Originally Posted by dr bob

It's not quite that easy. The original fitting depends on a steel ball bearing to mate up with the flared end on the fuel rail, providing the seal. The threads are straight onto the rail, so there's no sealing available from them. Some foilks have put layers of Teflon tape on the rail threads, hoping that would be enough to seal. It might be "good enough" for a quick pressure test on a cold engine, but not suitable for longer-term use, like using it to support FMU tuning under extended load.

I drilled and tapped a stock cap to accept a very common GM fuel pressure test connector, and use a commonly-available gauge on a short hose when I need to check fuel pressure. The GM fitting uses a 1/4-28 straight fitting, and I added an o-ring where the hose now butts up against the cap. Inside the threaded rail cap, a 1/4" brass tubing ferrule sits over the stub of the fitting that passes through the cap, effectively replaceing the ball-bearing duty while allowing fuel to pass through to the gauge port.

Theoretically, since the ball bearing does the original sealing at the flared end of the rail, you could easly drill & tap your existing rail cap, then install it again with the original ball seal after testing is done. Theoretically. For me the cap was cheap and an easy add-on to a parts pick-up from 928 International, plus it gets used on others' cars a lot more often than on mine, so having a dedicated piece in the gauge kit made more sense for me.