Hello knowledgable listmembers

A friend of mine gave me two new rotors he had bought for his 90 vanagon.  I
have an 85 and would like to use them if possible.  I checked the part numbers,
and they are different for the two years.  The only physical difference that I
can see is the 90 rotor is concave between the rotor surface and the threads for
the lugs along the length of the hub.  My 85 is flat in this area.  They use the
same bearings.  Does anyone know if the rotor was changed to provide clearance
for a larger caliper on the 90?  If this is the case I could see how the early
rotor would not fit the later design, but the later rotor may fit the earlier
setup without any problems.  Has anyone out there tried this?

Thanks,
Graham
85 Westy