The Castair P2B625-13/8 is a 2-groove (2B) replacement motor pulley with a fixed 1-3/8" bore shaft and 6.25" outer diameter, built for Cast Air belt-drive air compressor systems. If your existing motor pulley is worn, cracked, or out of spec, this is the direct replacement — same groove profile, same bore, same OD — so you can get your compressor back in service without fabricating or modifying anything.
Note: Cast Air did not provide a dedicated manufacturer product page for this pulley. Specifications below are drawn from the product listing and standard 2B-groove pulley engineering conventions. Contact us if you need confirmation for a specific compressor application.
What Is a 2B Motor Pulley?
The 2B designation tells you this pulley has two B-section V-belt grooves. B-section belts are the standard for single-phase and three-phase compressor motors in the 1-5 HP range. Two grooves let you run a matched belt pair, which doubles the power transmission capacity versus a single-groove setup and reduces belt slip under load — important for compressors that cycle frequently in a busy shop.
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): 6.25"
- Application: Motor-side pulley for belt-drive air compressors
- Compatibility: Castair belt-drive compressor systems using 1-3/8" motor shafts and B-section belts
- Part Number: P2B625-13/8
Why Motor Pulley OD Matters for Compressor Performance
The motor pulley OD and the pump flywheel OD together define the belt drive ratio — and that ratio determines how fast the compressor pump turns. A 6.25" motor pulley paired with a larger flywheel on the pump creates the speed reduction needed to run most single-stage Cast Air pumps at their correct RPM (typically 900-1,100 RPM on the pump side). Using the wrong pulley OD shifts this ratio, which can over-speed the pump and shorten its life, or under-speed it and cost you CFM. Always verify the OD matches the original before ordering.
Fixed Bore vs. Taper-Lock (SK/SD) Pulleys
- Fixed bore pulleys like the P2B625-13/8 are machined to a specific shaft diameter — in this case 1-3/8". They mount directly to the motor shaft with a keyway and set screw. The correct choice for a direct replacement on an existing motor with the same shaft diameter.
- Taper-lock (SK/SD) pulleys use a separate bushing that allows one pulley to fit multiple shaft diameters. Preferred when future motor swaps are anticipated or shaft diameter may vary.
- If your original pulley was a fixed bore 1-3/8", the P2B625-13/8 is the drop-in replacement. Do not substitute a taper-lock pulley — they require matching bushings and the hub geometry differs.
When Does a Shop Need This Pulley?
- Motor pulley cracked, damaged, or showing V-belt groove wear causing belt slippage or squealing
- Replacing a motor where the new motor has a 1-3/8" shaft — this pulley restores the original drive ratio
- Rebuilding a Cast Air belt-drive compressor and sourcing OEM-spec replacement parts
- Compressor pump RPM is off (verified with a tachometer) and pulley OD needs correction
2B Fixed-Bore 1-3/8" Motor Pulley Size Comparison
Cast Air offers this pulley in multiple OD sizes. Here is how the 2B fixed-bore 1-3/8" bore lineup compares:
- P2B625-13/8 — 6.25" OD — $68.34 (this unit) — smallest in the series; lowest pump RPM for a given motor speed
- P2B675-13/8 — 6.75" OD — $71.23 — slightly larger diameter, increases pump speed for the same motor RPM
- P2B80-13/8 — 8.0" OD — $88.55 — larger pulley, higher pump RPM, used on higher-CFM applications
- P2B875-13/8 — 8.75" OD — $103.95 — largest in the fixed-bore 1-3/8" lineup
Measure the original pulley OD before ordering. If you are not sure, call us — we can help verify the correct replacement for your Cast Air compressor model.
Technical Specifications
- Part Number: P2B625-13/8
- Manufacturer: Cast Air (Castair)
- Groove Type: 2B (two B-section V-belt grooves)
- Bore: Fixed, 1-3/8" (1.375")
- Outer Diameter: 6.25"
- Bore Style: Fixed (keyway and set screw mount)
- Belt Section Compatibility: B-section V-belts (two grooves, matched pair)
- Price: $68.34
- Manufacturer Data: Product page not available — specifications from product listing data
What Is Included
- 1x Castair P2B625-13/8 motor pulley
- Set screw(s) for shaft retention
Belts are not included. Order the appropriate B-section matched belt pair separately for your drive length.
Why Buy From Air Compressor Factory Direct?
We have specialized in Cast Air compressor parts and accessories since 2016. When you order from us, you get the correct part — not a generic substitute that may not match your groove profile or bore diameter. Our team knows the Cast Air lineup and can confirm fitment before you order. Every order ships free, and our tech support line is staffed by people who know compressor drive systems.
- Cast Air specialist since 2016 — not a general parts warehouse
- Free shipping on all orders
- Free tech support — call before you order if you are not sure about fitment
- Full manufacturer warranty on all Cast Air parts
- Honest guidance — we will tell you if a different size is the right call for your application
Call 866-217-0063 for quick answers and help!
