Some pictures would help, not sure what you are calling a "top hat assembly"?

The dished grease caps, are they what hold the axle shafts in the hubs? like this https://lh3.googleusercontent.com/-uiuF7fsgjTY/UG3ktL81kHI/AAAAAAAAARA/czrxlrXFILc/s912/IMG_2104.jpg
If so then a thin paper gasket and very lightly some Permatex #2 is what I use. In a pinch I have also used a little silicone and no gasket, also seems to work.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-