Caterpillarengine - industrialgas engine

Caterpillar G398

Maintenance schedule, common problems & OEM parts breakdown

The Caterpillar G398 is a bare industrial spark-ignited natural-gas V12 engine, not a mobile machine: no chassis, undercarriage, or attachments come with it. It shares its 48.3 L (2,945 cu in) V12 block, 159 mm x 203 mm (6.25 in x 8 in) bore and stroke, with the diesel D398; the two differ mainly in fuel delivery and ignition, carburetor and magneto on the gas engine versus fuel injection on the diesel. Output runs roughly 350 kW (470 hp) at 1200 rpm naturally aspirated, up to about 520 kW (700 hp) on turbocharged-aftercooled (TA, SINA, or SITA) arrangements — actual rating varies by arrangement, confirm the nameplate. Bare-engine weight lands around 6,275-6,730 kg (13,830-14,825 lb); packaged genset weight varies with the enclosure, radiator, and switchgear built around it. Cat built the G398 from roughly the 1960s into the 1980s alongside V8 (G379) and V16 (G399) siblings sharing the same bore and stroke, and sold it as a bare engine for generator sets in the 280-500 kW class, gas-compression packages, and pump-drive service.

Within the family, Cat offered naturally aspirated and turbocharged-aftercooled arrangements that trade compression ratio and boost for a wide swing in rated output, plus a later-series revision of the same basic architecture. Every version predates electronic engine control: ignition timing, fuel mixture, and governing run through a magneto, carburetor, and mechanical flyweight governor, with no ECM and nothing resembling a fault code to read. In today's used and parts market the G398 survives mainly as a rebuildable core for standby and continuous-duty gensets and gas-compression packages, where mechanical simplicity is the selling point: no electronics to fail, and rebuild parts still circulate through the secondary market even though Cat moved gas-engine development on to the electronically governed G3400 and G3600 families long ago.

Below: full specifications, fluids & capacities, the factory service schedule, common service parts, verified fault codes, what owners discuss, attachment guidance, the complete assembly directory, and a serial-number reference. Complete parts lists with full OEM part numbers, exploded diagrams, quantities, and fitment data are available free in Heavy Parts AI.

Caterpillar G398 specifications

Engine

Engine family	Natural-gas, spark-ignition version of the V-12 diesel-based D398 engine block; factory service literature groups the G398 together with its diesel D398 counterpart as one V-12 family.
Configuration	V-12, 4-stroke-cycle, water-cooled, carbureted spark-ignition industrial gas engine, with magneto ignition and a mechanical governor-to-carburetor linkage.
Cylinders	12 (V-12)
Bore x stroke	159 mm x 203 mm (6.25 in x 8.00 in), per the shared V-12 block used across the diesel and gas versions of this family.
Displacement	48.3 L (2,945 cu in)
Compression ratio	10:1
Gross power	Naturally aspirated units run roughly 350 kW (470 hp) gross at 1200 rpm with a 10:1 compression ratio; turbocharged-aftercooled (TA/SITA) units run higher, documented up to roughly 520 kW (700 hp). Output varies by arrangement and application; confirm against the engine's own rating plate.
Emissions tier	Pre-emissions-tier design. Mechanically governed and carbureted, with no electronic engine control or aftertreatment.

Weights

Engine dry weight	Approximately 6,275-6,730 kg (13,830-14,825 lb) for the bare V-12 engine, based on data for the shared block family. Exact figure depends on the arrangement (naturally aspirated vs. turbocharged-aftercooled), ignition, and accessory package fitted to the gas version.
Packaged generator set	Skid- or enclosure-mounted generator packages built on this engine weigh considerably more than the bare engine. Total package weight depends on the alternator, radiator, and enclosure selected, not on the engine alone.
Ground pressure	Not applicable. Stationary industrial engine with no undercarriage.

Dimensions

Bare engine footprint	Approximately 3.0 m long x 1.6 m wide x 2.2 m high (118 in x 62 in x 86 in). Figures are approximate and can vary with exhaust manifold and accessory-drive fitment.
Wheelbase / track gauge / ground clearance	Not applicable. Bare industrial engine with no chassis or undercarriage dimensions.

Performance

Rated speed	Commonly rated at 1,200 rpm for 60 Hz generator duty or 1,000 rpm for 50 Hz duty. Other industrial drive speeds are possible depending on the driven load.
Gross power at rated speed	Roughly 350 kW (470 hp) naturally aspirated; up to about 520 kW (700 hp) on turbocharged-aftercooled (TA/SITA) arrangements. Output varies by arrangement; verify against the specific unit's rating plate.
Fuel quality requirement	Requires natural gas of at least about 95% methane purity, with a heating value at or above roughly 900 Btu/cu ft (33.5 MJ/m3), per factory fuel-quality guidance for this engine family.
Gradeability / drawbar / lift / dig	Not applicable. Stationary industrial power unit, not a mobile working machine.

Service capacities (summary)

Fuel system	Natural gas fed from an external plant or pipeline supply. No onboard fuel tank.
Engine oil system	Approximately 94 L (25 US gal), based on service data for the shared V-12 block family. Confirm against the engine's own arrangement and serial-number range.
Cooling system	Water-cooled (jacket-water) system. Exact system capacity is not documented for this arrangement in available sources; verify with the shop manual or dealer for the specific serial-number range.
Hydraulic system	Not applicable. No onboard hydraulic system on this industrial engine.

Values vary by configuration, region, and serial range — confirm against your machine before planning transport or lifts.

G398 fluids & capacities

System	Capacity	Recommended fluid
Engine crankcase (with filter)	Approx. 94-110 L (25-29 US gal), based on the shared D398/G398 V12 block (48.3 L displacement, 6.25 in bore x 8 in stroke). An exact factory refill figure specific to the G398 spark-ignited configuration is not documented in currently available literature -- verify against the engine dataplate or dealer records before servicing.	Cat NGEO (Natural Gas Engine Oil), a low-ash oil for spark-ignited gas engines (sulfated ash at or below 0.45% wt, BN near 4.8). Offered in SAE 30 and SAE 40. Use SAE 30 in cooler ambient conditions and SAE 40 in warmer ambient or continuous-duty stationary operation.
Cooling system	Not documented in currently available literature for this configuration. Genset packages used either a horizontal radiator-and-fan arrangement or a remote heat-exchanger loop depending on the installation -- confirm capacity against the dataplate or dealer records.	Cat NGEC (Natural Gas Engine Coolant), supplied as a 50/50 premix of ethylene glycol and water, meeting the ASTM D6210 heavy-duty antifreeze/coolant standard. Suitable for natural-gas and diesel variants of this engine family.
Fuel system (natural gas supply)	No onboard fuel tank. The engine draws pipeline-quality natural gas through an external gas train/regulator skid rather than a stored liquid fuel supply.	Dry, low-H2S natural gas, minimum about 95% methane purity and at least roughly 900 BTU/scf heating value, per the fuel-gas quality requirements for spark-ignited Cat gas engines of this family. Not a lubricant/coolant system, so no oil-type recommendation applies.
Grease points (accessory drives, generator coupling/bearings)	Spec only -- no published refill volume for this application.	Multipurpose lithium-complex grease, NLGI 2 grade (equivalent to Cat MPGM).

Capacities are refill values from factory literature — always fill to the dipstick/sight gauge, not the number.

Caterpillar G398 maintenance schedule

Service interval	Tasks
Every 10 h	Check crankcase oil level before starting; top off with the specified oil grade. Check coolant level and look for leaks at the water pump, hoses, and aftercooler on turbocharged units. Drain water and sediment from the fuel-gas scrubber or moisture trap ahead of the carburetor. Walk around and soap-test fuel-gas fittings for leaks; listen for abnormal knock or misfire. Weekly: inspect the air-cleaner restriction indicator and service the element if flagged. Weekly: check battery condition and charging output for the starting and control circuits.
Every 250 h	Change crankcase oil and filter element(s); pull an oil sample first if running on a condition-based extension. Clean or replace the fuel-gas filter/strainer element ahead of the carburetor. Inspect spark plugs; clean, re-gap to 0.36 mm (.014 in), or replace as needed. Check breaker-point gap and dwell in the magneto; dress or replace pitted contacts. Inspect drive belts for the fan, water pump, and charging circuit; adjust tension or replace. Pull a coolant sample and check the antifreeze/corrosion-inhibitor concentration.
Every 500 h	Inspect and adjust the carburetor-to-governor linkage for full, unrestricted throttle travel. Check the governor oil level and lubrication points. Inspect turbocharger mounting, boost lines, and aftercooler core for leaks or fouling on TA/SINA arrangements. Verify magneto timing against the flywheel timing mark and correct any drift. Inspect the exhaust manifold and expansion joints for cracking from thermal cycling.
Every 1,000 h	Check and adjust valve lash: exhaust 0.89 mm (.035 in), intake 0.38 mm (.015 in); torque the locknut to 55 ± 7 N·m (40 ± 5 lb ft). Remove and clean the aftercooler core if air-inlet temperature has been trending up on turbocharged units. Review oil-pressure trend against baseline and investigate any steady decline. Overhaul the magneto's breaker points, condenser, and drive coupling. Re-torque cylinder-head hold-down bolts to the factory sequence if oil or coolant consumption is rising.
Every 2,000 h	Pull cylinder heads or borescope the bores to check valves, seats, and liner wear. Send oil and coolant samples for full lab analysis to trend wear metals and additive depletion. Inspect and recondition or replace the water pump and thermostat(s). Check the crankshaft vibration damper for cracking or bonding failure and verify mounting torque. Rebuild or exchange the carburetor and fuel-gas pressure regulator if performance has drifted.
Every 4,000 h	Perform a full top-end rebuild: recondition or replace valves, seats, and guides per wear found at the last borescope inspection. Recondition the turbocharger cartridge and aftercooler tube bundle rather than just cleaning them on TA/SINA arrangements. Rebuild the carburetor and fuel-gas pressure regulator as a matched set. Overhaul the magneto and replace the ignition wiring harness end to end. Re-torque and inspect the crankshaft vibration damper and flywheel housing.
Every 12,000 h	Plan a full inframe overhaul (crankshaft, main/rod bearings, liners, pistons) once oil-analysis trends, not hours alone, indicate it's due. Line-bore or replace worn main-bearing saddles and re-torque the block to the factory sequence. Replace coolant- and lube-system hoses, seals, and gaskets as a set rather than piecemeal. Re-certify the governor, magneto, and carburetor as a matched set before returning the engine to service. Weigh the actual interval against duty cycle: standby gensets run far longer between major overhauls than continuous gas-compression or pump-drive service.

Servicing the G398 beyond the schedule

Predictive Maintenance & Fluid Analysis

Pre-ECM engines like the G398 have no onboard diagnostics, so scheduled oil and coolant sampling is the main early-warning tool. Track wear metals, TBN depletion, and moisture at every oil change to catch bearing or liner wear before it shows up as a pressure drop. Trend cylinder balance and exhaust temperature by hand: a rising spread points to carburetor mixture drift, magneto timing wander, or a sticking valve. Log jacket-water temperature and boost on turbocharged-aftercooled units to catch aftercooler fouling early.

Corrective & Common Repairs

Most G398 driveability complaints trace back to the ignition and fuel-mixture side: worn breaker points, a fouled or mis-gapped spark plug, or a carburetor that has drifted off the governor linkage. Check magneto timing and point dwell before chasing a misfire elsewhere. Persistent detonation on this 10:1-compression engine usually means fuel gas below the required heating value or methane number, not a mechanical fault. Water pump seals, exhaust manifold gaskets, and governor linkage bushings are the other recurring wear points on engines this age.

Overhaul & Rebuild Points

There's no undercarriage or chassis to service. Overhaul planning centers on the cylinder heads, liners, and main/rod bearings. Borescope the bores and pull a head at the first sign of rising oil consumption or falling compression; liner glazing and valve-seat recession are the typical failure modes on high-hour units. A turbocharged-aftercooled arrangement also needs the turbo cartridge and aftercooler core evaluated at overhaul, since boost-side fouling raises detonation risk. Most surviving units are rebuildable cores today, so confirm liner and bearing availability before committing to a rebuild.

Seasonal & Environment Servicing

Fuel gas quality matters more on the G398 than ambient weather: pipeline gas below the specified heating value or methane number causes detonation on this 10:1-compression engine regardless of season. For standby gensets, exercise the unit under load periodically so the carburetor, magneto, and governor don't seize from disuse, and keep the crankcase breather and gas scrubber or moisture trap drained to stop water carryover. In cold climates, verify coolant and antifreeze concentration and block-heater function before startup, since there's no ECM-managed cold-start strategy to compensate for a sluggish crank or lean mixture.

G398 common service parts

Part number	Part	Notes
1R-1808	Filter As-Engine Oil	Lubrication service	Check fitment →

Always confirm against your machine's serial number — cross-check any part in Heavy Parts AI before ordering.

G398 fault codes & troubleshooting

Code	Meaning	Likely cause	What to do
LOW OIL PRESSURE (shutdown switch/lamp trip)	Oil pressure switch has tripped the fuel shutoff and lit the shutdown lamp on the engine control panel, stopping the engine.	Low oil level, plugged oil filter or suction screen, worn oil pump, or a misadjusted/failed pressure switch.	Check crankcase oil level and filter condition, verify switch setting and wiring, and find the cause before restarting.
HIGH JACKET WATER TEMPERATURE (shutdown switch/lamp trip)	Coolant temperature switch has exceeded its set point and tripped the fuel shutoff/alarm circuit.	Low coolant level, restricted radiator or heat exchanger, failed water pump, or a stuck thermostat.	Check coolant level and flow, inspect the water pump and thermostat, and verify switch calibration before restarting.
OVERSPEED TRIP	Mechanical overspeed safety device shuts off fuel when engine rpm exceeds the safe limit; shown by a tripped lever/flag at the governor, not a panel code.	Governor linkage binding or misadjustment, sudden loss of load, or a worn/failed flyweight or spring in the governor.	Inspect the governor and linkage for binding, confirm the fuel rack/throttle returns freely, then reset the trip per the service manual before restart.
LOW COOLANT LEVEL (alarm/shutdown, where panel-equipped)	An auxiliary coolant-level switch, if fitted on the control panel, signals that jacket water has dropped below a safe level.	External coolant leak, loose hose or clamp, or loss of coolant from a prior overheating event.	Inspect the cooling system for leaks, refill with the correct coolant, and check the float/level switch before returning the unit to service.
IGNITION MISFIRE / DEAD CYLINDER (manual diagnosis, no panel code)	Not an electronic code - a rough-running or dead-cylinder condition traced by hand to the magneto or ignition wiring, since this engine has no onboard display for ignition faults.	Magneto timing drift, worn breaker points or condenser, damaged/wet high-tension leads, or fouled or worn spark plugs.	Retime the magneto to the engine, inspect and service the breaker points, wiring, and plugs per the ignition section of the service manual.
GOVERNOR HUNTING / SPEED SURGE (manual diagnosis, no panel code)	Engine speed oscillates or surges under load; a commonly documented mechanical symptom rather than a displayed code on this engine.	Worn governor linkage, incorrect governor sensitivity or compensation adjustment, or a sticking carburetor throttle plate.	Check linkage for wear and free travel, adjust governor sensitivity/compensating needle per the manual, and confirm the throttle plate moves freely.

Codes and remedies are general guidance for this model family — always confirm with diagnostic tooling and your dealer before major repairs.

G398 attachments & work tools

Machine type / attachment applicability

The G398 is a bare industrial gas engine (V12, built on the D398 diesel block family), not a mobile machine chassis. It has no bucket, blade, coupler, or ground-engaging work-tool ecosystem. Factory options are engine-side arrangements for the driven equipment it powers (generator, compressor, or pump), not bolt-on attachments.

Aspiration / power arrangement

Offered as naturally aspirated (NA) and turbocharged-aftercooled (TA, also marketed SINA) arrangements, each with its own arrangement number rather than a field-installable option. Rated output varies by arrangement and site rating, roughly 350-520 kW (470-700 hp) at 1200 rpm; treat exact figures as varying by arrangement and verify against the nameplate.

Cooling package

Available with an engine-driven fan and radiator for self-contained skid installs, or with a jacket-water heat exchanger (copper-nickel tube bundle) for enclosed compressor-house or plant installations where raw water or remote cooling is used. TA arrangements add an aftercooler circuit on the intake side; NA units do not.

Driven-end coupling / mounting

Power take-off is through an SAE-pattern flywheel housing with a flexible coupling to the driven unit, not a quick-coupler system. Genset builds mount a generator end directly to the flywheel housing; compressor and pump-drive builds use a coupling or gear drive matched to the driven equipment, sized per application rather than offered as an accessory list.

Ignition and fuel system

Uses magneto ignition with distributor, wiring, and spark plugs, paired with a gas carburetor whose throttle links to the mechanical governor. Fuel gas quality (natural gas, field gas, or associated gas) must meet a minimum purity for reliable operation; gas train sizing is application-specific and set up at commissioning, not a bolt-on kit.

Governor and instrumentation

Speed control is by mechanical (or later add-on electronic) governor tied to the carburetor throttle linkage. Engine-mounted gauge panels typically cover oil pressure, jacket-water temperature, RPM, and running hours; these are standard instrumentation rather than optional attachments.

Control panel / switchgear (genset builds)

For generator-set packages, Caterpillar-style switchgear and paralleling controls (remote paralleling cabinets or isolated-bus emergency paralleling panels) can be specified alongside the engine-generator, but these are packaging-level electrical options for the driven generator, not engine attachments.

Application packaging

The same base engine is packaged three ways: generator-set drive (skid- or pad-mounted, with or without weatherproof enclosure), gas-compression drive (drilling rig or field compressor stations), and general pump/industrial drive. Each packaging choice determines cooling, coupling, and control selections above rather than adding separate work tools.

All G398 assemblies by section

Every catalogued assembly group for the Caterpillar G398. Open an assembly to preview the parts inside — full OEM part numbers are available in Heavy Parts AI.

Service Equipment And Supplies

324-8859 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	8
1F***31	Gasket	3
1F***19	Gasket-Cover	4
1F***23	Gasket Side Cover--Cylinder Block	4

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324-8852 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	12
1F***31	Gasket	3
1F***19	Gasket-Cover	6
1F***23	Gasket Side Cover--Cylinder Block	6

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324-8857 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	12
1F***31	Gasket	3
1F***19	Gasket-Cover	6
1F***23	Gasket Side Cover--Cylinder Block	6

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324-8853 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	12
1F***31	Gasket	3
1F***19	Gasket-Cover	6
1F***23	Gasket Side Cover--Cylinder Block	6

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324-8855 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	16
1F***31	Gasket	3
1F***19	Gasket-Cover	8
1F***23	Gasket Side Cover--Cylinder Block	8

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324-8850 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	16
1F***31	Gasket	3
1F***19	Gasket-Cover	8
1F***23	Gasket Side Cover--Cylinder Block	8

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324-8851 Kit-Engine Overhaul

0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	16
1F***31	Gasket	3
1F***19	Gasket-Cover	8
1F***23	Gasket Side Cover--Cylinder Block	8

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351-1742 Kit-Maintenance

2N***84	Gasket-Housing Cover	1
2N***97	Seal-Valve Cover	4
35***42	Kit-Maintenance	1
8F***94	"O" Ring	8

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351-1737 Kit-Maintenance

10***15	Gasket-Plug; (Combustion Sensor)	1
1R***08	Filter As-Engine Oil; (Advanced Efficiency)(High Efficiency)	1
2N***39	Spark Plug	8
35***37	Kit-Maintenance	1

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350-9205 Parts Gp-Engine Overhaul

0L***24	Gasket Drain Tube Bushing To Flywheel Housing	1
0L***13	Gasket (.6456" Inside Diameter)	1
0L***77	Screw-Lock	2
0R***21	Rod As-Connecting	1

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350-9204 Parts Gp-Engine Overhaul

0R***88	Cylinder Head As	1
0R***70	Cylinder Head Group	8
0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	16
10***57	Kit-Engine Overhaul	1

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350-9207 Parts Gp-Engine Overhaul

0R***88	Cylinder Head As	1
0R***70	Cylinder Head Group	6
0T***29	Locking Plate,Nut A Pump Mtg Part Of Kit P/N 5r***25	12
10***61	Kit-Engine Overhaul	1

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350-9208 Parts Gp-Engine Overhaul

0L***24	Gasket Drain Tube Bushing To Flywheel Housing	1
0L***13	Gasket (.6456" Inside Diameter)	1
0L***77	Screw-Lock	2
0R***21	Rod As-Connecting	1

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350-9209 Parts Gp-Engine Overhaul

0L***24	Gasket Drain Tube Bushing To Flywheel Housing	1
0L***13	Gasket (.6456" Inside Diameter)	1
0L***77	Screw-Lock	2
0R***21	Rod As-Connecting	1

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350-9210 Parts Gp-Engine Overhaul

0L***24	Gasket Drain Tube Bushing To Flywheel Housing	1
0L***13	Gasket (.6456" Inside Diameter)	1
0L***77	Screw-Lock	2
0R***21	Rod As-Connecting	1

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350-9206 Parts Gp-Engine Overhaul

0L***24	Gasket Drain Tube Bushing To Flywheel Housing	1
0L***13	Gasket (.6456" Inside Diameter)	1
0L***77	Screw-Lock	2
0R***21	Rod As-Connecting	1

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G398 serial number reference

On this engine the serial number plate is typically mounted on the engine block or on the flywheel housing - Caterpillar's older industrial engines vary between these two spots, so check both before assuming a genset skid tag is the engine PIN. Read the first three characters as the prefix, which identifies the engine model and build series; the digits that follow are the sequential unit number within that prefix run.

Prefix	Identifies	Notes
73B	G398 V12 natural gas engine, naturally aspirated (SINA) and turbocharged-aftercooled (SITA/TA) arrangements	Covers the main production run of the G398 V12 natural gas engine. The naturally aspirated version is rated near 470 hp (350 kW) at 1200 rpm with a 10:1 compression ratio; turbocharged-aftercooled units run higher, documented from roughly 450 hp up to about 700 hp (335-522 kW) at the same speed - actual rating varies by configuration and application, so confirm the arrangement plate for an exact figure. Used as the power end in Cat generator sets from about 280 kW up to 500 kW output. Service and parts documentation for this prefix spans from the low serial numbers up through at least the 900s. Requires minimum 95% pure natural gas fuel.

Frequently asked questions

What engine powers the Caterpillar G398?

The G398 is the engine itself: a spark-ignited natural-gas V12 sharing its 48.3 L (2,945 cu in) block and 159 mm x 203 mm (6.25 in x 8 in) bore and stroke with the diesel D398. It runs naturally aspirated or turbocharged-aftercooled depending on the rating, fires through a magneto and breaker-point ignition system, and feeds gas through a carburetor rather than fuel injection.

What's the operating weight of the Caterpillar G398?

As a bare engine, the G398 weighs roughly 6,275-6,730 kg (13,830-14,825 lb), varying with the naturally aspirated or turbocharged-aftercooled arrangement. There's no chassis or undercarriage to add, but a packaged genset's total weight varies widely with its enclosure, radiator, and switchgear, so treat the bare-engine figure as a baseline rather than a shipping weight.

What replaced the Caterpillar G398?

Caterpillar never issued a direct one-for-one replacement. The G398 was retired along with the rest of the mechanically governed D379/D398/D399 and G379/G398/G399 engine family, and its role in gensets and gas-compression packages was taken over by later electronically controlled gas engines such as the G3400 and G3600 series, which use ECM-based ignition and fuel control instead of a magneto and carburetor.

What G398 owners discuss

What's the real difference between the naturally aspirated and turbocharged-aftercooled G398, and which one do most used units turn out to be?

Cat built the Series A G398 in two induction versions: a naturally aspirated version and a turbocharged-aftercooled version. Field-quoted output on the naturally aspirated version runs around 350 kW (470 hp) at 1200 rpm with roughly 10:1 compression ratio. The turbocharged-aftercooled version runs a deliberately lower compression ratio to control detonation on pipeline-quality gas, and its rated output varies by installation and boost setting rather than one fixed number. Buyers should read the dataplate and job-specific rating sheet instead of assuming a horsepower figure off a listing, since output on this platform varies by configuration/series.

Why does the G398 still run on magnetos instead of an ignition module, and what actually fails on them?

The G398 is magneto-fired, not electronically ignited. A V12 of this generation runs two magnetos, one firing each bank of six cylinders (the related V16 G399 used two 8-cylinder magnetos on the same principle). Reported wear items are the usual magneto items: worn breaker points, weakening coil output as the unit ages, and impulse-coupling wear that shows up as hard or uneven starting. A magneto that has drifted out of time on one bank tends to show up as a rough-running bank, popping in the intake, or a flat spot under load rather than a clean no-start. Because mistimed ignition on a large gas V12 can backfire through the intake or overheat a bank, have your dealer or a qualified gas-engine tech verify magneto timing and drop-test both magnetos before returning the engine to service.

What do owners say about the G398's carburetor and gas quality requirements?

The G398 uses a carburetor with mechanical linkage tying the carburetor throttle to the engine governor, and that linkage needs periodic inspection and adjustment for wear at the pivot points; a sloppy linkage is a recurring cause of hunting or surging at idle. Fuel quality matters more on this engine than on later electronically controlled gas engines. Field guidance calls for gas purity around 95% or better with heating value at or above roughly 900 BTU per standard cubic foot. Running on lean, wet, or low-BTU field gas is a recurring complaint tied to rough idle, detonation, and shortened spark plug life. Carburetor mixture adjustment is treated as routine recurring maintenance on these units, not a one-time setup.

What wears out first on a high-hour G398, and is that different from the diesel D398?

The G398 shares its basic V12 block, crankshaft, and wet-liner architecture with the diesel D398; only the head, ignition, and fuel system differ. That shared bottom end carries the same known wear points: connecting-rod bearing wear (the V layout puts two rods on each crank journal, so bearing condition is a standard overhaul-interval check), tube-type oil cooler cores that clog and cause a slow oil pressure decline, and liner-seal (wet-liner O-ring) seepage that lets coolant migrate into the oil over time. A liner-seal leak usually shows up first as a rising coolant-in-oil trend on an oil analysis report, or as an unexplained coolant level drop, well before it becomes a visible external leak. Owners treat oil analysis as the early-warning tool rather than waiting for it to show on the gauges.

How many hours does a G398 typically run before it needs an overhaul, and is some oil leakage and burning at that point normal?

Owners running these engines at their rated 1200 rpm report long service lives before a major overhaul; accounts of this platform (in both diesel and gas form) still running at around 20,000 hours are common, often still making full power but starting to weep oil externally and burn a bit through the rings by that point. That combination, external seepage plus a modest rise in oil consumption at high hours, is generally treated as normal end-of-service-life behavior rather than a red flag. A sudden jump in consumption or a change in exhaust smoke color is not normal and should be checked. Oil change intervals are commonly run around every 250 hours or every three months, whichever comes first, given the size of the oil system.

The genset won't build voltage, or the output is unstable under load. What do owners check first on a G398 set?

Most G398 gensets pair with a Cat SR4 generator end. When the engine cranks and holds correct speed but produces no output, the first check is the exciter and voltage regulator, not the engine. A dead exciter field (lost residual magnetism) is a well-known cause of zero output and is corrected by briefly re-energizing ("flashing") the field from an external DC source using the regulator maker's procedure, not by working on the engine. Unstable or drifting voltage under load more often traces to a worn or failing automatic voltage regulator, a loose sensing lead, or deteriorated stator or rotor winding insulation than to anything in the engine. Owners recommend isolating engine speed and governor behavior from AVR and exciter behavior before tearing into either. Flashing the field feeds voltage into a live winding, so have your dealer confirm polarity and procedure before attempting it.

What should a buyer check before purchasing a used G398?

Confirm from the dataplate which induction version it is, naturally aspirated or turbocharged-aftercooled, since that changes the compression ratio, expected output, and fuel-quality tolerance; don't rely on a listing's advertised horsepower alone. Ask for oil analysis history and hours since major overhaul rather than trusting the hour meter by itself, since these platforms are commonly resold well past 20,000 hours and still running. Pull valve covers to check magneto condition and timing marks, inspect the carburetor-to-governor linkage for slop, and look for any coolant-in-oil trend on past oil analysis as a sign of liner-seal wear. If it's part of a compressor package, a common pairing for the naturally aspirated version, verify the skid, coupling, and compressor cylinder condition match the package rating and not just the engine. Because ignition timing and fuel-train integrity on a large gas V12 are safety-relevant, have your dealer or a qualified gas-engine shop verify magneto timing, ignition wiring, and fuel-system integrity before startup on any used unit.

Compiled from owner and technician discussions across the industry — experiences vary by serial range and machine history.

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