Got birds? Click Here!

Belt length is estimated. It's accurate for the stock pulleys in the new transmission (21:42, 15:73), but it's probably off a bit for anything else. I think it's close enough, though... it estimates 387.5mm for 27:42, and the 390mm belt I used with that setup was just a bit loose.

The part numbers shown above for the 42 and 48 tooth secondary pulley are for the aluminum versions. They are also available in plastic, S4176/42t, S4116/48t. There is also a plastic 25t clutch, S4085.

If your Futura has the original 10/50 secondary gearing, you can upgrade to the new 15/73 gearing with part S2999, which includes new pulleys, bearing strips, motor mount, etc. At the time of this writing, it's roughly $220 US.

Here's a partial list of belt part numbers:
S4177 - 340mm (probably useless with anything but the oldest Futura)
S4113 - 375mm (stock belt for the Futura SE with 15/73 gearing)
S4111 - 375mm 8mm (narrow width)
S4112 - 390mm

If what you need is slightly smaller than what's available, the S2930 - belt tensioner might be able to make up the difference. I used it with a 27t clutch and 390mm belt. It appears to have been intended for the old gold-style Futura frameset, but with a bit of Dremel work it fits the new frames reasonably well.

Double-check the part numbers before you commit to them. I think they are all correct, but I make no promises. I believe that all of the clutch part numbers are for the newer 43mm clutch, but I can't be certain.