Mercedes-Benz OM636 Specifications
| Engine Model: Mercedes-Benz OM636 |
| Engine Type: 4-cycle inline 4-cylinder OHV 8-valve liquid-cooled diesel |
| Total Displacement: 103.6-107.8 cu.in (1.69-1.76 L) |
| Rated Engine Power: 38-46 Hp (28-34 kW) at 3300 rpm |
| Fuel System Type: Indirect injection |
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The Mercedes-Benz OM636 is a water-cooled, inline-four
diesel engine produced by Daimler-Benz from 1948 through 1990. Originally
conceived during the final years of World War II, it emerged as the foundational
powerplant that established Mercedes-Benz’s reputation for diesel durability and
longevity. As the direct successor to the OM138 and OM143 pre-war diesel
engines, the OM636 became the “forefather” of the legendary Mercedes diesel
lineage that would follow, including the OM621, OM615, and OM616 families. Its
production run of over four decades - one of the longest in automotive history -
attests to the soundness of its engineering and its adaptability across
passenger car, commercial vehicle, marine, and industrial applications.
The OM636 is an indirect injection (IDI) diesel utilizing a pre-combustion
chamber design, a configuration that prioritized smoothness and reduced
mechanical stress over outright power output. Its architecture is defined by a
grey cast iron cylinder block and cylinder head, a gear-driven camshaft mounted
within the block, and a forged crankshaft supported by three main bearings.
This robust construction, combined with conservative power ratings, yielded
exceptional service life, with documented units exceeding 1,250,000 km (780,000
mi) in daily commercial service. The engine’s versatility was demonstrated by
its adoption in passenger cars such as the W136 170D and W120 180D Ponton,
utility vehicles including the early Unimog and L319 transporter, and countless
industrial installations ranging from marine propulsion to refrigeration units
and agricultural equipment.
Engine Type: Inline 4-cylinder, 4-cycle, naturally aspirated diesel,
water-cooled.
Displacement: 1,697 cc (103.6 cu.in) for early variants.
Displacement: 1,767 cc (107.8 cu.in) for later variants.
Bore: 75.0 mm (2.95 in).
Stroke: 100.0 mm (3.94 in).
Compression Ratio: 19.0:1.
Valvetrain: Overhead Valve (OHV) with pushrods and rocker arms, 8 valves (2 per
cylinder).
Camshaft is gear-driven from the crankshaft and mounted within the cylinder
block.
Firing Order: 1-3-4-2.
Maximum Power Output (1949 passenger car): 28 kW (38 hp) @ 3,300 rpm.
Maximum Power Output (1954 passenger car): 29 kW (40 hp) @ 3,300 rpm.
Maximum Power Output (industrial / marine): 31-34 kW (42-46 hp) @ 3,500-3,600
rpm.
Maximum Torque Output: 100-110 Nm (74-81 lb-ft) @ 1,600-2,000 rpm.
Idle Speed: 500-600 rpm.
The cylinder block is constructed from grey cast iron,
a material selected for its exceptional rigidity, vibration damping
characteristics, and wear resistance. The block incorporates an open-deck design
with cast-in-place cylinder bores machined directly into the casting. No
removable cylinder liners are utilized; instead, the bores are finished with a
precision honing process that establishes the final surface finish for piston
ring sealing. Bore diameter measures 75.0 mm (2.95 in) across all displacement
variants.
The piston stroke is 100.0 mm (3.94 in), a long-stroke configuration that favors
low-end torque generation - the operational priority for diesel engines of this
era. Total displacement for early production engines is 1,697 cc (103.6 cu.in).
Later variants achieved a displacement increase to 1,767 cc (107.8 cu.in)
through bore enlargement while retaining the same stroke length.
The crankshaft is forged from chromium-nickel steel, a high-strength alloy
selected for fatigue resistance under sustained diesel compression loads. Three
main bearings support the crankshaft, with bearing journals measuring 65.0 mm
(2.56 in) in diameter. Thrust washers positioned at the center main bearing
control axial crankshaft play. Connecting rods are forged from high-carbon steel
with an I-beam cross-section and a bearing journal diameter of 50.0 mm (1.97
in). Pistons are cast aluminum with three piston rings: two compression rings
and one oil control ring. The piston crown incorporates a combustion bowl that
aligns with the pre-combustion chamber in the cylinder head.
The camshaft is mounted within the cylinder block on three bearings and is
driven directly from the crankshaft via a helical gear set. This gear-driven
arrangement eliminates the need for a timing chain or belt, providing precise
valve timing over an extended service life. The camshaft operates the pushrods
through cam followers (tappets) that ride directly on the cam lobes.
The cylinder head is manufactured from grey cast iron,
consistent with the cylinder block material. This all-iron construction
contributes to the engine’s substantial dry weight of 158 kg (348 lb) but
provides superior thermal stability and gasket sealing characteristics compared
to aluminum designs of the era. The head is secured to the cylinder block using
eight head bolts arranged around each cylinder.
Each cylinder features a pre-combustion chamber (swirl chamber) located within
the cylinder head casting. This chamber receives fuel from the injection nozzle,
and initial combustion occurs within this confined space before expanding into
the main cylinder. The pre-combustion chamber design reduces combustion noise,
lowers peak cylinder pressures, and enables the use of a lower compression ratio
than would be required for direct injection, though it imposes a penalty on
absolute thermal efficiency.
The valvetrain consists of eight valves: four intake valves and four exhaust
valves. Intake valve diameter measures 35.0 mm (1.38 in), while exhaust valve
diameter measures 32.0 mm (1.26 in). Valve stems measure 9.0 mm (0.35 in) in
diameter. Valves are actuated by rocker arms mounted on a shaft running the
length of the cylinder head. Pushrods transmit motion from the camshaft-mounted
cam followers to the rocker arms, which in turn open the valves against coil
spring pressure. Valve clearance adjustment is accomplished via threaded
adjusters at the rocker arm-to-valve stem interface.
Valve seat inserts are manufactured from heat-resistant alloy steel, pressed
into the cylinder head casting. Valve guides are cast iron, pressed into the
head with an interference fit. The valve cover is stamped steel, secured with
acorn nuts and sealed with a cork gasket.
The OM636 employs an indirect injection fuel system
utilizing a Bosch inline injection pump. The pump is the PES series,
specifically the PES4A50 or later PES4A60 variants depending on production date.
This mechanically governed pump is mounted to the side of the cylinder block and
driven by the camshaft gear train. The injection pump contains individual
plunger-and-barrel assemblies for each cylinder, with fuel delivery volume
calibrated through adjustment of the pump’s control rack.
Injection timing is adjustable by rotating the injection pump relative to its
drive gear. Specification for static injection timing is 15-18 degrees before
top dead center (BTDC) measured at the injection pump drive. Timing is set using
a spill timing method or dial indicator inserted into the pump’s delivery valve
holder.
Fuel injectors are of the pintle type with a single-hole nozzle designed
specifically for pre-combustion chamber systems. Nozzle opening pressure is set
to 100-110 bar (1,450-1,600 psi). Injectors are installed into the cylinder head
with a copper sealing washer, secured by a clamp and retaining nuts.
The fuel system incorporates a hand primer lever on the injection pump to
facilitate system priming after filter replacement or fuel system servicing. A
primary fuel filter with water separator is mounted upstream of the injection
pump, with a secondary filter positioned between the primary filter and the
injection pump inlet.
The lubrication system is a forced-feed, full-flow
design utilizing a gear-type oil pump mounted to the front of the crankshaft.
The pump is driven by the crankshaft gear train and draws oil from the sump
through a wire mesh strainer. A pressure relief valve integrated into the pump
maintains system pressure within design parameters.
Total oil capacity is 4.5-5.0 liters (4.8-5.3 qt) including filter. Recommended
oil viscosity is 20W-40 or 15W-40 for most operating conditions, with API
classification CD or higher for diesel service. Oil pressure specifications
require a minimum of 1.0 bar (15 psi) at warm idle and 3.0-4.0 bar (44-58 psi)
at 2,000 rpm.
The oil filter is a full-flow, replaceable cartridge type housed in a cast
aluminum housing mounted to the cylinder block. Early production engines
utilized a bypass oil filter arrangement; later production standardized on
full-flow filtration. A separate oil filler cap is located on the valve cover.
The cooling system is pressurized and utilizes a
centrifugal water pump driven by the accessory drive belt. The pump is mounted
to the front of the cylinder block and features a cast iron housing with a
pressed steel impeller. Coolant circulation is controlled by a wax-pellet
thermostat located in the coolant outlet housing at the front of the cylinder
head.
Thermostat opening temperature is 75-80 °C (167-176 °F) for early production
engines, with later variants utilizing an 82-88 °C (180-190 °F) thermostat for
improved thermal efficiency. The system incorporates a bypass circuit that
allows coolant circulation within the engine during warm-up.
Total coolant capacity is 8.5-9.5 liters (9.0-10.0 qt), inclusive of the engine
block, radiator, and heater core where equipped. The radiator is a down-flow
copper and brass construction with a finned tube core. Cooling fan is a
mechanical unit with four or six blades, driven directly from the water pump
pulley. System pressure is controlled by a radiator cap rated at 0.5-0.7 bar
(7-10 psi).
Aspiration System: The OM636 is naturally aspirated,
with intake air drawn through a dry paper air filter element or, in early
applications, an oil-bath air cleaner. The intake manifold is cast iron with a
single plenum feeding all four cylinders. There is no turbocharger or
supercharger on production OM636 engines.
Timing System: The timing system consists entirely of helical gears. The
crankshaft gear drives an intermediate gear mounted on an idler shaft, which in
turn drives the camshaft gear and the injection pump drive gear. This gear train
is enclosed within the timing case at the front of the engine. No timing belt or
chain is present. Timing marks are stamped on the gears and the timing case
cover for synchronization verification.
Starting System: The engine is equipped with a Bosch electric starter motor
mounted to the bellhousing or the cylinder block depending on chassis
configuration. Cold-start assistance is provided by glow plugs installed in the
pre-combustion chamber of each cylinder. Glow plugs are of the pencil-type, with
manual activation via a push-pull switch on the dashboard in early vehicles and
automatic control via timer relay in later applications. Glow plug operating
voltage is 6 or 12 volts depending on vehicle electrical system.
Generator: The engine drives a generator (dynamo) via a V-belt from the
crankshaft pulley. Output is 12 volts, with current ratings from 130 to 250
watts depending on application. Later vehicles equipped with alternators instead
of generators.
Exhaust System: The exhaust manifold is cast iron, mounted to the cylinder head
on the left side of the engine. The system incorporates a single exhaust pipe
routing to a muffler and tailpipe.
Engine Management: No electronic engine management is present. All engine
functions are mechanical or electromechanical, including injection pump
governing, glow plug control, and generator voltage regulation.
Overall Length: 710 mm (28.0 in) measured from
accessory drive pulley face to transmission mounting flange.
Overall Width: 540 mm (21.3 in) inclusive of the injection pump and exhaust
manifold.
Overall Height: 760 mm (29.9 in) from oil pan bottom surface to valve cover top
surface.
Dry Weight: 158 kg (348 lb) without transmission, starter motor, or generator.
Valve Clearance (cold): Intake valves: 0.15-0.20 mm
(0.006-0.008 in). Exhaust valves: 0.20-0.25 mm (0.008-0.010 in). Adjustment
interval: 5,000 km (3,100 mi). Adjustment performed via threaded adjuster and
locknut at rocker arm. Timing Gear: No scheduled replacement interval.
Inspection recommended if unusual noise from timing case is observed.
Fuel Filter Replacement Interval: 10,000 km (6,200 mi) or 12 months. Water
separator draining recommended at each oil change. Engine Oil Change Interval:
5,000 km (3,100 mi) for standard driving conditions; 3,000 km (1,900 mi) for
severe service. Coolant Change Interval: 20,000 km (12,400 mi) or 2 years.
Coolant specification requires ethylene glycol-based formulation with corrosion
inhibitors suitable for cast iron components.
Glow Plug Inspection Interval: 10,000 km (6,200 mi). Resistance specification:
0.5-1.0 ohms at 20 °C (68 °F) for pencil-type plugs. Idle Speed (warm): 500-600
rpm, adjustable via idle speed screw on injection pump. Injection Timing
(static): 15-18 degrees BTDC at injection pump drive. Adjusted by rotating pump
body relative to drive gear.
Valve Timing Synchronization: Crankshaft at top dead center (TDC) No. 1 cylinder
on compression stroke; camshaft gear timing mark aligned with timing case
pointer; injection pump gear timing mark aligned with pump mounting flange
reference.
Compression Pressure: Minimum 20.0 bar (290 psi) with no cylinder varying more
than 2.0 bar (29 psi) from others. Measured with engine warm, throttle fully
open, glow plugs removed.
Cylinder Head Bolts (cold), Step 1: 50 Nm (37 lb-ft).
Cylinder Head Bolts (cold), Step 2: 70 Nm (52 lb-ft).
Cylinder Head Bolts (cold), Step 3: 90 Nm (66 lb-ft).
Crankshaft Damper Pulley Bolt: 180-200 Nm (133-148 lb-ft).
Connecting Rod Cap Bolts: 60-65 Nm (44-48 lb-ft).
Main Bearing Cap Bolts: 100-110 Nm (74-81 lb-ft).
Flywheel Bolts: 80-90 Nm (59-66 lb-ft).
Camshaft Bearing Cap Bolts: 25-30 Nm (18-22 lb-ft).
Camshaft Gear Bolt: 80-90 Nm (59-66 lb-ft).
Valve Cover Nuts: 5-8 Nm (4-6 lb-ft).
Intake Manifold Nuts: 20-25 Nm (15-18 lb-ft).
Exhaust Manifold Nuts: 25-30 Nm (18-22 lb-ft).
Oil Pan Bolts: 15-20 Nm (11-15 lb-ft).
Injection Pump Mounting Nuts: 20-25 Nm (15-18 lb-ft).
Injection Pump Drive Gear Nut: 60-70 Nm (44-52 lb-ft).
Fuel Injector Nozzle Holder: 50-60 Nm (37-44 lb-ft).
Glow Plugs: 20-25 Nm (15-18 lb-ft).
Water Pump Mounting Bolts: 20-25 Nm (15-18 lb-ft).
Thermostat Housing Bolts: 10-15 Nm (7-11 lb-ft).
Starter Motor Mounting Bolts: 30-40 Nm (22-30 lb-ft).
Generator Mounting Bracket Bolts: 25-30 Nm (18-22 lb-ft).
Passenger Cars: Mercedes-Benz W136 (170D) from
1949-1953, the first diesel passenger car offered by Mercedes-Benz after World
War II. Mercedes-Benz W191 (170DS and 170SD) from 1949-1952, representing the
extended-wheelbase luxury diesel variant. Mercedes-Benz W120 (180D “Ponton”)
from 1954-1961, the first unit-body diesel passenger car and the last
application of the OM636 in a Mercedes passenger vehicle before replacement by
the OM621.
Utility Vehicles: Unimog models 2010, 401, 402, and 411 from 1950-1965, where
the OM636 provided reliable power for agricultural and off-road applications.
Mercedes-Benz L319 transporter from 1955-1967, a light commercial van and truck
chassis utilizing the OM636 in diesel configurations.
Marine Applications: Extensively marinized by Volvo Penta as the MD4, MD5, and
MD6 series for sailboat and motor yacht propulsion. Sole Diesel and other marine
converters also produced OM636-based marine engines for fishing vessels and
pleasure craft.
Industrial Applications: Thermo King refrigeration units (reefers) for transport
refrigeration, where the OM636’s durability and ability to sustain
constant-speed operation proved advantageous. Stationary generators, forklifts,
agricultural tractors, combine harvesters, and industrial pumps received the
OM636 as a power unit, with production continuing in these applications into the
1990s.
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