Oil seal 55x80x10 (alternate)

This is the original thickness seal for the rear of the crankshaft as well as for the 2WD final drive sidecar output. Which means you can install it with no special tools, since it installs flush in the rear main seal application, and it installs all the way in in the final drive application.

Why did Ural stop using the 10mm thick oil seal in those two places? Well, it was a procurement and supply decision, not an engineering or manufacturing decision. (But engineering had to figure out the installation depth, and manufacturing had to figure out how to accomplish that. In the end everyone was probably a little grumpy about it.)

This is not the OEM seal from Ural and is not made by NAK. It is the same dimensions and it's still FKM/Viton with a double lip. Should work fine and costs less.

Rear main seal for the crankshaft. This presses into the rear main bearing carrier. It's also used in the 2WD final drives on the sidecar output side. 

Seal is made of Viton, aka FKM. Size is 55 x 80 x 10 mm. So 55 mm inside diameter, 80 mm outside, and 10 mm thick.