3/8” mechanical pump question

Kendog 170 said

You want one with 3/8" NPT size fittings (not 1/4") or you are restricting down from your 3/8 line . JMO

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Different measurements, to a degree.
A 1/4" NPT Pipe/fitting has about a 5/16" passage; a 3/8" barb fitting, on the other hand, only has about 1/4".
NPT, hose, flares, and lines are all measured differently. Makes piecing things together a convoluted mess sometimes... :)