Single-stage recip pump rebuild
Top-end rebuild on a single-stage reciprocating pump: valves, rings, gaskets, and break-in.
This guide is general field reference for trained, qualified service technicians. It is not a substitute for the original equipment manufacturer's service manual, applicable codes (NEC, OSHA, ASME, local), or the judgment of a licensed professional. Procedures, torque values, pressures, refrigerants, and safety requirements vary by make, model, jurisdiction, and revision. Always verify against current OEM documentation and follow your employer's safety program. Working on compressed-air, electrical, pressure-vessel, and refrigerant systems can cause serious injury or death. By using this guide you accept all risk; AirCompDX and its authors disclaim all warranties and are not liable for any damage, injury, loss, or code violation arising from its use.
LOTO the disconnect, bleed tank to 0 psi, and let the head cool below 120°F before disassembly. Crankcase oil is hot — drain into a catch pan.
- Metric/SAE socket set, 1/4 and 3/8 drive
- Torque wrench (10–80 ft-lb)
- Feeler gauges (0.0015"–0.020")
- Ring compressor and ring expander
- Plastigauge or bore micrometer
- Brass drift, dead-blow hammer
- Lapping plate or fine flat stone for valve plate
- OEM head gasket kit (head, valve plate, base)
- Reed/disc valve set or full valve plate assembly
- Piston ring set (compression + oil)
- Crankcase oil — ISO 100 or OEM compressor oil
- Intake filter element
- 1LOTO and depressurize
Open the disconnect, apply LOTO, verify zero volts to ground, then bleed the tank to 0 psi through the drain. Disconnect the discharge tube at the head.
- 2Remove shrouds and intake filter
Pull the belt guard, flywheel shroud, and intake filter housing. Mark the head bolt pattern with a sharpie to keep orientation on reassembly.
- 3Unbolt the head in cross pattern
Loosen head bolts in two passes, working corner-to-corner. Lift the head straight up — never pry on the gasket surface. Set the valve plate aside in order.
- 4Inspect valve plate and reeds
Check reeds for chips, curl, or carbon glaze. Lay the valve plate on a glass surface and check flatness with a feeler — any gap >0.002" means replace, not lap. Carbon on the discharge side = overheating or low oil.
- 5Pull piston and inspect bore
Rotate the flywheel to BDC, unbolt the connecting rod cap, push the piston up and out the top. Inspect the cylinder bore for scoring, glaze, and taper. >0.005" taper or visible scoring = sleeve or replace pump.
- 6Ring service
Roll old rings off with a ring expander. Clean the ring grooves with a broken piece of the old top ring. Install new rings with the gaps spaced 120° apart, oil ring last.
- 7Reinstall piston
Oil the bore and rings, compress with a ring compressor, tap the piston in with a wood dowel. Torque the rod cap to spec (typically 12–18 ft-lb on cast pumps) in two passes.
- 8Refit valve plate and head
New gaskets dry unless OEM specifies sealant. Lower valve plate onto dowels, then head. Torque head bolts in a star pattern in three passes to final spec (typically 20–30 ft-lb on cast iron heads).
- 9Refill oil and prime
Drain old crankcase oil. Refill to sight glass center with OEM-spec compressor oil (ISO 100 non-detergent on most cast splash-lube pumps). Bar the pump over by hand 10 revolutions to wet the bore.
- 10Break-in run
Restore power. Run unloaded with discharge open for 30 min, then loaded to 50% pressure for 1 hour. Cycle to full pressure and check head temp (<350°F at head, <180°F crankcase). Re-torque head bolts cold after 24 hours of run time.