Castair S755 — 3.5 HP Single-Stage Air Compressor Bare Pump
The Castair S755 is a 3.5 horsepower, single-stage, two-cylinder bare compressor pump built for shops and contractors who need a reliable cast iron pump head — either as a replacement for a worn pump or as the core of a custom compressor build. With a 12-inch flywheel OD and 16 oz. oil capacity, this is a splash-lubricated, oil-lubricated cast iron pump sized for continuous service in auto shops, body shops, small fabrication operations, and general commercial environments.
A bare pump is exactly what it says: the pump assembly only — no motor, no tank, no pressure switch, no controls. You bolt it to your existing tank and motor setup. That is the practical reason shops buy bare pumps — when the pump fails but the motor and tank are still serviceable, you replace the pump head and extend the life of your compressor infrastructure without buying a complete unit.
Pump Construction — What Makes Cast Air Different
- All Cast Iron: Cylinders, crankcase, heads, and valve plates are cast iron — not aluminum. Cast iron dissipates heat better, handles thermal cycling, and holds up to the continuous compression cycles a shop compressor runs every day
- Oil-Lubricated Splash System: The 16 oz. crankcase oil lubricates cylinder walls, rings, and rod bearings on every stroke. Oil-lubricated pumps last significantly longer and run cooler than oil-less designs
- I2 Cylinder Design: Inline twin cylinders deliver balanced compression with less vibration than a single-cylinder pump at equivalent HP
- Disc and Spring Valves: More durable and easier to service than reed valves. When you eventually service the valves, they come apart cleanly and install correctly
- 4-Ring Pistons: Four rings per piston means better sealing, less oil carry-over into the air stream, and longer intervals between ring replacement
- 12-Inch Flywheel OD: Factory spec for standard belt-drive sizing in the 3–4 HP range. Sized for correct belt selection when paired with a 1725 RPM motor using standard sheave ratios
Technical Specifications
- SKU: S755
- Horsepower: 3.5 HP
- Pump Type: Single-stage, reciprocating piston
- Cylinder Configuration: I2 (inline 2-cylinder)
- Flywheel OD: 12 inches
- Oil Capacity: 16 oz.
- Lubrication: Oil (splash lubricated)
- Component Type: Bare pump — no motor, tank, or controls included
- Manufacturer: Cast Air (Castair), family-owned since 1991
- Price: $445.06
Note: CFM output and max PSI ratings are dependent on RPM and pulley configuration. Contact us at 866-217-0063 with your motor HP, motor RPM, and intended tank size for correct sizing guidance before ordering.
What's in the Box
- Cast Air S755 bare pump assembly
- No motor, tank, belt, or controls — these are sourced separately or existing components are reused
Available Configurations — Where S755 Fits in the Castair Pump Lineup
- S755 — 3.5 HP, I2 cylinder, 12" flywheel, 16 oz. oil: Entry-level commercial pump — suits smaller single-phase motor setups and replacement pumps on 30–60 gallon tanks ($445.06)
- VA80 — 4–5 HP, V2 cylinder, 12.5" flywheel, 48 oz. oil: Step up in oil capacity and HP range — better for higher-demand single-phase applications with a larger tank ($1,800.56)
When Does a Shop Need the S755 vs. Stepping Up or Down
The S755 is right for you when:
- Your existing pump on a 30–60 gallon tank has failed and the motor and tank are still serviceable
- You are building a compact custom compressor for a specific 3–4 HP application
- You run a two-bay shop with intermittent air demand — impact wrenches, air ratchets, blow guns, tire inflation — not continuous spray painting or sandblasting
- You want a Cast Air pump at the entry price point
Step up to the VA80 when:
- You need more oil capacity (48 oz. vs. 16 oz.) for longer run cycles
- Your application is in the 4–5 HP range rather than 3–3.5 HP
- You are running spray equipment or DA sanders with extended duty cycles
Consider a complete compressor unit when:
- Your motor or tank is also at end of life — buying individual components adds up quickly at that point
- You need more than 5 HP — at that level, a Cast Air complete industrial-series unit is typically more cost-effective
Why Buy From Air Compressor Factory Direct?
- Authorized Cast Air dealer since 2016 — we know the lineup and can help match the right pump to your application
- Free shipping on qualifying orders
- Free technical support — call 866-217-0063 before you order if you need help confirming flywheel sizing, belt selection, or pump-to-motor compatibility
- Full manufacturer warranty on every Cast Air pump we sell
- Honest guidance — if the S755 is not the right pump for your application, we will tell you before you order
Call 866-217-0063 for quick answers and help!
