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

Castair NG-1 Three-Way Pilot Valve — 1/2" NPT Gas Engine, 25 CFM

The Castair NG-1 is a three-way pilot valve built for gas engine-driven air compressor systems that operate in the 25 CFM range. It uses 1/2" female NPT connections on both inlet and outlet, with a 3/8" NPT vent port for exhaust. If your shop runs a gas-powered compressor — portable or stationary — and you need the unloader circuit to operate mechanically without relying on an electrical control signal, the NG-1 is the correct valve. The gas engine version does not use a solenoid. It responds to pneumatic pilot pressure rather than an electrical switch, which is how gas engine control systems work.

The NG-1 and NDC-1 share identical port dimensions — both use 1/2" female NPT inlet and outlet, 3/8" NPT vent. The difference is in the actuation mechanism. If you are replacing a failed valve and are not sure which version you have, look at the wiring: if there are electrical leads going to the valve body, it is the NDC-1 (electric). If the control line is purely pneumatic with no wiring, it is the NG-1 (gas engine). Installing the wrong type will cause the unloader circuit to malfunction.

What the NG-1 Does in the Control System

• Load/Unload Cycling: The NG-1 controls when the compressor head is loaded (compressing air) and unloaded (running free). When tank pressure reaches the set point, the valve shifts and vents the unloader line, allowing the head to unload without stopping the gas engine.
• Mechanical Operation: Unlike electric dual valves, the NG-1 is operated by pneumatic pilot pressure — no solenoid, no electrical connection. The gas engine runs continuously, and the valve cycles the compressor head in and out of load based on pressure signal alone.
• Three-Way Function: Routes flow between three ports — inlet (from control/tank), outlet (to unloader), and vent (exhaust on unload). The vent function is what cleanly exhausts trapped pressure from the unloader line when the compressor shifts to unloaded state.
• Check Valve Function: The three-way design prevents backflow through the pilot circuit when the compressor is in the loaded condition.
• 25 CFM Capacity: Sized for mid-range compressors. Verify your compressor's actual control-circuit flow demand does not exceed 25 CFM before ordering. Undersizing the valve causes pressure drop across the valve itself, degrading control response.

Port Configuration and Plumbing

• Inlet: 1/2" female NPT — connects to the compressor discharge or tank pilot line
• Outlet: 1/2" female NPT — connects to the unloader mechanism on the compressor head
• Vent Outlet: 3/8" NPT — exhausts trapped pressure on unload; connect the Castair M38 muffler here to reduce noise
• Pilot Ports: 1/8" female NPT — pilot pressure signal input for valve actuation
• PSI Range Options: Order NG-1100 for 100–120 PSI systems, or NG-1140 for 140–170 PSI systems. The spring and seat configuration are different between the two — using the wrong pressure range will cause early cycling or failure to unload at the set point.

Gas Engine Compressor Control — Why No Solenoid

• Engine Runs Continuously: A gas engine-driven compressor does not stop and start with pressure like an electric motor-driven unit. The engine runs at governed RPM and the compressor head is loaded or unloaded to control air delivery.
• Pneumatic Signal Only: The control signal is a pneumatic pressure signal derived from the tank or discharge line. When pressure rises to the cut-out point, the pneumatic pilot signal shifts the NG-1 and the compressor head unloads.
• No Electrical Dependencies: The NG-1 has no coil, no wiring, no solenoid to fail due to moisture, voltage spikes, or vibration. For outdoor or remote-site applications where electrical reliability is a concern, this is an advantage.
• Direct Comparison to NDC-1: Same port sizes, same CFM rating, same vent port size. The NG-1 is mechanically (pneumatically) actuated; the NDC-1 is solenoid (electrically) actuated. Ports are interchangeable in terms of plumbing, but the function and wiring requirements are completely different.

How a Three-Way Pilot Valve Works

A three-way pilot valve manages three flow paths: inlet, outlet, and exhaust. In loaded operation, the valve holds the inlet-to-outlet path open, supplying the control signal to the unloader and keeping the compressor head engaged. When the pilot pressure signal reaches the cut-out threshold, the valve spool shifts: it blocks the inlet and connects the outlet to the vent port, exhausting the unloader line pressure. This allows the unloader mechanism to open and the compressor head to run free (unloaded) while the gas engine continues running.

When pressure in the tank drops to the cut-in threshold, the pilot signal drops and the spool shifts back to the loaded position. The vent port closes, the inlet re-pressurizes the outlet, and the unloader closes, putting the compressor back to full compression. This cycle repeats continuously during operation. The vent port must always be open to atmosphere or connected to a muffler — any back-pressure on the vent will cause delayed or failed unloading.

Pilot Valve Selection Guide — Complete Castair Lineup

Use this table to confirm the NG-1 is correct for your application, or to identify an alternative if your system specs differ:

PSI Suffix: Add 100 (for 100–120 PSI) or 140 (for 140–170 PSI) to the part number. Example: NG-1100 or NG-1140. Muffler: Castair M38 fits the 3/8" NPT vent port.

When Does a Shop Need the NG-1 vs. Alternatives

• NG-1 is right when: You have a gas engine-driven compressor, 1/2" NPT control lines, and the compressor draws 25 CFM or less through the valve. It is also the correct replacement when the original NG-1 has failed and existing plumbing is 1/2" NPT.
• Use NG-18 instead when: Your inlet line uses 3/4" compression tubing rather than 1/2" NPT pipe. The NG-18 has a 3/4" compression inlet and 1/2" NPT-F outlet — same CFM rating and vent size as the NG-1, just with a different inlet fitting. Gas engine application only.
• Step up to NLG-7 when: Your gas engine compressor exceeds 25 CFM at the valve, or the control lines are 3/4" NPT. The NLG-7 handles 50 CFM with 3/4" male compression inlet and 3/4" male NPT outlet, and a 1/2" NPT vent port.
• Use NDC-1 instead when: Your compressor is electric motor-driven with a dual-pressure-switch control system. The NDC-1 is the solenoid-operated equivalent — same port sizes as the NG-1, but electrically actuated.
• Step down to BG2-M when: Your gas engine system only needs a 2-way function with 1/4" NPT connections and 20 CFM or less. Simpler and less expensive for smaller systems.

Technical Specifications

• Part Number: NG-1
• Valve Type: 3-way pilot valve
• Application: Gas engine-driven air compressor control systems
• Operation: Pneumatically actuated (no solenoid)
• Inlet Port: 1/2" female NPT
• 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 (order as NG-1100) or 140–170 PSI (order as NG-1140)
• Manufacturer: Castair
• Price: $115.50
• Compatible Muffler: Castair M38 (3/8" vent port)

Why Buy From Air Compressor Factory Direct?

Air Compressor Factory Direct specializes in compressor controls and replacement parts. We have stocked Castair pilot valves since 2016 and understand the gas engine vs. electric dual distinction that catches a lot of buyers off guard. When you call us, you get someone who knows why the NG-1 and NDC-1 have identical port sizes but are not interchangeable — and who will make sure you order the right valve and the right pressure range the first time.

• Free Shipping: Every order ships free. No handling fees, no minimums.
• Free Tech Support: Call 866-217-0063 before you order if you need help confirming part number and pressure range. We will walk through the application with you.
• Full Manufacturer Warranty: Every Castair valve we sell carries the full manufacturer warranty. No grey-market parts, no refurbished valves.
• Correct Part, First Time: If you are not sure whether your system needs the NG-1 or the NG-18, or whether to order the 100 PSI or 140 PSI version, call us. We would rather spend five minutes on the phone than have you wait for a return and reorder.

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