Fuel Rail Adapter – Making an Adapter for Non-Metric Thread Fuel Pressure Gauges

Overview

This guide shows you how to make a custom adapter to connect a non-metric (SAE) fuel pressure gauge to the fuel rail of a Porsche 944 S / S2 with a 16v engine. It’s a budget-friendly solution if you already own a standard 0-100 psi gauge for US-made cars, which typically has a 1/4″ flare fitting. This adapter bridges the metric threads on the S / S2’s fuel rail to the SAE threads on your gauge, letting you check fuel pressure without buying an expensive metric-specific tool. Two DIY options are provided—using a spare fuel rail cap nut or crafting an adapter from round stock.

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Tools Required

• Drill with bits: 10mm (13/32 inch), 11/32 inch (Option 2)
• 1/8″ NPT tap (Options 1 and 2)
• 12mm x 1.5 tap (Option 2)
• Center punch (Option 2)
• 17mm wrench (Optional for Option 2, if flats are cut)
• File or mill (Optional for cutting flats in Option 2)

Parts Required

• Spare fuel rail cap nut, 928 110 475 01 (Option 1)
• Round stock, aluminum or mild steel, 7/8″ to 1″ diameter, about 1″ long (Option 2)
• Brass male-to-male fitting (1/8″ NPT to 1/4″ flare) (Options 1 and 2)
• Teflon thread tape (Options 1 and 2)

Step-by-Step Procedure

This procedure offers two options for making the adapter. Choose Option 1 for a simpler approach using a spare part, or Option 2 for a fully custom piece. Both are tailored to the Porsche 944 S / S2’s 16v engine fuel rail.

1. Option 1: Using a Spare Fuel Rail Cap Nut

1. Get a spare fuel rail cap nut (928 110 475 01) from an aftermarket vendor. This is the same cap used on the S / S2’s 16v engine fuel rail.
2. Drill a 1/8″ NPT-sized hole in the center of the cap nut. Start with a small pilot hole, then use the correct bit size for a 1/8″ NPT tap (typically 11/32 inch or slightly smaller, depending on your tap).
3. Tap the hole with a 1/8″ NPT tap to create threads for the brass fitting.
4. Wrap Teflon thread tape around the 1/8″ NPT end of the brass male-to-male fitting (1/8″ NPT to 1/4″ flare).
5. Thread the wrapped fitting into the tapped cap nut by hand, then tighten it snugly. This end will connect to your gauge’s 1/4″ flare fitting.
6. Screw the cap nut onto the fuel rail test port on your S / S2 to use your SAE gauge.

2. Option 2: Crafting a Custom Adapter from Round Stock

7. Take a piece of round stock (aluminum or mild steel), at least 7/8″ in diameter and about 1″ long.
8. Mark the center of one end with a center punch, then drill a small pilot hole all the way through the stock.
9. From one end, drill halfway through using a 10mm (13/32 inch) bit. This will match the S / S2 fuel rail’s metric threads.
10. Tap the hole you just drilled with a 12mm x 1.5 tap to create threads that fit the S / S2’s fuel rail test port.
11. Flip the stock over and drill halfway through from the other end with an 11/32 inch bit (sized for a 1/8″ NPT tap).
12. Tap this second hole with a 1/8″ NPT tap for the brass fitting.
13. Wrap Teflon thread tape around the 1/8″ NPT end of the brass male-to-male fitting (1/8″ NPT to 1/4″ flare).
14. Thread the fitting into the 1/8″ NPT side of your adapter and tighten it.
15. (Optional) Cut flats on the sides of the adapter with a file or mill to fit a 17mm wrench, making it easier to install.
16. Screw the 12mm x 1.5 side onto the S2’s fuel rail test port, and attach your gauge to the 1/4″ flare side.

Post Procedure Checklist

• Check that the adapter is securely threaded onto the fuel rail test port with no wobbling.
• Inspect for fuel leaks around the adapter and fitting connections after attaching the gauge and pressurizing the system.
• Confirm the gauge reads fuel pressure correctly (for the S / S2, expect around 2.5-3.0 bar at idle, per Porsche specs).

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manual, Volume 1
• Youtube