A lot depends on what you want to measure. The two fitting in the picture are in and out. The "In" comes from a hose and the out goes straight to the fuel pressure regulator. We use a Bosch adapter and take our system reading at the inlet. Our fitting is a tee that is inserted between the hose and the fitting and the gauge.

The screw that is in the picture allows access to lower chamber pressure. With two gauges calibrated to each other (both used to measure the same pressure to gauge their relative accuracy) the system pressure minus the lower chamber pressure gives differential pressure.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician