|Converting 6-hole rear axle to 5-hole: by Mark Brazelton ( firstname.lastname@example.org)
| I redrilled the original 6 lug axles on my 1960 GMC. The later 5-lug drums fit perfectly after the axle was repatterned and the brake drum registration stub on the axle was turned down to match the new drum center hole.
Although the brake shoes themselves changed over the years, GM used the 11x2 shoes from about 1951 up to the mid 70's. The important part is buying drums that are designed to fit the 11x2 shoes, and have the same front to back dimension in the brake drum lining/contact area.
I went to several different part stores with a tape measure, to confirm the depth of the shoe contact area. I was handed a variety of similar looking drums, all specified for the 11x2 shoes, but some were much deeper and would collide with the backing plate, while others would barely reach the back of the shoes. Some were made in Argentina, Paraguay, and a couple of other foreign countries I can't remember, so make sure to measure before taking them home. Just make sure the dimension from the axle mounting flange to the rear of the shoe contact area is the same dimension as your originals.
Although the newer drums look much bigger because of the fins, you can see in pictures below that the rear of the drums where the shoes ride are the same drum dimensions. The fins merely extend around and beyond the backing plate. The fins both cool the brakes and also provide better blockage from crud entering the brake assembly.
If you want to upgrade to larger shoes and brakes, as offered by some of the specialty suppliers, then the backing plates will also have to be changed, as the extra shoe width pushes the backing plates further over the axles to make up the space needed.
Original 6-hole brake drum
Later model (1975) 5-hole brake drum