The Castair PUK65 is a 4 to 5 HP single-stage cast-iron bare pump with a W3 (three-cylinder) configuration, 10-inch 1A flywheel, and 28-ounce oil capacity. It weighs 70 pounds. This is the pump for building a custom compressor unit from major components, replacing a failed pump in an existing tank-and-motor setup, or upgrading from an underpowered pump to a larger cylinder configuration. At $605.99, the PUK65 is the starting point for a serious piston compressor build in the 4-5 HP range.
Why Cast Iron vs. Aluminum
Cast-iron cylinders dissipate heat better than aluminum, resist bore wear longer, and hold machined tolerances through thousands of thermal cycles. An aluminum pump on a busy shop compressor runs hotter, wears the bore faster, and loses compression ring seal earlier. Oil-less pumps avoid lubrication maintenance entirely but sacrifice longevity significantly — Castair specifies that their cast-iron oil-lubricated pumps offer up to 10 times the service life of oil-less designs. For a production shop or contractor running this pump daily, the cast-iron construction pays for the price difference within the first couple of years of service through reduced downtime and parts replacement.
Key Specifications
- HP rating: 4 to 5 HP (motor supplied separately)
- Stage: Single stage
- Cylinder configuration: W3 (three cylinder)
- Flywheel: 10-inch diameter, 1A
- Oil capacity: 28 oz
- Weight: 70 lbs
- Warranty: 1 year parts when properly installed and maintained
- Price: $605.99
Construction Features
- Cast-iron cylinder construction: Superior heat dissipation and wear resistance
- W3 cylinder design: Three-cylinder configuration for smooth airflow and compact dimensions relative to output
- Two-piece connecting rod: Independent wrist pin for proper oil distribution and reduced piston side loading
- Three-ring piston: Oil control, compression, and wiper rings for clean, efficient compression
- Ball bearings at crankshaft ends: Reduced friction and longer bearing service life
- 10-inch flywheel: Proper belt drive sizing for 4-5 HP motor range and standard single-groove belt
- 28-oz oil capacity: Adequate for extended run cycles without overheating the crankcase oil
Castair Bare Pump Lineup — Where the PUK65 Fits
Castair publishes the following bare pump options. The PUK65 sits in the middle of the single-stage range:
- PK20 — 1.5 HP, 20 lbs, single, 1 cyl, 7.5" flywheel, 8 oz oil
- PK30 — 1.5 HP, 45 lbs, single, 1 cyl, 10" flywheel, 8 oz oil
- PK50 — 2 HP, 50 lbs, single, V2 cyl, 10" flywheel, 16 oz oil
- PK60 — 3.5 HP, 55 lbs, single, V2 cyl, 10" flywheel, 24 oz oil
- PUK65 — 4-5 HP, 70 lbs, single, W3 cyl, 10-1A flywheel, 28 oz oil ($605.99) — this unit
- VA80 — 3.5-5 HP, 110 lbs, single, V2 cyl, 12.5-2A flywheel, 1 qt oil
- C1 — 4-7.5 HP, 225 lbs, two-stage, inline 2 cyl, 16.875" flywheel, 2 qt oil
- C2 — 7.5-10 HP, 320 lbs, two-stage, V4 cyl, 19.0" flywheel, 2 qt oil
When Do You Need the PUK65 vs. Adjacent Pumps?
The PUK65 is correct when you have a 4 or 5 HP motor and need a matched single-stage pump. The W3 cylinder design makes it more compact than the VA80, which uses a larger V2 configuration at similar HP ratings — useful when fitting into an existing belt-guard configuration where space is constrained. If your motor is 3.5 HP, the PK60 is a better match. If you need two-stage compression — for working pressures above 125 PSI or better efficiency at high continuous output — step to the C1.
For a replacement application, match the flywheel diameter to your existing belt guard and sheave setup. The PUK65 uses a 10-inch 1A flywheel, which fits the standard belt arrangements on Castair units in the 4-5 HP range. Confirm your pulley groove count before ordering.
Technical Specifications
- Part Number: PUK65
- Manufacturer: Castair (Cast Air)
- HP range: 4-5 HP
- Stages: Single
- Cylinders: W3 (three-cylinder)
- Flywheel OD: 10" — 1A
- Oil capacity: 28 oz
- Weight: 70 lbs
- Warranty: 1 year parts
- Price: $605.99
What's in the Box
- 1x Castair PUK65 cast-iron bare pump with 10-inch flywheel installed
- Note: Motor, tank, pressure switch, belts, and motor pulley are sold separately
Why Buy From Air Compressor Factory Direct?
We carry Castair bare pumps for build and replacement work. If you are building a compressor from components or replacing a failed pump, call us before ordering — we can confirm the PUK65 is the right pump for your motor HP and RPM rating, and check whether you need matching belts and pulleys for your specific setup. Free shipping on qualifying orders. Full Castair warranty applies.
Call 866-217-0063 for quick answers and help!
