If you ever wanted to use it then keeping it plugged and clean is a good idea.

I'd guess it's 3/8" inverted flare. Plugs are available, try local auto parts store and ask for Weatherhead or Eaton part number 131-6

•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-