Cast Air P1A40-5/8 — 4" OD Single-Groove A-Section Motor Pulley, 5/8" Fixed Bore
The Cast Air P1A40-5/8 is a 4" outside diameter motor pulley with a single A-section belt groove and a 5/8" fixed bore. It mounts directly onto a 5/8" motor shaft — no separate hub required — and drives an A-section V-belt to the compressor pump pulley. This is a common motor pulley size on fractional-horsepower and small single-phase compressors. If your existing pulley is cracked, the bore is wallowed out, or a keyway has stripped, this is a direct replacement for any Cast Air or compatible compressor that uses a 5/8" motor shaft and an A-belt drive.
Getting the pulley selection right matters for compressor performance. The motor pulley diameter — in combination with the pump pulley diameter — sets the pump RPM. Too small a motor pulley and the pump runs slower than designed, reducing output CFM. Too large and the pump runs overspeed, increasing wear and potentially overloading the motor. The P1A40-5/8 is a 4" motor pulley, which is a standard size for this class of compressor. If you are replacing like-for-like, match the 4" OD and 1A groove specification to your existing pulley.
Pulley Specifications
- Part Number: P1A40-5/8
- Outside Diameter: 4"
- Bore Size: 5/8" fixed bore (no hub required)
- Groove Type: 1A — single groove, A-section belt profile
- Belt compatibility: A-section V-belt (e.g., A-29, A-31, A-34 — belt length depends on center distance)
- Mounting: Fixed bore — mounts directly on 5/8" motor shaft with setscrew
- Material: Cast iron (standard for compressor drive pulleys)
A-Section vs. B-Section Belt Grooves — Why It Matters
A-section and B-section belts are not interchangeable. The groove profile (angle and depth) is different between the two belt sizes. Running an A-belt in a B-groove or vice versa results in the belt riding too high or too low in the groove, causing slippage, rapid belt wear, and pulley wear.
- A-section belt: 1/2" wide at the top, used on lighter-duty drives and smaller compressors
- B-section belt: 21/32" wide at the top, used on heavier-duty drives and larger compressors
- Match the groove to the belt already on your compressor. If your compressor shipped with an A-belt, use the P1A40-5/8. If it shipped with a B-belt, you need the P1B40-5/8 instead.
Fixed Bore vs. Hub Pulleys
- Fixed bore (this pulley): The bore is machined to a specific size — in this case 5/8". The pulley mounts directly on the shaft. No hub is ordered separately. Simple, common, and lower cost.
- Hub pulleys: Bore is not machined — a separate hub (LX, SD, or SK series) is ordered to match your shaft size and pressed or bolted into the pulley. Used when the same pulley needs to fit different shaft sizes.
- If your motor shaft is 5/8": The P1A40-5/8 mounts directly. No additional parts needed beyond a key and setscrew (typically included or already on the shaft).
When to Replace the Motor Pulley
- Cracked pulley body: A cracked cast iron pulley is a safety hazard. Replace it immediately — do not run a cracked pulley under tension.
- Worn or damaged keyway: If the keyway is chipped, rounded, or the key spins in the slot, the pulley is slipping on the shaft. You may hear squealing during startup or notice the pump not keeping up with demand.
- Wallowed bore: A bore that has worn oversize — common when the pulley has run without a key — cannot be fixed by tightening the setscrew. Replace the pulley.
- Groove wear: A groove that has worn to a rounded profile allows the belt to slip under load. If the belt is in good condition but still slips at full load, measure the groove depth and compare to a new pulley.
- Belt replacement: When replacing a belt after extended service, inspect the pulley groove for wear at the same time.
Belt Guard Compatibility
Cast Air motor pulleys are used with specific belt guards depending on the compressor model. For reference, the 2BG guard (24x16x5) is used with Cast Air models A2, A3, A4, and A8. If you are ordering a replacement pulley for one of these models, confirm that a 4" OD motor pulley matches the original specification before ordering.
Available Motor Pulleys in This Series
- P1A40-5/8 (this pulley): 4" OD, 5/8" bore, 1A groove — $30.80
- P1B40-5/8: 4" OD, 5/8" bore, 1B groove — $30.80
- P1B40-7/8: 4" OD, 7/8" bore, 1B groove
- P2B series: Dual B-groove pulleys in multiple OD and shaft size combinations for larger compressors
Technical Specifications
- Part Number: P1A40-5/8
- Outside Diameter: 4.0"
- Bore: 5/8" fixed bore
- Number of Grooves: 1
- Groove Profile: A-section
- Bore Type: Fixed (no hub required)
- Material: Cast iron
- Price: $30.80
Why Buy From Air Compressor Factory Direct?
We specialize in Cast Air compressor parts and have the full lineup of motor pulleys in stock. When you call us before ordering, we can confirm the right pulley for your specific compressor model — the right OD, the right bore, and the right groove profile — so you are not guessing and hoping it fits. A wrong pulley means another order, more downtime, and more frustration. We would rather spend two minutes on the phone helping you get it right the first time.
- Free shipping on qualifying orders
- Free tech support: Call 866-217-0063 — we will help you identify the correct pulley for your compressor
- Full manufacturer warranty on all parts
- Honest guidance: If you need to know the original pulley spec for your Cast Air model, call us and we will look it up
Call 866-217-0063 for quick answers and help!
