Yanmar 2GM20 Specifications
| Engine Model: Yanmar 2GM20 |
| Engine Type: 4-stroke 2-cylinder OHV raw water cooled diesel |
| Total Displacement: 38.8 cu.in (0.64 L) |
| Rated Engine Power: 18 Hp (13.4 kW) at 3600 rpm |
| Fuel System Type: Indirect injection |
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The Yanmar 2GM20 is a two-cylinder, vertical,
four-stroke marine diesel engine produced by Yanmar Marine Systems,
introduced in the early 1980s as part of the GM series. This powerplant
was engineered specifically for mid-sized sailing yachts, motor sailers,
and small workboats, delivering 13.4 kW (18 hp) from a compact
0.636-liter displacement. The 2GM20 gained recognition for its
lightweight construction, quiet operation, and exceptional reliability
in confined engine compartments characteristic of sailing vessels.
The engine’s architecture incorporates a swirl pre-combustion chamber
indirect injection system, which reduces combustion noise and permits
operation with a range of marine diesel fuel qualities. The vertical
two-cylinder configuration with square bore and stroke dimensions - 75
mm bore x 72 mm stroke - produces a compact power package suited to
sailboat installations where space is limited.
The Yanmar 2GM20 was offered in two cooling configurations: direct
seawater-cooled (2GM20) and freshwater-cooled with a heat exchanger
(2GM20F). A hand-crank backup starting system supplements the 12V
electric starter, providing redundancy critical for marine applications.
This engine became one of Yanmar’s most popular small marine diesels,
powering sailboats such as the Catalina 30, Hunter 31, and Beneteau
First 30.
The Yanmar 2GM20 is a four-stroke, water-cooled, naturally aspirated
marine diesel engine configured for inboard installation with either
shaft drive or saildrive transmission. The engine utilizes indirect
injection with a swirl pre-combustion chamber.
Engine Type: Inline-2, 4-cycle, water-cooled, naturally aspirated marine
diesel
Displacement: 0.636 L (636 cc / 38.8 cu.in)
Bore: 75.0 mm (2.95 in)
Stroke: 72.0 mm (2.83 in)
Compression Ratio: 23.0:1
Firing Order: 1-2
Valvetrain: OHV (Overhead Valve), 4-valve (2 valves per cylinder)
Maximum Output (Crankshaft): 13.4 kW (18 hp) at 3,600 rpm
Continuous Rating (Crankshaft): 11.8 kW (16 hp) at 3,400 rpm
Rated Speed: 3,400 rpm (continuous); 3,600 rpm (maximum)
Low Idle Speed: 850-950 rpm
Injection Timing: 14° before top dead center
Injection Pressure: 12.0-13.0 MPa (1,740-1,885 psi)
Combustion System: Indirect injection (IDI) with swirl pre-combustion
chamber
The cylinder block is manufactured from cast iron using a high-pressure
die-casting process, selected for its dimensional stability, wear
resistance, and noise-dampening characteristics critical for sailboat
installations where engine noise is a concern. The block features a
deep-skirt design extending below the crankshaft centerline.
The cylinder bores are machined directly into the iron casting or
utilize replaceable dry cylinder liners depending on production variant.
The block incorporates a siamesed bore configuration where adjacent
cylinders share common walls, reducing overall engine length. Water
jackets surround each cylinder bore, with coolant flow directed from the
raw water pump or fresh water pump into the block before rising into the
cylinder head.
The crankcase incorporates three main bearing supports with the bearing
webs integrated into the block casting. Main bearing caps are
manufactured from cast iron and secured with two bolts per cap. The
block includes mounting provisions for the injection pump and the oil
filter housing. A cast aluminum oil pan is bolted to the lower crankcase
flange.
Block Material: Cast iron
Bore Diameter: 75.0 mm (2.95 in)
Cylinder Configuration: Inline, siamesed bores
Main Bearing Quantity: 3
Main Bearing Cap Design: Cast iron, two-bolt attachment
The cylinder head is constructed from cast iron using a permanent mold
casting process, selected for its thermal stability and compatibility
with the cast iron cylinder block. The head uses a single casting
covering both cylinders. The OHV valvetrain operates four valves - two
per cylinder (one intake, one exhaust) - via pushrods and rocker arms.
The camshaft is located within the cylinder block and driven by a gear
train from the crankshaft. Camshaft lobes actuate mechanical lifters
(tappets) that translate motion upward through pushrods to the rocker
arms mounted on a rocker shaft. Valve clearance adjustment is performed
manually using a screw-and-locknut adjuster at the rocker arm. The
combustion chambers incorporate a swirl pre-combustion chamber
(spherical cavity) for each cylinder, where initial fuel evaporation and
ignition occur before the burning mixture expands into the main chamber.
The cylinder head incorporates pressed-in valve seat inserts, with
exhaust seats manufactured from a nickel-based alloy. Valve guides are
pressed into the head. The pre-combustion chamber insert is manufactured
from heat-resistant steel and is replaceable.
Head Material: Cast iron
Valve Configuration: OHV, 4-valve (2 valves per cylinder)
Intake Valve Diameter: 30.0 mm (1.18 in)
Exhaust Valve Diameter: 28.0 mm (1.10 in)
Valve Stem Diameter: 7.0 mm (0.28 in)
Valve Seat Angle: 45 degrees
Camshaft Drive: Gear train from crankshaft
Valve Clearance (Cold, Intake): 0.20 mm (0.008 in)
Valve Clearance (Cold, Exhaust): 0.20 mm (0.008 in)
Pre-Combustion Chamber: Replaceable swirl-type insert per cylinder
The fuel system is a mechanically controlled indirect injection design
utilizing an inline injection pump. The pump is a Yanmar or Bosch inline
jerk-type pump with a mechanical governor, mounted on the side of the
cylinder block and driven by a gear train from the crankshaft.
The mechanical governor provides droop characteristics suited to marine
propeller loads, maintaining stable engine speed under changing load
conditions. A manual shut-off lever stops the engine by cutting fuel
supply to the injection pump. A mechanical lift pump mounted on the
cylinder block draws fuel from the marine fuel tank through a primary
water-separating filter and delivers it to the injection pump. A
secondary spin-on fuel filter is mounted between the lift pump and
injection pump.
The injectors are of the pintle nozzle type, calibrated to open at
pressures between 12,000 kPa and 13,000 kPa (1,740-1,885 psi). Each
injector is retained in the cylinder head by a clamp and sealed with a
copper washer. Fuel is injected into the pre-combustion chamber rather
than directly into the main combustion chamber.
Injection Type: Indirect injection (IDI) with swirl pre-combustion
chamber
Injection Pump Type: Yanmar or Bosch inline 2-cylinder, mechanical
governor
Injector Opening Pressure: 12.0-13.0 MPa (1,740-1,885 psi)
Injection Timing: 14° BTDC
Fuel Filtration: Primary water-separating filter plus secondary spin-on
filter
Governor: Mechanical, variable-speed
Fuel Requirement: Diesel fuel meeting EN 590 or ASTM D975; minimum
cetane 45
The lubrication system is a fully pressurized, wet-sump design with a
trochoid-type oil pump mounted internally within the timing gear case
and driven by the camshaft gear. The trochoid pump utilizes an inner and
outer rotor to generate flow with minimal pulsation.
Oil is drawn from the cast aluminum oil pan through a mesh strainer
positioned at the sump’s lowest point. Pressurized oil passes through a
full-flow, replaceable oil filter mounted externally on the cylinder
block. The filter is of the spin-on type and incorporates a bypass
valve. From the filter, oil enters the main oil gallery, supplying the
crankshaft main bearings, connecting rod bearings, camshaft bearings,
and rocker arm assemblies.
A pressure relief valve bypasses excess oil flow back to the oil pan
when system pressure exceeds the calibrated limit. An oil pressure
switch provides low-pressure warning to the helm instrument panel. The
lubrication system is designed for compact packaging, with oil capacity
limited to 2.0 liters.
Oil Type: Diesel engine oil meeting API CD, CE, or CF; SAE 15W-40
viscosity (monograde SAE 30 suitable in warm climates)
Oil Capacity (with filter): 2.0 L (2.1 US qt)
Oil Pressure (at idle, warm): 70-140 kPa (10-20 psi)
Oil Pressure (at rated speed): 280-420 kPa (41-61 psi)
Oil Pump Type: Trochoid, camshaft-driven
The Yanmar 2GM20 was offered in two cooling configurations. The standard
2GM20 uses direct seawater cooling: raw water drawn through a seacock
and strainer circulates through the engine block and cylinder head, then
discharges overboard. The 2GM20F uses indirect freshwater cooling: a
closed loop of antifreeze coolant circulates through the engine while
seawater flows through a heat exchanger to remove heat.
In the 2GM20 (raw water cooled), a flexible impeller-type raw water pump
(rubber impeller) is mounted on the front of the engine and driven by
the camshaft. This pump draws seawater and circulates it through the
engine block and cylinder head. A wax-pellet thermostat regulates
seawater flow. A bypass circuit permits warm-up circulation. The raw
water system includes a mixing elbow where seawater is injected into the
exhaust stream, cooling the exhaust gases.
In the 2GM20F (freshwater cooled), a centrifugal fresh water pump
circulates antifreeze coolant through the engine block and heat
exchanger. A separate raw water pump circulates seawater through the
heat exchanger. This configuration reduces internal corrosion and
permits engine operation at more stable temperatures.
Coolant Type (Raw Water): Seawater (no antifreeze in block).
Coolant Type (Freshwater): Ethylene-glycol based antifreeze.
Thermostat Opening Temperature: 60-70 °C (140-158 °F) raw water; 70-80
°C (158-176 °F) freshwater.
Raw Water Pump Type: Flexible impeller (rubber), camshaft-driven.
Raw Water Pump Impeller: Service life 200-500 hours; inspect annually.
Heat Exchanger (2GM20F): Shell-and-tube, cupronickel construction.
| Intake and Exhaust Systems |
Intake System: The intake system incorporates a flame arrester mounted
on the intake manifold, which prevents engine backfire from igniting
flammable vapors in the engine compartment. The intake manifold is
integrated into the cylinder head.
Exhaust System: The exhaust manifold is cast into the cylinder head. A
water-cooled exhaust elbow (mixing elbow) injects raw cooling water into
the exhaust stream, cooling the exhaust gases before they exit through
the vessel’s exhaust system. A waterlock prevents seawater from flowing
back into the engine.
Starting System: The engine is equipped with a 12V electric starting
system (1.0 kW starter motor). A manual hand-crank backup starting
system is provided for emergency starting in the event of battery
failure. The hand crank engages with the front of the crankshaft. Glow
plugs (one per cylinder) provide cold-start assistance.
Governor: The mechanical governor is integrated into the injection pump
and provides droop characteristics suited to marine propeller loads. A
remote throttle cable connects to the governor control lever.
Transmission / Saildrive Options: KM2P-1 Marine Gear: Mechanical reverse
gear transmission with parallel servo cone clutch. Forward reduction
ratios available: 2.21:1, 2.62:1, or 3.22:1. Reverse ratio: 3.06:1. SD20
Saildrive: Constant mesh gear with clutch dog, reduction ratio of
2.64:1. The saildrive provides a vertical output shaft passing through
the hull, eliminating shaft alignment concerns.
Timing Drive: The camshaft and injection pump are driven by a gear train
from the crankshaft. This gear-driven system requires no periodic
replacement and provides precise synchronization throughout the engine’s
service life.
Overall Length: 638 mm (25.1 in)
Overall Width: 455 mm (17.9 in)
Overall Height: 495 mm (19.5 in)
Dry Weight (2GM20 raw water): 106 kg (234 lb)
Dry Weight (2GM20F freshwater): 114 kg (251 lb)
Dry Weight with KM2P-1 gearbox: Add 22-28 kg (48-62 lb)
Engine Oil Change: 100 operating hours or annually.
Engine Oil Filter Change: At every oil change.
Fuel Filter Replacement: 200 operating hours or annually.
Raw Water Pump Impeller Inspection: 200 operating hours.
Raw Water Pump Impeller Replacement: 500 hours or if cracked.
Valve Clearance Inspection: 200 operating hours; adjust as required.
Glow Plug Inspection: 500 operating hours; test resistance and function.
Injector Service: 500 operating hours inspection and calibration.
Injection Pump Timing: Inspect following any pump removal or drive gear
disturbance.
Zinc Anode Replacement (Heat Exchanger): 200 operating hours; inspect
annually.
Transmission Oil Change (KM2P-1): 200 operating hours or annually.
Saildrive Oil Change (SD20): 200 operating hours or annually.
Cylinder Head Bolts (M10 x 1.25), 1st pass: 20 Nm (15 lb-ft)
Cylinder Head Bolts (M10 x 1.25), 2nd pass: 40 Nm (30 lb-ft)
Cylinder Head Bolts (M10 x 1.25), Final pass: 70-85 Nm (52-63 lb-ft)
Main Bearing Cap Bolts (M10 x 1.25), 1st pass: 20 Nm (15 lb-ft)
Main Bearing Cap Bolts (M10 x 1.25), Final pass: 45-55 Nm (33-41 lb-ft)
Connecting Rod Cap Bolts (M8 x 1.0), 1st pass: 10 Nm (7 lb-ft)
Connecting Rod Cap Bolts (M8 x 1.0), Final pass: 25-30 Nm (18-22 lb-ft)
Rocker Arm Shaft Bolts (M8 x 1.25): 20-25 Nm (15-18 lb-ft)
Intake Manifold Bolts (M8 x 1.25): 20-25 Nm (15-18 lb-ft)
Exhaust Manifold Nuts (M8 x 1.25): 20-25 Nm (15-18 lb-ft)
Injection Pump Mounting Nuts (M8 x 1.25): 15-20 Nm (11-15 lb-ft)
Fuel Injector Holders (M20 x 1.5): 50-60 Nm (37-44 lb-ft)
High-Pressure Fuel Line Fittings: 15-20 Nm (11-15 lb-ft)
Oil Pan Bolts (M8 x 1.25): 15-20 Nm (11-15 lb-ft)
Water Pump Bolts (M8 x 1.25): 15-20 Nm (11-15 lb-ft)
Thermostat Housing Bolts (M8 x 1.25): 15-20 Nm (11-15 lb-ft)
Flywheel Bolts (M10 x 1.25): 35-40 Nm (26-30 lb-ft)
The Yanmar 2GM20 engine was deployed exclusively in marine propulsion
applications for sailing yachts, motor sailers, and small workboats.
Sailing Yachts (25-33 ft / 7.6-10 m): The 2GM20 was widely installed in
mid-sized cruising and racing sailboats of the 1980s through the 2000s.
Common applications included the Catalina 30, Catalina 27, Hunter 31,
Hunter 28, Beneteau First 30, Beneteau Oceanis 321, Pearson 31, and
O’Day 35. The 2GM20F freshwater-cooled variant was preferred for
saltwater use to reduce internal corrosion. The SD20 saildrive
configuration was popular for sailboats with under-hull profiles
unsuitable for shaft installations.
Motor Sailers: Heavy displacement motor sailers receiving auxiliary
power from the 2GM20 included the Nonsuch 30 and various Fisher designs,
where the engine’s 13.4 kW (18 hp) provided adequate propulsion under
power without excessive weight.
Small Workboats: Harbor launches, fishing skiffs, and small pilot
vessels up to 8 m (26 ft) used the 2GM20 as primary propulsion. The
hand-crank backup starting system was valued for commercial applications
where battery failure could not be tolerated.
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