The Castair P2B725-11/8 is a 2-groove (2B) replacement motor pulley with a 1-3/8" fixed bore and 7.5" outer diameter, for use in Cast Air belt-drive air compressor systems. Note: the product name in the listing references both "1 3/8" bore and "7.5" OD" — this description reflects both dimensions as listed. The 7.5" OD is a larger motor pulley than the 6.25" and 6.75" units in the Cast Air lineup, which means it drives the pump faster for a given motor RPM, increasing CFM output where the pump design supports it.
Note: Cast Air did not provide a dedicated manufacturer product page for this pulley. Specifications are drawn from the product listing. Confirm fitment with your specific compressor model before ordering — call us if you need help.
What Makes This Pulley Different From the Smaller OD Options
The key difference between motor pulleys in the Cast Air lineup is the outer diameter. The OD sets the belt drive ratio relative to the pump flywheel. A larger motor pulley OD increases the ratio, spinning the pump faster. A 7.5" motor pulley paired with a standard 14" pump flywheel gives a drive ratio of approximately 1.87:1 — so a 1,750 RPM motor drives the pump at roughly 935 RPM. This is toward the upper end of the normal range for single-stage Cast Air pumps and is appropriate for applications where higher CFM output is needed from the same motor.
Pulley Specification Details
- Groove Profile: 2B (two B-section V-belt grooves)
- Bore Type: Fixed bore — 1-3/8" (1.375") shaft diameter
- Outer Diameter (OD): 7.5"
- Application: Motor-side pulley for belt-drive air compressors
- Compatibility: Cast Air belt-drive compressor systems with 1-3/8" motor shafts and B-section belts
- Part Number: P2B725-11/8
Fixed Bore vs. Taper-Lock — Which Do You Have?
- Fixed bore (this pulley): Machined for a specific shaft diameter. Mounts with keyway and set screw directly on the motor shaft. Correct for direct replacement when your original was also a fixed bore 1-3/8" unit.
- Taper-lock (SK/SD): Uses a removable bushing that adapts to multiple shaft sizes. A different part entirely — not interchangeable with fixed bore without modification.
- If your original pulley was fixed bore and measured 7.5" OD with a 1-3/8" bore, the P2B725-11/8 is the direct replacement.
When Does a Shop Need the 7.5" OD Pulley?
- Original motor pulley measured 7.5" OD with 1-3/8" fixed bore and is now worn or damaged
- Application requires higher pump RPM than the 6.25" or 6.75" pulleys provide
- Rebuilding a Cast Air compressor that originally shipped with a 7.5" motor pulley
- Pump CFM output is below spec and tachometer confirms the pump RPM is too low — a larger motor pulley corrects this
2B Fixed-Bore 1-3/8" Motor Pulley Size Comparison
- P2B625-13/8 — 6.25" OD — $68.34 — lowest pump RPM in the fixed-bore series
- P2B675-13/8 — 6.75" OD — $71.23 — mid-range OD for standard single-stage applications
- P2B725-11/8 — 7.5" OD — $84.70 (this unit) — larger OD, higher pump RPM, higher CFM potential
- P2B80-13/8 — 8.0" OD — $88.55 — even larger, for applications requiring maximum pump speed ratio
- P2B875-13/8 — 8.75" OD — $103.95 — largest fixed-bore option
Technical Specifications
- Part Number: P2B725-11/8
- Manufacturer: Cast Air (Castair)
- Groove Type: 2B (two B-section V-belt grooves)
- Bore: Fixed, 1-3/8" (1.375")
- Outer Diameter: 7.5"
- Bore Style: Fixed (keyway and set screw mount)
- Belt Section: B-section (two matched belts)
- Price: $84.70
- Manufacturer Data: Product page not available — specifications from product listing data
What Is Included
- 1x Castair P2B725-11/8 motor pulley
- Set screw(s) for shaft retention
B-section matched belts sold separately. Measure your drive center distance to determine the correct belt length.
Why Buy From Air Compressor Factory Direct?
Since 2016, we have stocked Cast Air replacement parts across the full pulley lineup. If you are not certain whether the 7.5" OD is the correct size for your compressor, call us before ordering. We can help you identify the right pulley based on your compressor model and original specifications. Free shipping, free tech support, and full manufacturer warranty on every part we sell.
- Cast Air specialist since 2016
- Free shipping on all orders
- Free tech support — we confirm fitment before you order
- Full manufacturer warranty
- Honest guidance on which OD is correct for your application
Call 866-217-0063 for quick answers and help!
