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From RedsHeaders.

What can you do if you want a rope rear main seal in your 1932-41 flathead V8?
If you are working on a 1932-41 flathead engine that is late enough to have the pot metal labyrinth seal inserts, both top and bottom, it is easy to make the change. You may just change the crankshaft and seal pieces for the later parts.
If your engine is a 221 cubic inch, you will probably want to stay with a 1.998" journal and early full floating rods. The rods for 2.138" journals will not pass through the 3 1/16" cylinder bores.
You can use a 1949-53 crankshaft in the early engines. You would have to turn the main journals down to 2.398" to fit in the mid-1936 through 1938 block. You also have to relieve either the block or the crankshaft at the rear of the front main and both sides of the center main journal. If it is a 4" stroke crankshaft with the rod journals offset ground on 4 1/8" stroke to 1.998" size, you can use 21A and/or 91A rods which will pass through the 3 1/16" cylinder bores.
When mixing crankshafts, rods, and blocks, use the correct bearing set for the housings (block and rods) in the correct undersize for the journals of the crankshaft.
For the 1932 through mid-1936 blocks, you can grind the crankshaft rear labyrinth slinger away to a 2.498" surface. Then install an upper seal retainer for a rope and an aftermarket adaptor type seal retainer in the main cap. That way you can use the stock rope seals.