Cast Air P1B40-5/8 — 4" OD Single-Groove B-Section Motor Pulley, 5/8" Fixed Bore
The Cast Air P1B40-5/8 is a 4" outside diameter motor pulley with a single B-section belt groove and a 5/8" fixed bore. It mounts directly onto a 5/8" motor shaft — no hub required — and drives a B-section V-belt to the compressor pump pulley. This pulley fits the same motor shaft size as the P1A40-5/8, but uses a B-groove profile rather than an A-groove. The distinction is critical: if your compressor runs a B-belt, you need this pulley. If it runs an A-belt, you need the P1A40-5/8. Ordering the wrong groove type means the belt will not seat correctly, slippage is immediate, and belt wear is accelerated.
The B-section belt is wider and deeper than an A-section belt — 21/32" wide at the top versus 1/2" for an A-belt. B-belts handle higher torque loads. Compressors that run on 5/8" motor shafts but use B-belts are typically in the higher end of the small-compressor class, where the drive transmits more torque than a single A-belt can reliably carry. If your compressor shipped with a B-belt and you are replacing the motor pulley, this is the correct part.
Pulley Specifications
- Part Number: P1B40-5/8
- Outside Diameter: 4"
- Bore Size: 5/8" fixed bore
- Groove Type: 1B — single groove, B-section belt profile
- Belt compatibility: B-section V-belt (e.g., B-26, B-29, B-31 — belt length depends on center distance)
- Mounting: Fixed bore — mounts directly on 5/8" motor shaft with setscrew and key
- Material: Cast iron
Why Belt Section Matters
B-section and A-section belts differ in cross-section dimensions. A B-belt is wider and has a different flank angle than an A-belt. When you put a B-belt in an A-groove, the belt sits too high in the groove and makes minimal contact with the flanks, reducing the friction surface that transmits power. When you put an A-belt in a B-groove, the belt drops too deep and the flanks bottom out. Either way, slippage occurs under load. Use the belt section that matches the original drive specification — and match the pulley groove to that belt section.
- A-section belt + 1A groove: P1A40-5/8
- B-section belt + 1B groove (this pulley): P1B40-5/8
- B-section belt + 1B groove, 7/8" shaft: P1B40-7/8
Fixed Bore vs. Hub Pulleys
- Fixed bore (this pulley): The bore is machined to 5/8". Mounts directly on the motor shaft with a key and setscrew. No additional parts needed.
- Hub pulleys: The pulley has no machined bore — a separate hub (LX, SD, or SK series) is ordered for the specific shaft size. Useful when the same pulley frame must fit multiple shaft diameters.
- For 5/8" motor shafts: This pulley is the straightforward choice — install directly, no separate hub order required.
When to Replace the Motor Pulley
- Belt slipping under load: If the belt slips when the compressor builds pressure despite being correctly tensioned, inspect the groove for wear. A worn groove loses the sharp flank angles that grip the belt.
- Visible groove wear: Look at the groove flanks. If they have worn to a rounded, smooth profile rather than the original angled shape, the pulley needs replacement.
- Cracked body: Any crack in the cast iron body is a replacement indicator. A spinning cracked pulley can shed fragments at speed.
- Damaged keyway: If the keyway has rounded corners or the key has been spinning in the slot, the pulley cannot hold its angular position on the shaft reliably.
- Wallowed bore: An oversize bore that the setscrew cannot compensate for. This creates runout that accelerates belt wear and increases vibration.
Setting Proper Belt Tension
After installing a new pulley and belt, correct tension is critical. A belt that is too loose slips under load and wears rapidly. A belt that is too tight overloads the motor bearings and pump shaft bearings, shortening their service life significantly. The general guideline for V-belt drives on compressors: the belt should deflect approximately 1/2" per foot of span under moderate finger pressure (roughly 10 lbs force). Confirm this against the Cast Air service documentation for your specific model.
Belt Guard Compatibility Reference
Cast Air compressor models use specific belt guard sizes. For reference:
- 2BG guard (24x16x5): A2, A3, A4, A8 models
- 3BG guard (32x20x5): C1, D3, G11 models
- 4BG guard (40x20x5): C2 model
- 4BG7 guard (40x20x7): C3, G30 models
- 5BG guard (48x32x7.5): C3U, C4U models
Available Motor Pulleys in This Series
- P1A40-5/8: 4" OD, 5/8" bore, 1A groove — $30.80
- P1B40-5/8 (this pulley): 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 sizes for larger compressors
Technical Specifications
- Part Number: P1B40-5/8
- Outside Diameter: 4.0"
- Bore: 5/8" fixed bore
- Number of Grooves: 1
- Groove Profile: B-section
- Bore Type: Fixed (no hub required)
- Material: Cast iron
- Price: $30.80
Why Buy From Air Compressor Factory Direct?
A lot of shops order the wrong pulley because the A/B groove distinction is easy to miss when you are in a hurry. We can help you confirm the correct groove type for your compressor before you order. Tell us the compressor model number or describe the belt you are running, and we will point you to the right pulley. That is what the tech support line is for.
- Free shipping on qualifying orders
- Free tech support: Call 866-217-0063 to confirm you have the right groove type before ordering
- Full manufacturer warranty on all Cast Air parts
- Honest guidance: We stock the full pulley lineup — we will tell you which one is correct, not just sell you whatever is in your cart
Call 866-217-0063 for quick answers and help!
