These strong fiberglass-reinforced polypropylene adapters are commonly used with our fresh water tanks equipped with 1-1/2″ FPT fittings.
Just screw the 1-1/2″ MPT end into the tank and then attach either a 1-1/4″ or 1-1/2″ diameter (ID) clear vinyl or green corrugated hose. A little silicone caulk on the barbs will act as pipe dope for corrugated hose.
A hose clamp finishes the job.
No thread tape is needed when screwing plastic fittings into polyethylene tanks. Just go one turn past hand tight.
