Two-stage recip pump rebuild
HP + LP cylinder rebuild on a two-stage pump including intercooler service and stage balance.
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.
Intercooler tubes hold residual pressure even with the tank drained — crack the cooler unions slowly. LP head and HP head torque values differ; do not mix.
- Full socket set, torque wrench (10–120 ft-lb)
- Inside micrometer or bore gauge
- Ring compressor + expander
- Pin punch set
- Intercooler line wrench (often 7/8" and 1")
- Manometer or low-range pressure gauge (0–60 psi) for interstage check
- Complete OEM gasket kit (LP head, HP head, valve plates, base, intercooler)
- LP and HP valve plate assemblies (do not swap)
- LP and HP ring sets
- Intercooler O-rings or copper crush washers
- Compressor oil — ISO 100 (or OEM synthetic on continuous-duty units)
- 1LOTO, depressurize, drain intercooler
Apply LOTO, bleed tank to 0 psi, then crack each intercooler union 1/2 turn to release trapped interstage pressure before fully disconnecting.
- 2Document orientation
Photograph the intercooler routing and head orientation. LP and HP cylinders look similar on some twin-V pumps but the heads are not interchangeable.
- 3Pull both heads
Loosen head bolts in cross pattern, two passes. Keep LP and HP valve plates segregated — gasket thickness and reed lift differ.
- 4Service LP cylinder
Pull the LP piston, replace rings, inspect bore. LP bore is larger and tends to glaze before it scores; deglaze with a brake-hone for 10 seconds in a crosshatch pattern if rings are reusable.
- 5Service HP cylinder
Pull the HP piston. The HP bore runs hotter; expect more carbon at the top of the bore. Measure ring end gap on new rings (typ. 0.003"–0.005" per inch of bore).
- 6Inspect intercooler
Pressure-test the intercooler to 1.5× working pressure with shop air and soapy water. Replace if any tube weep or fin damage that blocks >25% of fin area.
- 7Reassemble in stages
Install both pistons, both valve plates, then both heads finger-tight. Torque LP head first to spec, then HP head — alternating between heads prevents block distortion.
- 8Reconnect intercooler
New O-rings or copper washers every time. Torque unions to spec; over-torque cracks the cooler casting.
- 9Oil change and pre-lube
Refill crankcase to mid-sight-glass. Bar over 10 revolutions by hand and check for binding before energizing.
- 10Stage balance check
After 1 hour of loaded run, measure interstage pressure at the intercooler tap. Healthy two-stage runs ~35–45 psi interstage with discharge at 175 psi. Low interstage = LP valves leaking; high interstage = HP intake restricted or HP valves leaking.
- 11Final commissioning
Re-torque all head bolts cold after 24 h. Log final discharge temp (<350°F), interstage psi, amp draw on all phases, and oil level. File with the unit PM record.