The Castair PK20 is a 1.5 HP single-stage, single-cylinder bare pump with a 7.5" (1A-series) flywheel and 8 oz. oil capacity. This is the pump assembly only — no motor, no tank, no belt. It is the replacement pump head for Cast Air single-cylinder compressor systems, or the core component when building a custom 1.5 HP compressor from components. At this power level, the PK20 handles light to medium shop air demand: inflation, air tools in low-cycle applications, and blow guns.
Note: Cast Air did not provide a dedicated manufacturer product page for this pump. Specifications are drawn from the product listing. Contact us to confirm CFM output, max PSI rating, and specific compatibility with your existing motor and tank configuration before ordering.
What Is a Bare Pump?
A bare pump — sometimes called a pump head or pump assembly — is the compression mechanism without the motor, tank, or associated hardware. You need to supply the motor (correctly matched to the pump's HP rating and RPM requirements), a pulley and belt drive, a tank sized to your CFM demand, and the connecting hardware. The PK20 is the correct choice when:
- The pump on an existing Cast Air compressor has failed and the motor and tank are still serviceable
- You are building a custom compressor from components and need the pump assembly
- You are upgrading from a smaller pump to a 1.5 HP single-cylinder unit while keeping the existing motor and tank
Pump Specification Details
- Part Number: PK20
- HP Rating: 1.5 HP
- Stage: Single stage
- Cylinders: 1
- Flywheel OD: 7.5" (1A-series flywheel)
- Oil Capacity: 8 oz.
- Drive: Belt drive (flywheel and belt not included)
- Price: $194.81
Single Stage vs. Two Stage — Which Applies Here
Single-stage pumps compress air in one step from atmospheric pressure to the tank pressure. For most shop air applications below 150 PSI, single-stage is the standard. The PK20 is a single-stage design, which is appropriate for typical shop pressures (90-135 PSI working pressure). If your application requires sustained pressures above 150 PSI, a two-stage pump is the correct choice — the single-stage PK20 is not rated for that.
Oil-Lubricated Design
The PK20 is an oil-lubricated pump with an 8 oz. oil sump. Oil-lubricated pumps run cooler, last longer under continuous duty, and are rebuildable — replacement rings, valves, and gaskets are available for Cast Air oil-lube pumps. The tradeoff compared to oil-free designs is the maintenance requirement (oil changes at specified intervals) and the minor oil carry-over into the air stream, which matters for some applications (food, paint). For general shop use, oil-lubricated is the standard and preferred choice.
Flywheel and Drive Configuration
The PK20 comes with a 7.5" 1A-series flywheel. The "1A" means the flywheel has one A-section or B-section V-belt groove. To complete the drive, you need:
- A motor pulley sized to achieve the correct pump RPM from your motor's output RPM (typically 900-1,100 RPM at the pump shaft)
- A matched V-belt in the correct length for your drive center distance
- A motor rated for 1.5 HP at the appropriate voltage (115V/230V single-phase or three-phase depending on your power supply)
The drive ratio is determined by the motor pulley OD relative to the flywheel OD. With a 7.5" flywheel and a 1,750 RPM motor, you need a motor pulley of approximately 3.9-4.7" OD to hit the 900-1,100 RPM pump speed range. If you are replacing an existing pump in a complete unit, the motor, motor pulley, and belt are typically already sized correctly.
When Does a Shop Need the PK20?
- Existing single-cylinder Cast Air pump has failed (scored cylinder, broken valve, cracked casting) and the motor and tank are in good condition
- Building a new belt-drive compressor station using components and a 1.5 HP motor is available
- Light to medium air demand: inflation, blow guns, pneumatic nail guns, low-cycle impact use
- Budget-conscious replacement where the motor and tank replacement cost is avoided by swapping only the pump
Cast Air Bare Pump Size Comparison
- PK20 — 1.5 HP, 1-cylinder, 7.5" flywheel, 8 oz. oil — $194.81 (this unit) — light to medium shop air demand
- PK30 — 1.5 HP, 1-cylinder, 10" flywheel, 8 oz. oil — $241.01 — same HP but larger flywheel; check application compatibility
- PK50 — 2 HP, V-2 cylinder, 10" flywheel, 16 oz. oil — $371.91 — step up for higher CFM demand
- PK60 — 3.5 HP, V-2 cylinder, 10" flywheel, 32 oz. oil — $432.74 — heavy shop air applications
Technical Specifications
- Part Number: PK20
- Manufacturer: Cast Air (Castair)
- HP: 1.5
- Stage: Single stage
- Cylinders: 1
- Flywheel OD: 7.5" (1A-series)
- Oil Capacity: 8 oz.
- Drive Type: Belt drive (flywheel included, belt not included)
- Price: $194.81
- CFM/PSI Rating: Not confirmed in listing — contact us for current performance data
- Manufacturer Data: Product page not available — specifications from product listing data
What Is Included
- 1x Castair PK20 bare pump with flywheel
- Oil fill (confirm if pre-filled or dry — do not run dry)
Motor, motor pulley, belt, tank, pressure switch, safety valve, regulator, and connecting hardware are not included. This is the pump assembly only.
Why Buy From Air Compressor Factory Direct?
A bare pump replacement is a more technical purchase than ordering a complete unit. You need to confirm the motor RPM, pulley sizing, and belt length to complete the installation correctly. We have been selling Cast Air pump assemblies since 2016 and can help you match the PK20 to your existing motor and tank configuration — or identify the correct supporting components if you are building from scratch. Free shipping, free tech support, full manufacturer warranty.
- Cast Air specialist since 2016
- Free shipping on all orders
- Free tech support — we help match the pump to your motor and tank
- Full manufacturer warranty
- Component-level expertise — we can source the complete drive kit
Call 866-217-0063 for quick answers and help!
