a little large. Try a 26t if possible. Campag stuff was pure racing stuff and racers didn't often need low gears such as a 42-52 and a 13/14-28. In the Gran Sport era, 47-52 or even 49-52 (or 49-54) was the norm and a 13/14-24 or 26 was frequently used in back if a straight block wasn't going to work. When racers commonly went to 42-52 in the late 60s, their steel freewheels (and alloy inner chainring) got smaller and this reduced overall weight, plus the rear gear jumps between gears got closer (often a good thing in racing). And if needed for a truly steep climb, better to have a 42-26 than a 47-26.
Good luck with your project, Bill Bryant Santa Cruz, California
> I was trying unsuccessfully today to get a Campagnolo Gran Sport mech from the
> early 1960s to accommodate the 28 tooth cog on a five-speed freewheel. Is it a
> question of footering about with chain length or am I wasting my time? The
> front chainrings are the usual 42/52.
> Since I've hardly got any time to waste these days, any advise/suggestions
> would be much appreciated.
> Bruce Robbins
> Near the hamlet of Lundie
> in the gloriously green and
> rolling Angus countryside