c4 question
 

how do you tell the difference between a 157 and a 164 tooth bell

On the flywheel just count the teeth, bellhousing? dont know on that one, googling it now.

why would the bell be different? :? :? :?

The bell housing is diiferent between the two, it is hard to visibly see the difference though, the easiest way is to just count the teeth, the diameter is also different but I cant remember the dimesnisons off the top of my head.