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Cummins/Onan BF Engine

The Cummins/Onan BF is a 40.3 L 2-cylinder engine rated at 3600 rpm, producing up to 11.9 kW. It powers generator sets from Cummins/Onan, including the 5.0BF - 34 3E and S.OSF - 34 IE. It holds 2 L (0.5 gal) of oil.

Also known as: Onan BF

40.3 L

Displacement

Cylinders

3600 rpm

Rated speed

11 kW

Gross power

On this pageSpecs Generator sets Maintenance FAQ Service intervals Torque & specs Fluids Procedures

Engine specifications

Oil capacity	2.0 L
Displacement	40.3 L
Cylinders	2
Rated speed	3600 rpm
Gross power	11 kW

Generator sets powered by this engine

Cummins/Onan 5.0BF - 34 3ECummins/Onan S.OSF - 34 IE

How to read your Cummins/Onan engine serial & model number →

Troubleshooting

Fault codes

Code	Meaning	Remedy
BF	BF
BFA	BFA
1	Engine does not crank.	Check remote start control wires for continuity between the generator set and the remote start panel. Are control wires sized large enough to avoid excessive voltage drop? Replace control wires with correct wire gauge if necessary.
2	Engine cranks but does not start.	Check all terminal connections on K2 crank ignition relay. If OK, replace K2 relay. Does the fuel pump operate during cranking? Check the fuel pump lead wire connections and repair or replace as necessary.
3	Engine starts but stops when start switch is released.	Add oil as required. If oil level is OK, disconnect the low oil pressure switch lead wire and ground it to check if the engine starts and runs. Check low oil pressure switch operation and replace if necessary.
4	Engine is running - then stops.	Connect a voltmeter between remote connector plug terminal 6 and ground to check battery voltage. Replace fuse F2 if open.
5	Low battery - no charge rate.	Check condition of battery and wire connection between CR1 diode, K1 start solenoid, R2 resistor, and battery. Replace defective CR1 diode or R2 resistor if necessary.
6	Running Time Meter Inoperative.	Replace defective running time meter if necessary.
7	Battery Condition Meter Inoperative.	Replace fuse F1. Check wire connections between remote connector plug terminal 5 and the battery condition meter, and between the battery condition meter and ground. Use voltmeter to measure voltage between battery charge meter positive terminal and ground.

Symptoms & remedies

Symptom	Cause	Remedy
NO AC OUTPUT	. Blow fuse or circuit breaker (if used). Then replace fuse or reset circuit breaker. . Disconnected wire or lead on brushes. . Brushes not making contact with slip rings may be excessively worn.	. Look for cause and repair.
LOW AC OUTPUT	. External short circuit on line. Circuit problem. . Generator overloaded. . Remove part of load. . Shorted or grounded circuit in field or armature winding.	. Test for open, grounded, or shorted windings. Repair or replace as necessary. . Engine not running properly causing generator to slow down.
NOISY GENERATOR	. Defective bearing in end bell. . Brush rig loose. . Armature and field frame rubbing together.	. Replace bearing. . Retorque. . Check for generator misalignment and for varnish lumps between armature and field. Repair as necessary. . Turn commutator on lathe; undercut mica insulation.
GENERATOR OVERHEATS	. Windings and parts covered with oil or dirt.	. Clean generator. . Take necessary steps to allow for proper cooling.
Air intake restricted or incoming air too hot.		. Take necessary steps to allow for proper cooling.
Shorted, open or grounded windings in armature or field.		. Test for open, grounded or shorted windings. Repair or replace as necessary.
IT STOPS	Faulty Ignition . See 4 above (CRANKS BUT WON'T START). Out of Fuel. Refill fuel tank. Low Oil Level. Check oil levei. if low or empty, refill to proper level.
LAMP INOPERATIVE		Replace F1 fuse on control.
Low or Discharged Battery		Faulty Starter Faulty Start Solenoid Ignition Timing Wrong Spark Plug Gap Worn Points or Improper Gap Setting Bad Ignition Coil or Condenser Faulty Spark Plug Wires
FUEL SYSTEM		Out of Fuel - Check Lean Fuel Mixture - Readjust Rich Fuel Mixture or Choke Stuck Engine Flooded Poor Quality Fuel Dirty Carburetor Dirty Air Cleaner Dirty Fuel Filter Defective Fue! Pump Wrong Valve Clearance Broken Valve Spring Valve or Valve Seal Leaking Piston Rings Worn or Broken Wrong Bearing Clearance Poor Air Circulation Dirty or Oily Cooling Fins Blown Head Gasker Insufficient Coolant Faulty Thermostat Worn Water Pump or Pump Seal Water Passages Restricted Defective Gaskets

Fault codes and symptom→remedy steps extracted from this engine’s service/owner documentation; verify against the official manual before servicing.

Common problems

Unsatisfactory or erratic operation of the generator
Cause: Improper governor operation causing over, under, and fluctuating engine speeds
Fix: Check the entire electrical system with a continuity tester or ohmmeter
Plugged or clogged cooling fins
Fix: Check and clean cooling fins at least every 50 hours. Remove any dust, dirt or oil which may have accumulated. Plugged or clogged cooling fins can cause overheating and engine damage.
Overheating
Cause: Plugged or clogged cooling fins
Fix: Check and clean cooling fins at least every 50 hours. Remove any dust, dirt or oil which may have accumulated.
Loss of power, flooding, hard starting and overheating
Cause: Air cleaner becomes too dirty
Fix: Clean the air cleaner every 25 hours of operation or more often under extremely dusty conditions. Remove, clean and oil the paper element and polyurethane pre-cleaner as needed.
Loss of auxiliary power
Fix: 1. Press the reset button(s) on the circuit breaker(s). If the button appears to reset, start the engine to determine whether auxiliary power is now available. 2. Measure the load being drawn. Total load should not exceed 5KW. Check appliance, tool, or cord plugs for shorts.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan BF.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Air Filter Element (3 inch height)		140-1216		Amazon eBay NAPA $16.72
Fuel filter			—
Oil filter			122-0338	100 hr	Amazon eBay $13.57
Battery	This cranking motor requires no oiling but when it is dismantled, lubrication should be done as instructed in Disassembly Section.		—
Belt			—
Coolant filter			—
Oil	API SE or SE/CC	2.13 L	—	50 hr
Spark plug	0.025 inch gap		167-0241	100 hr	Amazon Amazon eBay $34.86 eBay $17.97

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.

Repair guides

Step-by-step procedures compiled from the Cummins/Onan service manual for this engine.

Service intervals

Item	Action	Hours	Notes
Air Cleaner	Clean	25 hr	Clean and oil polyurethane pre-cleaner every 25 hours; more often in extremely dusty conditions
Crankcase Oil	Drain	25 hr	First service — Drain initial oil fill at 25 operating hours
Cooling Fins	Clean	50 hr	Check and clean cooling fins at least every 50 hours
Crankcase Oil	Change	50 hr	Change every 50 operating hours when engine is warm
Spark Plugs	Check	100 hr	Check, clean and reset every 100 operating hours
Breaker Points	Change	200 hr	Change every 200 hours of operation
Valve Clearance	Adjust	400 hr	Check and adjust at least every 400 operating hours or when poor performance is noticed
Generator Brushes, Collector Rings and Bearing	Inspect	500 hr	Periodic inspection every 500 hours

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Service specifications

Type	Item	Value	Unit	Condition
Clearance	Camshaft Bearing to Camshaft	0.0015-0.0030	in	at 70°F
Clearance	Camshaft Center Pin Extension	3/4	in	from end of camshaft
Clearance	Camshaft End Play	0.003	in	minimum
Clearance	Carburetor Float Clearance	1/8	in	float resting lightly against needle and seat
Clearance	Connecting Rod End Play	0.002-0.016	in	at 70°F
Clearance	Crankshaft End Play	0.006-0.012	in	at 70°F
Clearance	Crankshaft Main Bearing Journal - Standard Size	1.9992-2.0000	in	at 70°F
Clearance	Crankshaft Main Bearing Journal to Bearing Clearance	0.0025-0.0038	in	at 70°F
Clearance	Crankshaft Rod Bearing Journal - Standard Size	1.6252-1.6260	in	at 70°F
Clearance	Crankshaft Rod Journal to Rod Bearing	0.0020-0.0033	in	at 70°F
Clearance	Cylinder Bore - Standard Size	3.1245-3.1255	in	at 70°F
Clearance	Cylinder Out of Round Maximum	0.002	in	rebore and hone if exceeded
Clearance	Cylinder Taper Maximum	0.005	in	rebore and hone if exceeded
Clearance	Gear Cover Oil Seal Depth	31/32	in	from mounting face of cover
Clearance	Generator Brush Wear Limit	5/16	in	replace when worn to this length or less
Clearance	Governor Cup Travel	7/32	in	in and out travel
Clearance	Governor Linkage Clearance	1/32	in	engine stopped; tension on governor spring
Clearance	Oil Pump Gear Backlash	0.002-0.005	in	at 70°F
Clearance	Piston Clearance in Cylinder	0.001-0.003	in	at 70°F; measured .10 below oil control ring, 90° from pin
Clearance	Piston Pin in Piston	0.0002-0.0004	in	at 70°F
Clearance	Piston Pin in Rod	0.0002-0.0007	in	at 70°F
Clearance	Piston Ring Gap in Cylinder	0.010-0.020	in	at 70°F
Clearance	Piston Ring Side Clearance (Top Compression Ring)	0.008	in	replace piston when reached
Clearance	Roll Pin Protrusion	3/4	in	from gear cover mounting surface
Clearance	Starter Brush Wear Limit	.35	in	replace when worn to less than this length
Clearance	Tappet to Cylinder Block Clearance	0.0015-0.0030	in	at 70°F
Clearance	Throttle Stop Screw Clearance	1/32	in	no load
Clearance	Timing Gear Backlash	0.002-0.003	in	at 70°F
Clearance	Valve Seat Width	1/32-1/8	in
Clearance	Valve Stem to Guide - Exhaust	0.0035-0.0040	in	at 70°F
Clearance	Valve Stem to Guide - Intake	0.0010-0.0025	in	at 70°F
Gap	Breaker Point Gap	.025	in	Full Separation
Gap	Spark Plug Gap	.025	in
Other	Ignition Timing	21° BTC	° BTC	HOT
Other	Ignition Timing	25° BTC	° BTC	COLD
Other	Starter Maximum Current Draw	32	Amperes	no-load test
Other	Valve Face Angle	44	°
Other	Valve Seat Angle	45	°
Speed	Idle Speed	2200-2300	rpm
Speed	Maximum Full Load Speed	3600	rpm	continuous operation
Speed	Starter No-Load Test RPM	4800-6000	rpm
Torque	Connecting Rod Bolt	14-16	ft-lb
Torque	Cylinder Head Stud Nuts	14-16	ft-lb	Cold
Torque	Flywheel Cap Screw	35-40	ft-lb
Torque	Gearcase Cover	8-10	ft-lb
Torque	Manifold Mounting Stud Nuts	6-9	ft-lb
Torque	Oil Base	18-23	ft-lb
Torque	Oil Pump	7-9	ft-lb
Torque	Rear Bearing Plate Screws	25-27	ft-lb
Torque	Spark Plug	15-20	ft-lb
Torque	Starter Mounting Bolts	18-20	ft-lb
Torque	Stud and Nuts	8-10	ft-lb
Valve clearance	Exhaust Valve Tappet Adjustment	0.012-0.014	in	Cold
Valve clearance	Intake Valve Tappet Adjustment	0.007-0.009	in	Cold

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Recommended fluids

Fluid	Grade	Standard	Capacity	Notes
Engine oil	5W30 or 10W	API SE or SE/CC		For temperatures below 30°F
Engine oil	SAE 30	API SE or SE/CC	2.0 U.S. quarts	For temperatures above 30°F; use 4 pints (4-1/2 if equipped with oil filter)
Fuel	Regular grade automotive gasoline		5.6 U.S. gallons	Nonleaded or regular grade; do not use highly leaded premium. Tank capacity 5.6 U.S. gallons.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Procedures

Carburetor Adjustment adjustment

⚠ Damage will result if needle is forced against its seat.

Run engine to normal operating temperature.
Make idle adjustment under no load.
Open main jet until engine runs smooth under acceleration with no load.
Open about 1/4 turn further if needed for sudden load.
Set throttle stop screw to 1/32 in clearance.
If hunting, open main needle a little more; do not exceed 1/2 turn beyond maximum power.

Carburetor Float Adjustment adjustment

Invert float assembly and casting.
With float resting lightly against needle and seat, clearance between bowl cover gasket and free end of float should be 1/8 in.
If reset needed, bend float tangs near pin to obtain 1/8 in clearance.

Governor Adjustment adjustment

Tools: tachometer

Adjust carburetor main jet for best full load mixture.
Adjust carburetor idle needle with no load.
Adjust length of governor linkage.
Check governor linkage and throttle shaft for binding or looseness.
Adjust governor spring tension for nominal no-load speed.
Check rpm drop from no-load to full-load and adjust sensitivity.
Recheck speed adjustment.
Set carburetor throttle stop screw.

Tappet Adjustment adjustment

Tools: 7/16 in open end wrench, 9/16 in open end wrench, feeler gauge

Remove parts to gain access to valve tappets.
Remove spark plugs to ease turning crankshaft.
Rotate flywheel until left intake valve opens and closes; align TC mark. Both valves closed.
Intake: .007 in gauge should pass freely; .009 in should not.
Exhaust: .012 in gauge should pass freely; .014 in should not.
Use 7/16 in open end wrench to turn adjusting screw; hold tappet with 9/16 in wrench.
For right cylinder, turn engine one revolution and align TC mark; repeat.
Replace all parts and tighten screws; torque manifold bolts.

Breaker Points Replacement (Cold Setting) maintenance

Tools: screwdriver, flat thickness gauge

Remove control box cover screws and cover.
Disconnect wire from control box to ignition coil; remove front panel.
Remove breaker box cover.
Remove spark plugs to rotate crankshaft by hand.
Remove points mounting nut and disconnect leads.
Replace condenser and points.
Rotate crankshaft clockwise to align 25° BTC mark; turn 1/4 turn more.
Set point gap to .025 in; tighten nut and recheck.

Generator Brush Inspection maintenance

⚠ Do not use a substitute brush.

Remove generator fan guard and fan.
Detach brush guide by removing end bell screws.
Tag leads 5 and 6; unscrew brush retaining screws and pull brushes.
Inspect brush faces for smooth coppery cast; no deep grooves.
Replace brushes if worn to less than 5/16 in length.
Work brushes up and down in guide to ensure no binding.

Out-of-Service Protection (More Than 30 Days) maintenance

Run engine until thoroughly warm.
Turn off fuel supply and run until engine stops.
Drain oil while warm; refill and attach warning tag.
Remove spark plugs; pour 1 ounce rust inhibitor or SAE50 oil into each cylinder; crank and reinstall plugs.
Service air cleaner.
Clean governor linkage and wrap with clean cloth.
Plug exhaust outlets; wipe unit and coat rustable parts.
Disconnect battery; provide suitable cover.

Electric Starting operation

⚠ Do not touch welder terminals while operating.

Adjust choke according to temperature conditions.
Move ignition switch to ON.
Push START switch firmly. If unit does not start within a few seconds, release and wait.
If unit does not start after two attempts, open choke and repeat starting sequence.

Fuel Pump Disassembly repair

⚠ Use care when reassembling; all parts must be perfectly aligned.

Remove vacuum line and fuel line.
Remove two fuel pump attaching screws.
Grasp pump and carefully pull apart.
Diaphragm, plunger, return spring, pump body and gaskets will be loose.
Internal parts are available in a repair kit.

Extracted from the manufacturer documentation linked to this model. Values are copied as printed; confirm against your unit's manual and serial number.

Common questions

What is the oil capacity of the Cummins/Onan BF?

The Cummins/Onan BF engine has an oil capacity of 2.0 liters.

What is the displacement of the Cummins/Onan BF?

The Cummins/Onan BF has a displacement of 40.3 liters with 2 cylinders.

Which generators use the Cummins/Onan BF engine?

The Cummins/Onan BF powers 2 generator set(s), including Cummins/Onan 5.0BF - 34 3E, Cummins/Onan S.OSF - 34 IE.

How often should you change the oil in the Cummins/Onan BF?

The Cummins/Onan BF oil change interval is every 50 hours.

What is a common problem with the Cummins/Onan BF engine?

A commonly reported issue on generators using the Cummins/Onan BF is: Unsatisfactory or erratic operation of the generator. Cause: Improper governor operation causing over, under, and fluctuating engine speeds. Fix: Check the entire electrical system with a continuity tester or ohmmeter.