Air/oil cooling and vertically split engine housing.

Air/oil cooling and vertically split engine housing.

The "Big Boxer" is air/oil cooled. Its aluminium engine housing is split vertically. The crankshaft forged from heat-treated steel has an additional main bearing in the middle to avoid unexpected bending vibrations. Both heat-treated steel connecting rods with i-shafts are equipped with friction bearings like the crank shaft. The pistons are cast with aluminium, and the running surface of the light-alloy cylinders is coated with a NiCaSil finish. Wet sump lubrication guarantees the supply of oil for lubrication and cooling.

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Classic OHV valve train coupled with modern technology.

Classic OHV valve train coupled with modern technology.

The "Big Boxer" has four valves. This produces the best possible torque and optimal consumption and emission values.  Dual ignition, a modern combustion chamber architecture, as well as intake manifold injection and the BMS-O engine management system. The inspiration for the valve train was given by a legendary engine design from BMW Motrrad: the twin-cylinder boxer engine on the R 5/R 51 from 1936 – 1941 and the R 51/2 from 1950 – 1951. Compared to other OHV designs from BMW Motorrad, the Big Boxer has –  like its historic predecessors – two camshafts driven by the crankshaft using a bush chain. This provides a more solid valve train with improved precision for handling and greater high-speed performance.  Or even: more emotion on the road. 

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Fork rocker arm and manually adjustable valve play compensator.

Fork rocker arm and manually adjustable valve play compensator.

Also completely a BMW Motorrad boxer design: Both valve push rods activate one push rod each on the top and bottom of the cylinder in a sealed tappet rod for the intake and exhaust side. Both intake and exhaust valves are actuated in pairs via fork rocker arms. The valve play is not compensated for using hydraulic elements, rather using one adjustment screw with lock nut per valve. Just like what most common classic air-cooled BMW twin-cylinder boxers have had for decades. In this manner, the valve play (0.2 – 0.3 mm) can be adjusted in a very short time, even with the "Big Boxer". We are bringing the history of design back onto the road, and you will feel it in your whole body. 

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Six-speed claw transmission with anti-hopping feature.

A single-plate dry clutch transmits the torque generated by the engine to the transmission, which is typical of the vast majority of our boxer engines. What is new, however, is the anti-hopping feature that prevents the rear wheel from locking up during a hard downshift.  For greater safety and a more comfortable ride. 

The six-speed claw transmission is located in a two-part aluminium housing. It is constructed as a four-shaft drive with helical gear pairs. A reverse gear is also available as an option. This gear is driven by an intermediate transmission and electric motor and can be controlled manually. 

As with all BMW motorcycles with a boxer engine, torque is transmitted from the transmission to the rear wheel via a cardan shaft or cardan drive with universal joint.  A tripod joint on the transmission side ensures length compensation. With the Big Boxer – as was the custom at BMW Motorrad up to and including model year 1955 – the cardan shaft and universal joint are nickel-plated like the fascinating pieces of classic motorcycle technology that they are. They also run open – a real visual delight. 

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Heritage Hub.

Discover the new 1800cc boxer engine in action on the new Concept R 18 models and Guerilla bikes.

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