This article here offers some excellent information to get you started.

Diff article

Then when your all up with H145 this and H165 that...and H190.err well you get the idea.

If that don't help I'm sure we can take it from there.

Hi Guys

I have question and hope some one can help me out, my diff pinion ripped some teeth off today, rather then get a replacement pinion and save myself alot of time and $ im going to buy a new centre!! as far as im aware my diff is origanal just with the rally 4:1 ratio, i dnt need another centre with the same ratio i just need one thats goin to fit into my housing. How can i tell wat my diff is out of and where abouts to i measure from?? ? ? ?

Cheers Logan