Mercedes-Benz M186 Specifications
| Engine Model: Mercedes-Benz M186 |
| Engine Type: 4-cycle inline 6-cylinder OHC 12-valve liquid-cooled gasoline |
| Total Displacement: 182.8 cu.in (2.99 L) |
| Rated Engine Power: 115-125 Hp (86-93 kW) at 4500 rpm |
| Fuel System Type: Twin downdraft carburetors |
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The Mercedes-Benz M186 engine is a post-war,
inline-six cylinder gasoline engine. It succeeds the pre-war M159 series
and was introduced in 1951 for the prestigious 300-series (W186) saloon.
This powerplant represents a significant evolution, blending robust,
over-engineered construction with modernized features for the 1950s.
Known for its smoothness, durability, and substantial low-speed torque,
the M186 was engineered for luxury sedans.
It incorporates an aluminum cylinder head, a thermostatically controlled
oil cooler, and twin carburetors. The engine's design focused on
providing relaxed, high-speed cruising capability and long-term
reliability, establishing it as a cornerstone of Mercedes-Benz's
renaissance in the luxury automobile market during the 1950s.
Engine family: Mercedes-Benz M186.
Engine type: Inline-six cylinder, four-stroke Otto cycle, water-cooled.
Displacement: 2996 cc (182.8 cubic inches).
Bore: 85.0 mm (3.35 inches).
Stroke: 88.0 mm (3.46 inches).
Compression ratio: 6.4:1 for early versions; 7.4:1 to 7.5:1 for later
versions (M186.921).
Valvetrain: Single overhead camshaft (OHC).
Maximum power output: 115 Hp (86 kW) at 4600 rpm for type M186.920. 125
Hp (93 kW) at 4500 rpm for type M186.921.
Maximum torque output: 22.5 kgm (163 lb-ft / 221 Nm) at 2600 rpm.
Firing order: 1-5-3-6-2-4.
Engine orientation: Longitudinal.
Aspiration: Naturally aspirated.
Fuel type: Gasoline.
Recommended oil grade: SAE 20W-50 or equivalent monograde, per period
specifications.
Oil capacity: 7.0 liters (7.4 U.S. quarts).
Coolant capacity: 16.0 liters (16.9 U.S. quarts).
The cylinder block is a single, heavy casting from grey cast iron. It is
designed with deep water jackets for effective cooling. The block
incorporates six cylinder bores, cast integrally without liners in early
versions, with a bore spacing of 100 mm (3.94 inches). The crankshaft is
supported by seven main bearings, ensuring smooth rotation and
durability. The crankshaft is a forged steel component, hardened for
increased wear resistance.
Main bearing caps are cast iron and secured with two bolts each.
Connecting rods are forged from steel, with a robust I-beam design.
Pistons are cast aluminum alloy, featuring a flat crown design. Each
piston is fitted with three compression rings and one oil control ring.
The block's deep skirt design contributes to overall rigidity.
The cylinder head is a cast aluminum alloy unit. This material
represented a modern advancement, reducing weight and improving heat
dissipation. The valvetrain uses a single overhead camshaft (OHC),
driven by a duplex roller chain from the crankshaft.
The camshaft acts directly on inverted bucket-type tappets which operate
the valves, a design ensuring precise valve control. Valve clearance
adjustment is performed with shims placed under the tappets. The engine
uses two valves per cylinder (one intake, one exhaust). Valve sizes are
substantial relative to the displacement to support airflow. Intake and
exhaust valves are positioned in line. Combustion chambers are of a
wedge design.
The fuel system employs twin Solex 32 PAJAT downdraft carburetors. These
are single-choke carburetors with a fixed venturi. The dual-carburetor
setup is designed to improve volumetric efficiency and power output over
a single-carburetor system. The carburetors are manually synchronized
during service.
A mechanical, diaphragm-type fuel pump, driven by an eccentric on the
camshaft, delivers fuel from the tank to the carburetors. A manual choke
cable is operated from the dashboard for cold starting. The system
requires periodic adjustment and balancing to maintain optimal
performance and fuel mixture.
The lubrication system is a pressure-fed, full-flow wet-sump design. A
gear-type oil pump is driven from the crankshaft. Oil is drawn from the
sump through a pickup screen, pressurized, and forced through a
full-flow, replaceable cartridge-type oil filter. A defining feature is
the inclusion of a thermostatically controlled oil cooler.
This cooler regulates oil temperature, ensuring optimal viscosity during
warm-up and under high-load conditions. Oil from the main gallery
supplies the seven main bearings and connecting rod bearings. A separate
feed line supplies the overhead camshaft bearings and tappets. The
system maintains stable oil pressure across the operating range.
The cooling system is a pressurized, pump-circulated system. A
centrifugal water pump is driven by a V-belt from the crankshaft pulley.
Coolant circulates through the deep-jacket cylinder block and aluminum
cylinder head, exiting to a large, crossflow copper-and-brass radiator.
A wax-type thermostat regulates coolant flow to maintain efficient
operating temperature.
The ignition system is a conventional coil-and-distributor type. The
distributor is driven from the camshaft and features centrifugal and
vacuum advance mechanisms for spark timing control. The generator and
coolant fan are also V-belt driven.
The external dimensions of the M186 engine reflect its inline-six
configuration and substantial construction. Approximate length is 1100
mm (43.31 inches). Approximate width is 650 mm (25.59 inches).
Approximate height is 900 mm (35.43 inches). These dimensions include
the fan, generator, and carburetors. The weight of the complete M186
engine, with fluids and standard accessories, is approximately 250 kg
(551 lbs). This mass contributes to the vehicle's smooth and stable
driving character.
Maintenance intervals were more frequent than modern standards. Engine
oil and filter changes were recommended every 3000 km (1864 miles) or
quarterly. Spark plugs required cleaning and gap adjustment every 5000
km (3107 miles), with replacement at extended intervals.
Valve clearance inspection and adjustment via shims was a critical
service task, performed every 10,000 km (6214 miles). Carburetor
synchronization needed checking and adjustment at each major service.
The timing chain required periodic inspection for wear. Coolant should
be drained, flushed, and refilled annually. The oil cooler thermostat
function should be verified during servicing.
Torque values are specified for clean, lightly oiled threads. Cylinder
head bolts are tightened in a specific multi-stage sequence: final
torque 10.0 kgm (72 lb-ft / 98 Nm). Main bearing cap bolts: 8.5 kgm (62
lb-ft / 83 Nm).
Connecting rod cap bolts: 7.5 kgm (54 lb-ft / 74 Nm). Crankshaft pulley
bolt: 25.0 kgm (181 lb-ft / 245 Nm). Camshaft sprocket bolt: 8.0 kgm (58
lb-ft / 78 Nm). Spark plugs: 3.5 kgm (25 lb-ft / 34 Nm). Oil drain plug:
5.0 kgm (36 lb-ft / 49 Nm).
The Mercedes-Benz M186 engine had a singular, prestigious vehicle
application. It was developed specifically for and installed exclusively
in the Mercedes-Benz 300 (W186) "Adenauer" luxury saloon, produced from
1951 to 1957.
This includes the initial 300 model and its subsequent refined versions,
the 300b (1952) and the 300c (1955). The engine was not used in any
other Mercedes-Benz model series, making it unique to the flagship W186
line. It was succeeded by the M189 engine in the W189 300d model.
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