Hey Guys,
I'm trying to buy a new chain for my 91 907 but I cant find exactly what chain it is that I need. I know its supposed to be a 520 but all the chains I see are 520 x (insert #). Is it 520 x100? 120? Also the site says that the 520 chain is rated for a 250cc engine and that certainly isn't right.

Thanks in advance.

520x108 (chain links)
You can fit a 520 chain also on a 250 or 125cc, but that doesn`t mean that`s the max the chain is rated for.

G.

Probably "G man" mean, that IF you have normal size sprockets, so check them still before you buy new chain.... U can also count amount of links very easily from old chain.... its 520 surely, but U can choose now o-ring, or x-ring chain....

exactly, assuming he`s using original/standard sprockets.
However normally if you use a rear sprocket with 1 or 2 teeth more you still can use the original chain size and don`t need a longer one (yet).

So how many teeth are normally on the rear sprocket? Mine has 37, but I'm sure a 40 would really give it grreat low end power, but I wonder how it would effect cruising at 75 mph?

15/39 standard gearing.
I run 14/40. 4000RPM in 5th/100km/h, 5k/120/5th.
6th still overdrive gear, I like it.
& 108 fits this gearing.

I use 15 / 46 combo, as i dont need top speed so much...happy to acceleration... In my case original chain is too short...

The rule is two links added/removed from the chain for every four teeth up or down on the sprocket.