I bought a new caliper's, brake hose's & brake pads for my W-250 1st Gen w/Dana 60 and the new pads won't go on the rotor because they seem too thick. I looked on line and seems to only be one part number (149) for 81-93, I know the early rotors were thinner with the older Duel-Matic style lock outs which makes me think there should be two different pad set's. And "YES" the piston is all the way compressed but maybe there were two different calipers?

i ran into that situation several years ago, although on a different application, but i forget exactly what i did to rectify it.
i'm sure there has been other instances of the same thing out there.
it seems quality is just not there these days, and companies really don't seem to care, so they just list one part number that is in their "fitzall" catagory...........

I agree with that

The calipers changed to match the rotor thickness.

I'm not positive if the rotor change was model year or DRW related.

Be careful about putting wide calipers on thin rotors, the piston can overextend.

Check and see what your truck's GVWR is. The second gens had the 7500 and the 8800, Different rotors and pads. I imagine they did the same thing in the first gens.

Yes, you guys are right and looks like it happened between 89 to 90 which was when the lockout style changed too. If you have the Dual-Matic style lockout they have the thinner rotor and thicker went newer style lockout. Thanx