I've got an St170 manifold mated to GSXR TB's for my b/top zetec. I know a lot of people are using pipercross filters but what am I looking for when going to order one? Obviously it's not a standard fitting so if anyone can offer advice that would be brilliant.

Blank backing plate to mount on the carbs, then a choice of filter to suit. Obviously the length and width of the filter will determine which backplate you require, but the only real decided is the depth of the filter in relation to space under bonnet or whether you want it protruding!