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Cummins/Onan B48M Engine

Name: Cummins/Onan B48M Engine
Brand: Cummins/Onan

The Cummins/Onan B48M is a 0.78 L 2-cylinder air cooled (pressure) engine rated at 3600 rpm, producing up to 13.4 kW. It powers generator sets from Cummins/Onan, including the B48. It holds 1.6 L (0.4 gal) of oil.

Also known as: Onan B48M

0.782 L

Displacement

Cylinders

3600 rpm

Rated speed

13 kW

Gross power

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

Engine specifications

Oil capacity	1.6 L
Displacement	0.782 L
Cylinders	2
Aspiration	Air Cooled (Pressure)
Rated speed	3600 rpm
Gross power	13 kW

Generator sets powered by this engine

Cummins/Onan B48

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

Common problems

Weakness and Sleepiness
Fix: Get out into fresh air immediately, shut down the unit and do not use until it has been inspected.
Vomiting
Fix: Get out into fresh air immediately, shut down the unit and do not use until it has been inspected.
The reed valve must be flat with no sign of a crease.
Fix: Assemble using a new gasket. Torque capscrew to 1-2 ft-lbs (1 -3 Nm).
Oil pump is worn and needs replacement.
Fix: Install a new pump assembly if any parts are worn.
Muscular Twitching
Fix: Get out into fresh air immediately, shut down the unit and do not use until it has been inspected.
Inability to Think Coherently
Fix: Get out into fresh air immediately, shut down the unit and do not operate until it has been inspected and repaired.
Excess oil can cause high oil consumption, high operating temperatures, and oil foaming.
Fix: Do not overfill crankcase. Run engine until thoroughly warm before draining oil.
Dizziness
Fix: Get out into fresh air immediately, shut down the unit and do not use until it has been inspected.
By-pass valve is not operating normally.
Fix: Inspect plunger action by removing the 3/8 x 24 x 7/8 cap screw, and then remove spring and plunger with a magnet tool.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan B48M.

Maintenance schedule

Item	Spec	Capacity	Part #	Interval
Air filter	Cartridge Type		—	200 hr
Oil filter	Full Flow type		—	100 hr
Oil	API SE or SE/CC classification, SAE 20W-40 or 20W-50 or 10W-30 or 10W-40	2.84 L (without filter), 3.1 L (with filter)	—	100 hr
Spark plug	Gap 0.025 in. (0.64 mm)		—	100 hr

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

Service intervals

Item	Action	Hours	Notes
Engine general	Inspect	8 hr	X1 - Check for fuel leaks; with engine running, visually and audibly check exhaust system for leaks.
Oil level	Check	25 hr	First service — Check with engine off; allow 10 minutes drain-down. Do not operate below ADD mark.
Air Cleaner	Other	25 hr	First service — Perform more often under severe conditions
Air cleaner	Check	25 hr	X2 - Perform more often under severe operating conditions.
Air cleaner polyurethane precleaner	Clean	25 hr	More often under extremely dusty conditions; clean, oil and reinstall over paper element
Crankcase Oil	Change	25 hr	First service — Initial break-in change only
Inspect Engine Generally	Inspect	25 hr	First service — Check for fuel leaks. With engine running, check exhaust system visually and audibly.
Battery electrolyte level	Check	50 hr	First service
Cooling fins	Clean	50 hr	X2 - Perform more often under severe operating conditions.
Breaker Points	Check	100 hr	Replace every 200 hours or if pitted/burned
Crankcase oil (with filter)	Change	100 hr	X3 - Initial break-in check only (25 h).
Crankcase oil (without filter)	Change	100 hr	X2 - Perform more often under severe operating conditions.
Oil filter	Replace	100 hr	First service — X3 - Initial break-in check only.
Spark plugs	Check	100 hr	Regap; replace plugs showing fouling or electrode erosion
Air cleaner element	Replace	200 hr	X2 - Perform more often under severe operating conditions.
Breaker points	Change	200 hr	Change to maintain maximum engine efficiency
Breather valve	Clean	200 hr	X2 - Perform more often under severe operating conditions.
Carbon and Lead Deposits	Clean	200 hr	May extend to 400 hours for unleaded fuel engines
Carbon and lead deposits (cylinder head)	Clean	200 hr	X5 - For engines running on unleaded fuel this interval may be extended to 400 hours.
Compression	Test	200 hr	X4 - For detailed maintenance contact an Onan Service Center or refer to the SERVICE MANUAL.
Compression Check	Test	200 hr
Cylinder heads	Clean	200 hr	Remove for carbon cleaning and gasket change at least every 200 hours; extend to 400 hours for engines running unleaded fuel
Points and condenser	Replace	200 hr
Valve Clearance	Check	200 hr	Adjust when cold to specification
Valve clearance	Adjust	200 hr	X4 - For detailed maintenance contact an Onan Service Center or refer to the SERVICE MANUAL.
Valve tappet clearance	Check	200 hr	Adjust if necessary; also check when poor engine performance is noticed; adjust only at ambient temperature

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	Bearing to crankshaft clearance - aluminum rod	0.0020-0.0033 inch (0.051-0.084 mm)	inch
Clearance	Camshaft bearing bore (bearing installed)	1.3760-1.3770 inch (34.95-34.98 mm)	inch
Clearance	Camshaft bearing clearance	0.0015-0.0030 inch (0.038-0.076 mm)	inch
Clearance	Camshaft bearing journal diameter	1.3740-1.3745 inch (34.90-34.91 mm)	inch
Clearance	Camshaft end play	0.0030 inch (0.076 mm) minimum	inch
Clearance	Camshaft lift - exhaust	0.291-0.299 inch (7.39-7.59 mm)	inch
Clearance	Camshaft lift - intake	0.286-0.295 inch (7.26-7.49 mm)	inch
Clearance	Connecting rod journal diameter	1.6252-1.6260 inch (41.28-41.30 mm)	inch
Clearance	Connecting rod large bore diameter (rod bolts properly torqued)	1.6280-1.6285 inch (41.35-41.36 mm)	inch
Clearance	Connecting rod side clearance	0.0020-0.0160 inch (0.051-0.406 mm)	inch
Clearance	Crankshaft end play	0.006-0.012 inch (0.15-0.30 mm)	inch
Clearance	Cylinder bore honed diameter	3.2490-3.2500 inch (82.53-82.55 mm)	inch
Clearance	Cylinder taper / out-of-round maximum allowable	0.003 inch (0.08 mm)	inch
Clearance	Gear cover oil seal installed depth from front of gear cover	0.645 inch (16.4 mm)	inch
Clearance	Intake valve clearance (stem to guide)	0.0010-0.0025 inch (0.025-0.064 mm)	inch
Clearance	Intake valve stem diameter	0.3425-0.3430 inch (8.70-8.71 mm)	inch
Clearance	Main bearing clearance	0.0025-0.0038 inch (0.064-0.097 mm)	inch
Clearance	Main bearing inside diameter (installed)	2.0015-2.004 inch (50.84-50.90 mm)	inch
Clearance	Main bearing inside diameter (without bearing)	2.187-2.188 inch (55.55-55.58 mm)	inch
Clearance	Main bearing journal diameter	1.9992-2.0000 inch (50.78-50.80 mm)	inch
Clearance	Oil pump gear backlash	0.002-0.003 inch (0.051-0.076 mm)	inch
Clearance	Piston clearance in cylinder (measured 90 deg to pin, 1.187 inch below top of piston)	0.0033-0.0053 inch (0.084-0.135 mm)	inch
Clearance	Piston pin bore	0.68774-0.68814 inch (17.47-17.48 mm)	inch
Clearance	Piston pin bushing bore (finished bore)	0.6879-0.6882 inch (17.47-17.48 mm)	inch
Clearance	Piston ring clearance - top groove	0.003-0.008 inch (0.076-0.203 mm)	inch
Clearance	Piston ring groove width - No. 3 oil control ring	0.188-0.189 inch (4.775-4.801 mm)	inch
Clearance	Piston ring groove width - top 1 compression ring	0.080-0.081 inch (2.032-2.057 mm)	inch
Clearance	Piston ring groove width - top 2 compression ring	0.080-0.081 inch (2.032-2.057 mm)	inch
Clearance	Timing gear backlash	0.002-0.005 inch (0.051-0.127 mm)	inch
Clearance	Valve guide inside diameter	0.7475-0.7480 inch (18.99-19.05 mm)	inch
Clearance	Valve guide protrusion from rocker box side of block	11/32 inch (8.7 mm)	inch
Gap	Breaker Point Gap - Static	0.021	in	Full separation, engine cold
Gap	Breaker point gap	0.021	in (0.53 mm)
Gap	Breaker point gap	0.021 inch (0.53 mm)	inch	equivalent to 21 deg BTC
Gap	Piston ring end gap in cylinder	0.010-0.020 inch (0.254-0.508 mm)	inch
Gap	Spark Plug Gap	0.025	in
Gap	Spark plug gap	0.025 inch (0.635 mm)	inch
Gap	Starter pinion to ring gear lash (free travel)	0.020 +/- 0.010 inch (0.51 +/- 0.25 mm)	inch
Gap	Starter pinion to ring gear lash - maximum acceptable	0.035 inch (0.889 mm)	inch
Injection timing	Ignition timing (static setting)	21	degrees BTDC	static setting
Other	Battery electrolyte specific gravity, fully charged	1.260	specific gravity	77 F (25 C)
Other	Battery electrolyte specific gravity, recharge threshold	1.215	specific gravity
Other	Battery specific gravity	1.260	specific gravity	at 77 deg F (25 deg C)
Other	Camshaft center pin extension from end of camshaft	3/4 inch (19 mm)	inch
Other	Compression ratio	6.2 to 1
Other	Cylinder head temperature limit for torquing or removing heads	below 700 deg F (37 deg C)	deg F	as printed in manual
Other	Cylinder hone crosshatch included angle	23 degrees	deg
Other	Cylinder honing surface finish	20 to 40 RMS	RMS
Other	Governor cup in-and-out travel	7/32 inch (5.6 mm)	inch
Other	Idle mixture screw - Marvel Schebler (back out from lightly seated)	1-1/8	turns
Other	Idle mixture screw - Nikki full adjustment (back out from lightly seated)	3/4	turns
Other	Ignition Timing	21	degrees BTC
Other	Main mixture screw - Marvel Schebler (back out from lightly seated)	1-1/4	turns
Other	Main mixture screw - Nikki full adjustment (back out from lightly seated)	1-1/4 to 1-1/2	turns
Other	Oil by-pass valve plunger diameter	0.3105 to 0.3125 inch (7.89 to 7.94 mm)	inch
Other	Oil by-pass valve spring free length	1.00 inch (25.4 mm)	inch
Other	Oil by-pass valve spring load	2.6 +/- 0.2 lbs (11.6 +/- 0.9 N) when compressed to 0.5 inch (12.7 mm)	lb
Other	Starter brush wear limit - replace if worn shorter than	1/4 inch (6.35 mm)	inch
Other	Tappet clearance in bore	0.0015-0.003 inch (0.038-0.076 mm)	inch
Other	Valve face angle	44 deg	deg

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	Ambient	Capacity	Notes
Engine oil		API SF, SF/CC, or SF/CD		1.7 qts	Use SAE viscosity grade per temperature chart; straight weight oils recommended for severe duty and above 32 F (0 C). Capacity is crankcase without filter (1.6 litre); oil filter adds 0.3 qts (0.3 litre).
Engine oil	viscosity grade per oil viscosity chart in manual	API classification SF, SF/CC, or SF/CD		1.7 qt	Straight weight oils recommended for severe duty use and at temperatures above 32 deg F (0 deg C) for minimum oil consumption; oil capacity 1.7 qts (1.6 litre) without filter; oil filter capacity 0.3 qt (0.3 litre). Change oil more often in extremely dusty, high ambient or low ambient conditions
Engine oil	SAE 20W-40, 20W-50, 10W-30, or 10W-40	API SE or SE/CC classification	-20°C to 40°C	2.84 L without filter, 3.1 L with filter	Use same brand when adding between oil changes. When checking oil, allow minimum 10 minutes for oil to drain.
Fuel	Regular or Unleaded Gasoline	Clean, fresh, unleaded preferred			Avoid leaded premium gasoline. Do not use gasoline de-icers or fuel with alcohol concentration greater than 10%. High octane not necessary.
Grease	multi-purpose grease (seal lips); silicone base grease GE Versilube 322-L for starter armature shaft threads and spline				Fill space between oil seal lips with multi-purpose grease before installing seals

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

Procedures

Breaker Points Replacement and Adjustment adjustment

⚠ Make sure the feeler gauge is clean and free of any grease, oil or dirt.

Tools: Flat feeler gauge, Allen wrench

Remove the spark plugs.
Remove the breaker box cover; rotate the crankshaft clockwise (facing flywheel) until points are fully open.
Remove the condenser (screw A) and detach the condenser lead and coil lead (screw B).
Remove the two Allen screws (C) and lift the breaker assembly from the engine.
Replace the condenser and point assembly with new parts and reinstall in reverse order of removal.
Rotate the crankshaft clockwise by hand until points are fully open.
Using a clean flat feeler gauge, set the point gap to the specified value by adjusting the Allen socket head screw (D).
Replace the breaker box cover, coil wire, spark plugs, and spark plug cables.

Carburetor Adjustment adjustment

⚠ Overtightening the mixture adjustment screw will cause carburetor damage. Turn mixture adjustment screw in only until light tension can be felt.

Tools: screwdriver

Initial settings: Marvel Schebler - idle mixture screw out 1 turn, main mixture screw out 1-1/4 turns.
Walbro - idle mixture screw out 1-1/8 turns; main mixture screw out 1-1/2 turns.
Nikki - idle mixture screw out 3/4 turn; main mixture screw out 1-1/4 to 1-1/2 turns.
Nikki with semi-automatic choke and limited idle adjustment - no initial adjustments; when replacing idle screw turn out 1-1/4 turns.
Start engine and allow it to warm up thoroughly (at least 10 minutes).
Move engine speed control to slow position and adjust throttle stop screw to obtain specified rpm.
Determine best idle mixture setting; set about 1/8 turn outward (rich) from midpoint.
Readjust throttle stop screw and release governor arm.
Adjust governor low speed stop for 1100 +/- 100 rpm idle.
Move speed control to fast position and bend high speed stop to equipment manufacturer's recommended speed.
Check main mixture adjustment by rapidly accelerating from idle to full speed; adjust in 1/8 turn increments if needed.

Governor Adjustment - Fixed Speed adjustment

Tools: tachometer

Engine speed is controlled by adjusting governor spring tension.
On side pull governors the governor spring is factory set in the third hole of the governor arm.
On front pull governors the governor spring is factory set in the second hole of the governor arm adapter.
To increase sensitivity, move spring loop into a hole closer to the pivot.
To decrease sensitivity, move spring loop into a hole farther from the pivot.
After sensitivity is set, recheck low speed rpm setting and adjust if necessary.
Check governor arm, linkage, throttle shaft and lever for binding or excessive slack.

Governor Adjustment - Variable Speed adjustment

Tools: tachometer

Disconnect linkage (A) from one of holes (C).
Push linkage (A) and governor arm (B) as far back (toward carburetor) as they will go.
Holding linkage and governor arm toward carburetor, insert end of linkage into whichever hole (C) in governor arm lines up closest; if between two holes, insert in next hole out.
Adjust governor link length so that with engine stopped and tension on governor spring, throttle shaft stop almost touches carburetor side stop.

Marvel Schebler Carburetor Adjustment adjustment

⚠ Overtightening the mixture adjustment screw will cause carburetor damage; turn in only until light tension can be felt.

Turn both mixture screws in until lightly seated, then back the idle mixture screw out 1-1/8 turns and the main mixture screw out 1-1/4 turns.
Start the engine and allow it to warm up thoroughly (at least 10 minutes).
Move the engine speed control to the slow position.
Determine if the engine has a governor low speed adjustment screw.
Pull the governor back so the throttle stop screw is against its stop and hold the governor arm there.
Adjust the throttle stop screw to obtain 1200 RPM without a low speed screw, or 1100 RPM with one.
Turn the idle adjustment screw in until speed drops, then out until speed drops again; set about 1/8 turn outward from the midpoint.
Engines with a governor low speed adjustment screw: adjust it to obtain 1200 RPM.
Check main mixture by rapidly accelerating from idle to full; turn main screw out in 1/8 turn increments if needed, up to 1/2 turn beyond original.

Nikki Carburetor Adjustment adjustment

⚠ Overtightening the mixture adjustment screw will cause carburetor damage; turn in only until light tension can be felt.

Identify whether the carburetor is the full adjustment or limited idle mixture type.
Full adjustment: back idle mixture screw out 3/4 turn; back main mixture screw out 1-1/4 to 1-1/2 turns.
Limited idle mixture: idle fuel adjustment is restricted to 1/4 turn by a limiting cap; no main fuel adjustment screw.
Start the engine and allow it to warm up thoroughly (at least 10 minutes).
Move the engine speed control to the slow position; bend or turn the low speed stop so the throttle stop screw controls speed.
Adjust the throttle stop screw for 1000 rpm idle.
Full adjustment: set the idle mixture screw about 1/8 turn outward (rich) from the midpoint of the highest-speed range.
Readjust the throttle stop screw for 1000 rpm idle, then adjust the governor low speed stop for 1100 rpm idle.
Move the speed control to fast; bend the high speed stop so the engine runs at the manufacturer's recommended speed.
Check the main mixture by rapidly accelerating from idle to full; adjust in 1/8 turn increments if needed, up to 1/2 turn beyond original.

Oil By-Pass Valve Inspection adjustment

Tools: magnet tool, parts cleaning solvent

Remove the 3/8 x 24 x 7/8 cap screw located behind gear cover and under governor arm.
Remove spring and plunger with a magnet tool.
Check spring and plunger per measurements: plunger diameter 0.3105 to 0.3125 in (7.89 to 7.94 mm); spring free length 1.00 inch (25.4 mm); load 2.6 +/- 0.2 lbs (11.6 +/- 0.9 N) compressed to 0.5 inch
Check valve seat and clean away any accumulation of metal particles.
Verify valve seat is concentric with larger diameter valve bore.
Clean plunger and spring in parts cleaning solvent and install.

Valve Clearance Adjustment adjustment

⚠ Do not adjust when engine is warm

Tools: Feeler gauges, 7/16-inch open end wrench, 9/16-inch open end wrench

Remove ignition key to prevent accidental starting
Remove all parts necessary to gain access to valve tappets
Remove spark plugs to ease turning engine over
Turn engine flywheel by hand until left intake valve opens and closes
Continue turning until TC mark on flywheel aligns with TC mark on gear cover
Insert correct feeler gauge between valve stem and tappet
Feeler gauge should pass freely; thicker gauge should not
Turn adjusting screw using 7/16-inch wrench to obtain correct clearance
Rotate engine one complete revolution and repeat for right cylinder
Replace all removed parts and tighten securely

Valve Tappet Adjustment adjustment

⚠ Adjust the valve clearance only when engine is at ambient temperature.

Tools: feeler gauge, 7/16-inch open end wrench, 9/16-inch (14 mm) open end wrench, socket wrench

Remove ignition key to prevent accidental starting.
Remove all parts necessary to gain access to valve tappets.
Remove spark plugs to ease turning the engine over by hand.
Place a socket wrench on the flywheel capscrew and rotate crankshaft clockwise until left intake valve opens and closes; continue until TC mark on flywheel lines up with TC mark on gear cover (left pi
Verify left intake and exhaust valves are closed and there is no pressure on valve lifters.
Correct feeler gauge should pass freely between valve stem and tappet; a 0.002 inch (0.05 mm) thicker gauge should not.
To correct clearance, use a 7/16-inch open end wrench to turn adjusting screw; a 9/16-inch (14 mm) open end wrench is required to hold the tappet.
To adjust valves on the right hand cylinder, turn engine one complete revolution and again line up the marks.
Replace all parts removed and tighten all screws securely; torque manifold bolts to specified torque.

Compression Test diagnostic

Tools: compression gauge

Run the engine until thoroughly warm.
Stop engine and remove spark plugs.
Remove air cleaner and place throttle and choke in the wide open position.
Insert the compression gauge in one spark plug hole.
Crank the engine and note the reading.
Refer to SPECIFICATIONS section for compression pressures.

Connecting Rod Bearing Clearance Check with Plastigauge diagnostic

Tools: Plastigauge, torque wrench

Make certain all parts are marked or identified so they are reinstalled in their original positions.
Place a piece of correct size Plastigauge in the bearing cap the full width of the crankshaft rod surface and about 1/4 inch (6.35 mm) off center.
Rotate the crank about 30 degrees from bottom dead center and reinstall the bearing cap; tighten bolts to specified torque; do not turn the crankshaft.
Remove bearing cap; the flattened Plastigauge will adhere to either the bearing cap or crankshaft.
Compare flattened Plastigauge with the graduations on the Plastigauge envelope to determine clearance.

Flywheel Alternator Output Test diagnostic

⚠ Reversing positive and negative battery connections or allowing engine to run without being connected to the alternator will result in engine electrical system damage.

Tools: volt-ohmmeter such as Simpson 270, DC voltmeter

With engine running, check battery terminal voltage (regulator output) using a DC voltmeter; should be 13.6 to 14.7 VDC.
If voltage is greater than specified replace regulator-rectifier; if less, proceed to step 2.
Examine all wires for loose, corroded, broken connections, short circuits; check fuses; repair as needed.
Disconnect plug from regulator-rectifier and test AC voltage at plug with engine running near 3600 rpm.
If AC voltage reads more or less than specified, proceed to step 4; if AC voltage is as specified but DC voltage low, replace regulator-rectifier.
Use Rxl scale on ohmmeter to detect opens in stator (unit not running); refer to Table 1 for resistance specifications.
If resistance is not as specified, replace stator; if windings are grounded the stator must be replaced.

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

Voltage reconnection

Voltage	Phase	Connection	Leads	Notes
12V	Single	Battery ignition system with alternator charging	B+ to starter and ignition, AC stator output to regulator-rectifier, negative ground	Permanent magnet flywheel alternator with solid-state voltage regulator. 15-ampere system. Do not reverse battery leads.

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 B48M?

The Cummins/Onan B48M engine has an oil capacity of 1.6 liters.

What is the displacement of the Cummins/Onan B48M?

The Cummins/Onan B48M has a displacement of 0.782 liters with 2 cylinders.

Which generators use the Cummins/Onan B48M engine?

The Cummins/Onan B48M powers 1 generator set(s), including Cummins/Onan B48.

How often should you change the oil in the Cummins/Onan B48M?

The Cummins/Onan B48M oil change interval is every 100 hours.

What is a common problem with the Cummins/Onan B48M engine?

A commonly reported issue on generators using the Cummins/Onan B48M is: Weakness and Sleepiness. Fix: Get out into fresh air immediately, shut down the unit and do not use until it has been inspected..