What is my seal called?

Bad Sport said

Pipe dope, is pretty much the same thing just use that. Don't need a bunch of it just smear a thin layer on the splines.

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Will do. Thanks very much. I didn't know to do that. I am searching everywhere on the net. I can't find the nut size. I am gonna grab a 1 1/4. It seems to measure up to that size.