Crankshaft & connecting rod bearing service
Inspect and replace crank main and rod bearings on a splash- or pressure-lubricated reciprocating pump, including clearance checks.
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.
Fully drain hot oil into a rated container. Cast-iron pump bodies are heavy and awkward — get help or use a lift. A dropped crankshaft ends the job.
- Plastigauge or micrometer set and bore gauge
- Torque wrench with angle gauge if the OEM specifies torque-to-yield
- Bearing puller / press
- Dial indicator for end play
- Seal driver set
- Main and rod bearing set (verify standard vs undersize)
- Front and rear crank seals
- Complete gasket kit
- Oil pump kit for pressure-lubricated pumps
- OEM-spec crankcase oil
- 1Diagnose before disassembly
Knock under load, low oil pressure hot (below 15–20 psi on a pressure-lube pump), or bronze/aluminum glitter in the drained oil all point to bearings. A cold-only knock is more often a loose flywheel or wrist pin.
- 2Drain and open
LOTO, bleed the tank, drain the crankcase warm so contaminants come out with the oil. Remove belt guard, flywheel, and the crankcase cover.
- 3Measure end play
Dial-indicate crank axial float before removing anything — typically 0.004–0.012". Excess end play means thrust surfaces are worn.
- 4Check rod journal clearance
Lay Plastigauge across the journal, torque the cap to spec without rotating, then remove and read. Typical 0.0015–0.0025". Out of spec with a good journal = new bearings; scored journal = grind undersize or replace the crank.
- 5Inspect journals
Look for scoring, heat discoloration (blue), and taper. Any measurable taper or a scratch you can catch a fingernail on means the crank goes to a machine shop.
- 6Replace bearings
Install new shells dry into clean saddles with the tang seated in its notch. Never file a bearing cap or shim a shell.
- 7Service the oil pump
On pressure-lubricated pumps, check gear-to-body clearance and the pressure relief spring; replace the pickup screen. Prime the pump before closing up.
- 8Assemble and torque
Lube journals with assembly lube, torque caps in sequence, and confirm the crank turns smoothly by hand at every stage. Install new seals with a driver, square to the bore.
- 9Fill and verify
Refill to the sight-glass mid mark. Bar over ten revolutions, then run unloaded and confirm oil pressure (20–35 psi hot on pressure-lube pumps) and crankcase temperature below 180°F.