Castair SD138 — Replacement Motor Hub, 1-3/8" Shaft
The Castair SD138 is a replacement motor hub sized for motors with a 1-3/8-inch output shaft. This is the SD-series drive hub that connects the electric motor shaft to the belt-drive flywheel and pulley system on Cast Air belt-driven compressor units. When the hub cracks, strips, or wears to the point where it no longer holds the drive pulley securely, the correct repair is a direct OEM replacement — not an improvised substitute that is close but not engineered for this application.
The SD138 designation tells you everything: SD series, 138 = 1-3/8 inches. Measure your motor shaft with a caliper before ordering — 1-3/8 inches is 1.375 inches in decimal. If your shaft is larger at 1-5/8 inches, you need the SD158 instead.
What the Motor Hub Does
On a belt-driven compressor, the motor hub mounts to the motor's output shaft and carries the drive sheave (pulley) that transfers torque to the compressor pump via V-belt. Every time the motor starts, the hub absorbs the full startup torque load. Over time, the keyway can wear, the bore can develop slop, or the hub body can crack from fatigue. Any of these conditions cause belt drive problems: vibration, misalignment, slipping belts, or a compressor that simply will not run correctly. Replacing the hub with the correct OEM part solves all of these symptoms.
Key Features
- Shaft Size: 1-3/8 inch: Fits motor output shafts measuring 1.375 inches in diameter — the SD138 bore is machined specifically for this shaft size
- SD Series Hub: The SD designation identifies the hub style used on specific Cast Air compressor configurations — not interchangeable with SK-series hubs used on other models
- OEM Cast Air Replacement: Manufactured to Cast Air specifications — same tolerance fit as the original hub
- Precision Machined Bore: Correct bore tolerance prevents shaft slop and keeps the drive assembly running concentric — a worn or out-of-round hub bore causes vibration and belt alignment problems even with a new belt installed
- Belt-Drive Application: Designed for belt-driven compressor pump assemblies where the motor drives the pump via sheave and V-belt
Technical Specifications
- Part Number: SD138
- Shaft Diameter: 1-3/8 inch (1.375")
- Hub Series: SD
- Component Type: Replacement motor drive hub
- Application: Belt-driven Cast Air compressor pump assemblies
- Manufacturer: Cast Air (Castair)
- Price: $31.57
SD Series Hub Lineup — Choosing the Right Shaft Size
- SD138 — 1-3/8 inch (1.375") motor shaft. This product. ($31.57)
- SD158 — 1-5/8 inch (1.625") motor shaft — larger motors in 5 HP and above range ($31.57)
For SK-series hub applications, see SK158 and SK178. SD and SK hubs are different series and are not interchangeable.
When Does a Shop Need the SD138
Replace the motor hub when:
- The existing hub shows cracking, bore wear, or visible damage at the keyway or set screw seat
- The compressor produces unusual vibration, particularly at startup — often a hub that has gone loose on the shaft
- The belt keeps going out of alignment despite correct tension — a worn hub bore causes pulley runout that no amount of belt tensioning will fix
- You are installing a replacement motor and the new motor has a 1-3/8-inch output shaft
- Preventive maintenance stocking — having the correct hub on the shelf means you are not down while waiting for a part when the original fails
Why Buy From Air Compressor Factory Direct?
- Authorized Cast Air dealer since 2016 — we stock the complete SD and SK hub range
- Free shipping on qualifying orders
- Free technical support at 866-217-0063 — call if you need help confirming whether your compressor uses SD or SK series hubs, or to cross-reference the correct shaft size
- Full manufacturer warranty on every Cast Air part
Call 866-217-0063 for quick answers and help!
