1/2" NPT on the block doesn't always equate to a 1/2" ID hose. Often the NPT size is less than the hose used.
Even so I would guess 1/2" or 5/8" is the correct tube/hose size for the full flow filters. (for sure not 3/8", to much restriction)

Anyone have one to measure?

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-