Castair 3/4" Compression 1/2" NPT-F Outlet Vent Outlet 3/8" NPT 25 Max. CFM Gas Engine 3 Way Pilot Valve

Castair NG-18 Three-Way Pilot Valve — 3/4" Compression Inlet, Gas Engine, 25 CFM

The Castair NG-18 is a three-way pilot valve for gas engine-driven air compressors that use 3/4" compression tubing on the inlet side of the control circuit. It handles up to 25 CFM, routes the outlet through a 1/2" female NPT connection, and vents through a 3/8" NPT port. If you are replacing a pilot valve and the inlet line uses compression-style tubing rather than NPT-threaded pipe, the NG-18 is the correct part — the NG-1, its closest sibling, uses a 1/2" female NPT inlet and will not match your existing compression fitting.

The distinction between NG-18 and NG-1 is entirely about the inlet connection. Both valves are three-way, both handle 25 CFM, both use identical outlet (1/2" F NPT) and vent (3/8" NPT) ports, and both are mechanically operated for gas engine applications. The NG-18 exists specifically for systems where the inlet control line is run in compression tubing. If your inlet line is NPT pipe, order the NG-1 instead. If your inlet is compression tubing, order the NG-18. Getting this wrong means the valve will not connect to your existing plumbing without adapters — and adapters in a control circuit add leak points.

What the NG-18 Does in the Control System

• Load/Unload Control: The NG-18 manages the compressor head's load state. When tank pressure reaches the set cut-out point, the valve shifts and vents the unloader line, allowing the compressor head to run free while the gas engine continues at operating speed.
• Three-Way Routing: Three ports handle three flow states — inlet carries the control signal in loaded condition, outlet delivers that signal to the unloader mechanism, and vent exhausts trapped pressure when the valve shifts to unloaded position.
• Pneumatic (Mechanical) Operation: No solenoid, no electrical connections. The valve responds to pneumatic pilot pressure from the control circuit. This is the correct operation type for gas engine drives — gas engines run continuously and the valve cycles the compressor head based on pressure signal alone.
• 3/4" Compression Inlet: This is the defining feature of the NG-18. The 3/4" compression fitting connects to compression-style tubing without threads — the nut compresses a ferrule onto the tube for a seal. If your system was originally plumbed with compression tubing on the inlet, the NG-18 maintains that original plumbing configuration.
• 25 CFM Rating: The flow capacity through the valve. Confirm your compressor's control circuit does not require more than 25 CFM at the valve. For larger gas engine compressors with higher flow demands, see the NLG-7 (50 CFM).

Port Configuration and Plumbing

• Inlet: 3/4" compression — connects to 3/4" OD compression tubing; no threads, uses compression nut and ferrule
• Outlet: 1/2" female NPT — connects to the threaded unloader mechanism on the compressor head
• Vent Outlet: 3/8" NPT — exhausts trapped pressure on unload; connect Castair M38 muffler here for quiet operation
• Pilot Ports: 1/8" female NPT — pneumatic pilot pressure input for valve actuation
• PSI Range Options: Order NG-18 with suffix 100 for 100–120 PSI systems, or suffix 140 for 140–170 PSI systems. The internal spring and seat are calibrated differently for each range — using the wrong pressure version will cause the valve to cycle at the wrong pressure points.

Compression vs. NPT Fittings — Why This Matters for Ordering

• Compression Fittings: Use a nut and ferrule (sleeve) that compress around the outside of the tube to form a seal. No pipe threads. Common in older and portable gas engine compressor systems where tubing was used rather than pipe. The NG-18's 3/4" compression inlet accepts this style of connection directly.
• NPT Fittings: Threaded connections that seal on tapered threads with thread sealant. The NG-1 uses 1/2" female NPT on the inlet — if your inlet line is threaded pipe, the NG-1 is the correct valve.
• Cannot Interchange Without an Adapter: A 3/4" compression fitting and a 1/2" NPT fitting are not the same, and they are not the same size. If you install the NG-18 where the NG-1 is called for (or vice versa), you will need adapter fittings, which add potential leak points to a pressure-control circuit. Use the correct valve for your plumbing style.
• How to Identify Your Inlet Connection: Look at the existing valve's inlet. If there is a hex nut that slides onto tubing and a brass sleeve (ferrule) behind it, it is a compression fitting. If the line screws directly into threaded ports, it is NPT. If you are unsure, measure the OD of the tubing (3/4" OD for NG-18) or the thread size and pitch of the fitting.

How a Three-Way Pilot Valve Works

A three-way pilot valve controls the load/unload cycle of a compressor head using three flow paths. During loaded operation, the inlet and outlet ports are connected — the pneumatic control signal passes from the tank through the valve to the unloader mechanism, keeping the compressor head engaged. When the tank pressure rises to the cut-out point, the pilot signal pressure shifts the valve spool: the inlet closes, and the outlet connects to the vent port. This exhausts the pressure trapped in the unloader line, allowing the unloader to open and the compressor head to run free (unloaded state) while the gas engine continues running.

When tank pressure drops to the cut-in point, the pilot signal pressure drops, the spool shifts back, and the inlet-to-outlet path reopens. The unloader line re-pressurizes, the unloader closes, and the compressor head resumes full compression. The 3/8" NPT vent port must be open to atmosphere or connected to a muffler at all times — back-pressure on the vent will prevent proper unloading and can cause compressor head damage or motor overload.

Pilot Valve Selection Guide — Complete Castair Lineup

Confirm the NG-18 is correct for your system, or identify the right alternative if your specs differ:

PSI Suffix: Add 100 (100–120 PSI) or 140 (140–170 PSI) when ordering. Muffler: Castair M38 fits the 3/8" NPT vent port on NG-18, NG-1, and NDC-1.

When Does a Shop Need the NG-18 vs. Alternatives

• NG-18 is right when: You have a gas engine-driven compressor with a 3/4" compression-style inlet line on the control circuit, an outlet that connects to 1/2" NPT, and the compressor runs 25 CFM or less through the valve. If you are replacing the original NG-18 and the compression tubing inlet is already in place, keep the NG-18.
• Use NG-1 instead when: Your gas engine compressor has 1/2" female NPT on both inlet and outlet of the control circuit. Same CFM rating, same vent port, same mechanical operation — just threaded NPT inlet instead of compression. At $115.50 they are the same price.
• Step up to NLG-7 when: Your gas engine compressor exceeds 25 CFM at the valve, or you need 3/4" connections on both inlet and outlet. The NLG-7 handles 50 CFM with a 3/4" male compression inlet and 3/4" male NPT outlet, and a larger 1/2" NPT vent port.
• Use NDC-1 or NLDC-1 when: Your compressor is electric motor-driven, not gas engine. The NDC-1 (25 CFM, 1/2" NPT) and NLDC-1 (50 CFM, 3/4" NPT) are solenoid-operated for electric dual control systems.
• Do not use an adapter to convert NPT to compression: If your system has 1/2" NPT inlet plumbing, order the NG-1 and use it directly. Adding a compression-to-NPT adapter in a control circuit is an unnecessary leak point and complicates future service.

Technical Specifications

• Part Number: NG-18
• Valve Type: 3-way pilot valve
• Application: Gas engine-driven air compressor control systems
• Operation: Pneumatically actuated (no solenoid)
• Inlet Port: 3/4" compression fitting
• Outlet Port: 1/2" female NPT
• Vent Outlet Port: 3/8" NPT
• Pilot Ports: 1/8" female NPT
• Maximum Flow Capacity: 25 CFM
• Pressure Range Options: 100–120 PSI (suffix 100) or 140–170 PSI (suffix 140)
• Manufacturer: Castair
• Price: $115.50
• Compatible Muffler: Castair M38 (3/8" vent port)

Why Buy From Air Compressor Factory Direct?

Air Compressor Factory Direct has been stocking Castair pilot valves and compressor control components since 2016. The NG-18 is one of those parts that technicians get wrong on the first order because the product listings do not always make the compression vs. NPT inlet distinction clear. When you call us, we will confirm whether your system needs the NG-18 or the NG-1, and which pressure suffix to order, before we ship anything.

• Free Shipping: All orders ship free. No handling charges, no minimums.
• Free Tech Support: Call 866-217-0063 with your compressor make and model and we will confirm the correct valve and pressure range. This takes less time than a return shipment.
• Full Manufacturer Warranty: Every Castair valve we sell carries the full manufacturer warranty. No refurbished parts, no grey-market inventory.
• Correct Part Guidance: If the NG-18 is not the right call for your system, we will tell you — and direct you to the NG-1, NLG-7, or another valve that is. We do not benefit from selling you the wrong part.

Call 866-217-0063 for quick answers and help!