Rigid and hydraulic drums are 8" diameter. The rear brake arrangement changed in 1938 from a double spring
to a single spring
(at the bottom). The shoes widened in 1938 to 1-5/64". They stayed the same width until after 1962.
The cast drums and pressed steel drums were used different friction material. One was suitable for cast iron, one wasn't.
The lugs on the back of the cast iron drums were for (hammer) loosening the drum if it got stuck on the hub.
The only drums that are cast iron
(aftermarket) now, are the '63-66 with the raised cast-in dust ring. And they changed to wider shoes. "Mercury" Morse told me cast iron drums can warp
just sitting on a shelf. http://www.vintagebrake.com/
There is a difference in the pressed steel rigid ('37-57) and hydraulic ('58-62) pressed steel drums, but I don't know what it is, yet. I'll need to go out and make a side-by-side comparison
. Yea, then they started using cast iron in late '58.
They used 1/8" rivets (and dowel pins) through 1959. They changed the rivet diameter in 1960 to 3/16".
All of the chain-to-transmission alignment was accomplished by recessing material from the drum side of the '58-62 sprocket for drum inset, or reducing the width of the backing plate spacer when they widened the shoes for '63-66.
I measured both the rigid and hydraulic pressed steel drums. They are the same drum. The difference is, you see 5/16" more drum behind the sprocket on the rigid
than on the hydraulic. The reason for that, is because the (hydraulic) sprocket has that recess cut-out
so the hydraulic sprocket (on that same
rigid frame drum stock) will (move to the left) and line up with the transmission.
V-Twin has two different part numbers. One for rigid
and one for hydraulic
. The drums are the same depth and diameter, the faces of the two sprockets are different.
If you tried to use a rigid frame drum and sprocket
on a '58-62 hydraulic Big Twin, the rear sprocket wouldn't line up with the transmission sprocket and vise-versa.
Something's not right with my explanation though, if the '58-62 sprocket recess moves the sprocket to the left, then you'd see more drum on the hydraulic than on the rigid
. There's an answer in there somewhere. If I have to place them side-by-side, I'd have to remove a rear wheel and I've got header-slotting this morning. Gi' day mate