Fuel Pressure Tester Help - Schrader Valve Size?

[Erntster]

2000 Mercruiser 350 Mag MPI Horizon, MEFI 3 - Serial #OL394802
I’m trying to connect a fuel pressure tester to the schrader valve on the front/end of the fuel rail. This seemed simple enough...
I rented a tester from O’Reillys, with many sizes of adapters, clamps, hoses, etc. Nothing in the “kit” fit the threads on this schrader valve. Most were too big. The closest was a “Ford Schrader Valve Adapter.” It appears to be somewhere between 1/4” and 5/16”...or maybe an M8?
Are these an odd “marine” specialty size?

[Viper]

Merc's kit has the right size but so does one of the kits Snap-On sells. Don't recall the size though.

[tomschauer]

Its some crazy odd size, and not a regular flare type schrader. I removed that 3/8" mip fitting and installed a standard 3/8" mip to 1/4' Schrader fitting on my mercs. I did the same on several fuel injected outboards in the past so I could use my pressure gauge.

[edw]

If it is a normal Shrader valve, this is what you need: https://www.aircraftspruce.com/catalog/ ... seneck.php

Ed