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Cummins/Onan B48M Engine

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	Months	Notes
Engine general	Inspect	8 hr		X1 - Check for fuel leaks; with engine running, visually and audibly check exhaust system for leaks.
Exhaust system	Inspect	8 hr		Maintenance schedules recommend inspection every eight hours by owner/operator for RV generator set exhaust systems.
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
Engine generally	Inspect	25 hr		Check for fuel leaks. With engine running, visually and audibly check exhaust system for leaks.
Inspect Engine Generally	Inspect	25 hr		First service — Check for fuel leaks. With engine running, check exhaust system visually and audibly.
Precleaner	Clean	25 hr		or more under extremely dusty conditions
Battery electrolyte level	Check	50 hr		First service
Cooling fins	Clean	50 hr		X2 - Perform more often under severe operating conditions.
Spark arrester/muffler, screen type	Clean	50 hr		Remove and scrape clean or replace screen; may be blown clean with compressed air. Every 50 operating hours.
Spark arrester/muffler, spin-out type	Clean	50 hr		Remove pipe plug in bottom of muffler and run set for 5 minutes; replace plug. Every 50 hours of set operation.
Spark arresters	Clean	50 hr		Spark arresters should be cleaned every 50 to 100 hours; routine cleaning during servicing is good safety practice.
Spark plugs	Replace	75 hr		Annually or every 75 hours, whichever occurs first, for B43M/B48M/BF/BG/N52M engines.
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		lead cleaning and gasket change; extend to 400 hours for unleaded fuel
Points and condenser	Replace	200 hr
Valve clearance	Adjust	200 hr		X4 - For detailed maintenance contact an Onan Service Center or refer to the SERVICE MANUAL.
Valve clearance	Check	200 hr		Initial break-in check indicated; for detailed maintenance contact an Onan Service Center or refer to the Service Manual.
Valve tappet clearance	Adjust	200 hr		check and adjust if necessary at least every 200 operating hours or when poor engine performance noticed
Valve tappet clearance	Check	200 hr		Adjust if necessary; also check when poor engine performance is noticed; adjust only at ambient temperature
Stainless steel spark arrester screen	Replace	500 hr		For units using specially designed stainless steel screens.
Flexible fuel lines	Replace		60 mo	Replace every five years as a precautionary measure; check at each service.

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	0.0020 - 0.0160	in
Clearance	Bearing to Crankshaft Clearance (Aluminum Rod)	0.0020 - 0.0033	in
Clearance	Bearing to crankshaft clearance - aluminum rod	0.0020-0.0033 inch (0.051-0.084 mm)	inch
Clearance	Brush Wear Limit (Inertia Type Starter)	1/4	in	replace if shorter
Clearance	Brush Wear Limit (Solenoid Shift Starter)	11.5	mm	replace if shorter
Clearance	Camshaft Bearing Bore (Bearing installed)	1.3760 - 1.3770	in
Clearance	Camshaft Bearing Clearance	0.0015 - 0.0030	in
Clearance	Camshaft Bearing Journal Diameter	1.3740 - 1.3745	in
Clearance	Camshaft End Play	0.0030 - 0.0053	in
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	in
Clearance	Connecting Rod Large Bore Diameter	1.6280 - 1.6285	in	rod bolts properly torqued
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	in
Clearance	Crankshaft end play	0.006-0.012 inch (0.15-0.30 mm)	inch
Clearance	Cylinder Air Housing Clearance	0.187 to 0.313	in	B48G beginning Spec D, B43G, B43E
Clearance	Cylinder Air Housing Clearance	4.76 to 7.94	mm	B48G beginning Spec D, B43G, B43E
Clearance	Cylinder Bore Honed Diameter	3.2490 - 3.2500	in
Clearance	Cylinder Bore Out-of-Round Maximum	0.003	in
Clearance	Cylinder Bore Taper Maximum	0.005	in
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	Dashpot to governor arm bracket	0.050	in (1.27 mm)
Clearance	Front gear cover oil seal installation depth	1-1/32	in	rear spring side of casing from gear cover mounting face
Clearance	Fuel injection pump installation dimension	3.260	in	new camshaft; instead of 3.254
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	in
Clearance	Main Bearing Inside Diameter (Installed)	2.0015 - 2.0040	in
Clearance	Main Bearing Inside Diameter (Without bearing)	2.187 - 2.188	in
Clearance	Main Bearing Journal Diameter	1.9992 - 2.0000	in
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 By-Pass Valve Plunger Diameter	0.3105 to 0.3125	in
Clearance	Oil By-Pass Valve Spring Free Length	1.00	in
Clearance	Oil pump gear backlash	0.002-0.003 inch (0.051-0.076 mm)	inch
Clearance	Piston Pin Bushing Bore (Finished bore)	0.6879 - 0.6882	in
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	Starter Gear Lash	0.020 ± 0.015	in
Clearance	Starter Gear Lash	0.51 ± 0.38	mm
Clearance	Starter Pinion Clearance	0.020 to 0.080	in
Clearance	Starter Pinion Clearance	0.5 to 2.0	mm
Clearance	Starter gear lash	.020 - .010	in	pinion/ring gear engagement
Clearance	Timing gear backlash	0.002-0.005 inch (0.051-0.127 mm)	inch

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
Coolant	Ethylene glycol base anti-freeze mixed with water				Do NOT use straight 100% ethylene glycol. Normal mixture approximately 50% anti-freeze and 50% water; ratio varies with ambient temperature.
Engine oil		API SF, SF/CC, or SF/CD		1.5 qts	Straight weight oils recommended for severe duty use and at temperatures above 32°F (0°C) for minimum oil consumption. Refer to viscosity chart.
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 viscosity grade per temperature chart; straight weight oils recommended for severe duty and above 32°F (0°C)	API SF, SF/CC, or SF/CD	-30°C to 40°C	1.4 L	Crankcase capacity without filter 1.5 Qts (1.4 litre); oil filter capacity .3 Qts (.3 litre). Ambient range values reflect the viscosity chart axis (TEMPERATURE RANGE YOU EXPECT BEFORE NEXT OIL CHANGE). Use same brand when adding oil between changes.
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	ASTM 2-D or 1-D diesel fuel	Cetane number 45			Use Number 1-D diesel fuel when ambient temperatures are below 32 F (0 C) or during long periods of light load/cyclic operation.
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.
Fuel	Unleaded gasoline; regular leaded gasoline may also be used but not preferred				Do not use highly leaded premium gasoline. Do not use fuel containing alcohol concentrations greater than ten percent. Do not use fuel system de-icers.
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

Adjust anti-dieseling solenoid adjustment

With set stopped, turn lever counterclockwise; set screw so plates are closed but not binding.
Check lever has approximately 30 degrees travel counterclockwise from vertical.
Start set; lever should rotate approximately 60 degrees clockwise from stopped position.
Check solenoid plunger is fully seated.
If not, adjust distance between plunger and lever arm; repeat steps.

Breaker Points Replacement and Adjustment adjustment

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

Tools: Flat feeler gauge, Allen wrench

Remove spark plugs.
Remove breaker box cover. Rotate crankshaft clockwise (facing flywheel) until points are fully open.
Remove condenser (screw A) and detach condenser lead and coil lead (screw B).
Remove two Allen screws (C) and lift breaker assembly from engine.
Replace condenser and point assembly with new parts and reinstall in reverse order of removal.
Rotate crankshaft clockwise by hand until points fully open; set point gap with a clean flat feeler gauge to 0.016 in by adjusting the Allen socket head screw (D).
Replace 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.

Carburetor Mixture Adjustment adjustment

⚠ Over-tightening mixture adjustment screw will cause carburetor damage.

Tools: screwdriver, tachometer

Warm engine at least 10 minutes.
Set throttle to slow; adjust throttle stop screw for 1000 rpm idle.
Turn idle mixture screw in until speed drops, then out until speed drops again.
Set screw about 1/8 turn outward from midpoint.
Readjust throttle stop for 1000 rpm idle, then governor low speed stop for 1100 rpm idle.
Move to fast; adjust high speed stop to recommended speed.
Check acceleration; adjust main mixture in 1/8 turn increments if needed.

Carburetor adjustment for 6.5 NH RV adjustment

Turn idle mixture screw out 3/4 to 1 turn from seated.
Turn main mixture screw 1-1/2 to 1-3/4 turns out from seated.
Start set; adjust governor spring for 1860 RPM no load (62 Hz or 130 volts).
Hold governor arm so throttle lever rests on stop; adjust idle stop screw to 1500 RPM (50 Hz or 100 volts); release.
Adjust idle mixture screw to highest RPM or voltage; readjust governor spring to 1860 RPM no load.
Apply full load; adjust main mixture screw to highest RPM or voltage; readjust governor spring to 1770 RPM full load.
Remove and add load several times to check hunting; readjust governor spring if required.

Carburetor and Governor Adjustment adjustment

Start engine and allow it to warm up thoroughly (at least 10 minutes).
Move engine speed control to the slow position.
Bend or turn the low speed stop on the governor so the throttle stop screw on the carburetor controls engine speed.
Adjust the throttle stop screw for 1000 rpm idle.
Adjust the governor low speed stop for 1100 rpm idle.
Move engine speed control to the fast position.
Bend the high speed stop on the governor so the engine runs at the vehicle manufacturer's recommended speed.

Crankshaft Endplay Check adjustment

Tools: feeler gauge, torque wrench

Tighten rear bearing end plate to specified torque.
Measure crankshaft endplay.
If too much endplay, remove rear bearing end plate.
Add shim between thrust washer and plate.
Reinstall end plate with notches aligned to lock pins.
Torque and recheck endplay.

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.

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..