Sx200 gearing 15 x 47

[Norrie Bodge]

lads...im running Sx200 gearing in my GP
currently on refurbing the engine fitting a Mick Abbey tuned barrel modified for reed valve, using 30mm carb on LTH manifold & JL-4 pipe
& wanna give try with 15 x 47 drive setup
as in this Quote from MBDs site

[quote]
The SX 200 gearbox is a good all round gearbox, but the engine needs some pulling power to pull 4th especially two up!
Changing the crownwheel to a 47 tooth improves the top gear ratio too 4.91:1.
[quote]

thing is which length chain do i use?

cheers

[jason frost]

81 with pull down guide