i'm putting in my quaife diff. how do i know how many shims to put under it?

 

you only use one thrust shim. But you have to select the correct thickness one to use. This is done checking the preload on the diff, and using thicker or thiner shims to get the preoload where you want it. I suggest you get a helms manual it comes with good instructions to set the preload. It's not all that hard to do, just timing consuming.