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I replaced old rusty trailing arms in '79 528i with a clean set from a '78/'79 (?) 633. The 528i non-vented rotor, caliper combination will not bolt up, missing, overlapping by about 1/4 ".

It occurred to me 6 series cars used vented rear rotors with a spacer in the rear caliper. Is that the problem ? I.e. install the spacer in the calipers and order new vented rear rotors ?

Seems like I've heard something about a spacer in the rear hub/bearing assy, not just different rotors/calipers. Check Realoem, it should be there.

Seems like I've heard something about a spacer in the rear hub/bearing assy, not just different rotors/calipers. Check Realoem, it should be there.

I understand that the hub is different in order to position the rotor slightly outboard to accommodate the vented feature. The caliper is also different but only because of the wider spacer between the outboard and inboard sections.
If you buy the vented rotors, you'll also need to break apart the calipers and replace the spacer with a wider piece...or replace with the correct E24 calipers.

There are spacers in 6 series rear calipers to fit the thicker vented rotors.

528i rear calipers are just the two halves, no spacers, that fit the thinner non-vented rotor.

I bet the 633 trailing arms do use a shim or something in the hub to use the vented rotors. An extra part, not different hub or trailing arm.

Therefore, rather than removing rear axle bearings to change a shim, I'm thinking to change the rotors to vented and add the spacer in the calipers.

Great if someone who knows can affirm.