M2B15 owner’s manual

M2B15 BMW 500 cc engine

by Duane Ausherman

This is the 1921 owner’s manual for the very first BMW motorcycle engine made in 1920.   They were sold to motorcycle manufacturers Victoria, Helios, Corona, Heller, and Scheid.   The next year, BMW made the engine for their own motorcycle, the R32.   It was reported that the ignition and carbs are different on the R32.  I owned this manual for many years and just recently sold it.

The front cover lettering has faded, but the emblem is still nice and clear.

This is a translation provided by Cris of the /5 United group.  Thank you very much.   He is unsure of some of the terms, and so any help would be appreciated.   I have taken the liberty to add in American English and edit it for ease of reading.  In July 2018, I filtered this text with Grammarly and found more than 50 ways to improve it for greater readability.


The Bayern Small Engine embodies, despite its small size and low weight of only 31kg, all achievements of the modern engine building by which the Bayern Engines distinguish themselves.   The engine, of which the parts have been thought through into the finest detail, and been manufactured on first-class dedicated machines by modern processes, is one of the best examples of its class, and this is both in a technical and economical sense.  Its operation, though, as that of all precision machines, demands a thorough knowledge of its construction and operating procedure, which is why these are described here in a short and easy-to-understand way.   At the same time, a few practical tips are given to the user about the correct operation and maintenance of his engine.

A thorough study of the instructions and data in this booklet is urgently recommended.

I.  The Bayern Small Engine

A.  Description

Cylinder: the two gray cast-iron cylinders are horizontally opposed and amply provided with cooling fins, which, axially oriented on the cylinder, allow a complete flow of air and thus guarantee good cooling of the cylinder on all sides.   The cylinders are cast in one piece and attached to the crankcase with bolts.

Valves: Intake and exhaust valves are situated next to each other in an easily reachable location on top of the Cylinders:  For their fabrication, only first-class materials were employed, which means that valve grinding will only be necessary in very rare cases.   After the removal of a bolted-on cover, each valve can be easily removed.

Actuation: The actuation of the valves is taken care of by a camshaft parallel to and above the crankshaft.  Its movement is carried over from the cams to rollers, situated in ??Anhubkolben??- could be pushrods.  The valve train is enclosed and dust-free.   After the removal of a knurled thumb screw, the valve stems and pressure springs are easily accessible.

Any impurities present in the fuel settle down here and can be removed from time to time after the fuel filter is screwed out, while any water contained in the fuel can be drained off by turning the drain nipple h.

Pistons: The pistons are manufactured from a high-quality special aluminum alloy in the aluminum foundry of the Bayerische Motoren Werke.  These pistons, containing three cast iron piston rings and one oil scraper ring, are very reliable due to their lightness.  Additionally, the heat drain of aluminum pistons is very favorable, which is of great importance in air-cooled engines.

Piston Rods: The hollow piston rods are likewise manufactured out of the best materials and are very resistant.

Crankshaft: composed of special steel and fully balanced.  The fabrication of the double offset crank is extremely meticulous, and the surface load is relatively low due to the ample measures of the crankpins.  On the flywheel side, the crank runs in a double, and on the carburetor side in a single bearing.  The middle link is designed as a crank disk, provided with drilled holes for the redirection of lubricating oil.

Flywheel: The flywheel is situated on the left side of the engine and is attached to the crankshaft by means of a conical coupling and a nut.  For easier removal of the flywheel, a puller nut is incorporated.

Crank housing: The crank housing is made of excellent quality cast aluminum and is constructed in two parts so that the lower part serves at the same time as an oil container and an oil pump housing, while the drive wheels for the camshaft and the magneto are situated in the upper part.

Carburetor: The carburetor is an in-house construction and the result of extensive preliminary experiments.  It lives up to all the requirements of engine use and functions, in the first place, very economically.  In Figure IV, the carburetor is depicted in various cut diagrams.  It consists of float chamber a, in which the float b and a needle valve c are pressed down by a weighted lever, the fuel level is kept at the same height at all times, the mixing chamber d, and the air regulation e.  The fuel enters through the supply tube f and the fuel filter g into the lower part of the carburetor housing.

From the float chamber, the fuel passes through the channel i and the main jet k into the mixing chamber d, in which it mixes with the air that enters through the atomizer l and ends up as a flammable air-fuel mixture in the openings n, regulated by the throttle slider m, into the connection tube and finally into the cylinder.  The throttle slider is secured against shifting either by a screw that is reachable from outside or by a spring and nut.  When the slider is opened, the engine speed increases.  The slider must never be filed or sanded, as then the sides of the openings n will let air through, and the engine will not return to idle.  The slider o is for the regulation of supplementary air, which reaches the mixing chamber through the opening p.  When departing, the supplementary air is to be closed.  In cold weather, the engine needs less air than when warm; the right setting is easy to determine after riding a few kilometers.  Now, one can leave the air lever in the middle position and drive slowly without having to adjust the supplementary air, as the throttle slider covers the supplementary air channel when closed.  By the addition of a filter q in the inlet channel, large dirt is prevented from entering the carburetor or the cylinder with the intake air.  Depending on the position of the engine on the bike, the inlet channel can be replaced by a curved tube that leads to the engine cylinder to take in warm and, if possible, dust-free air in a sheltered position through a filter situated at the end of the tube.  With the engine idling, the fuel enters from the channel in the idle jet r, from which it enters the engine through the channel s, mixed with the air entering at t.  When any plugging of the jets k and r, they can easily be removed after loosening the jet screws u and v.  Cleaning the jets must only be done by blowing them through with the air pump; never must one try to do this with a needle or other tool, as the jets will be widened and the carburetor will use more fuel.  When the carburetor is set up right and under normal riding conditions, the main air slider is to be opened for about 3/4, while during steep uphill stretches, the air intake should be reduced more so that the engine, in order to deal with the heightened output, can draw in a larger amount of fuel.

Ignition: the ignition of the air-fuel mixture takes place by a high-tension ignition magnet, which is attached with straps on a flat plane on top of the engine to be easily detachable.

Lubrication and oil circulation: Since the safety of operation and the engine life depend highly on the lubrication of an engine, special attention was given to the careful construction of the lubrication apparatus of the Bayern Small Engine.  The lubrication is completely automatic and is provided by a gear pump situated in the lower part of the crankcase, which is also designed as the oil holder.  From here, the oil is pressed, through a calibrated oil jet, into the hollow crankshaft, and from there, through the first link of the crank, which is provided with a channel, into the first crankpin.  Part of the oil passes through the channel into the piston rod of the first cylinder and into the small end, while the other part enters into the middle part of the crank, which is disk-shaped, and from here through channels near the disk circumference into the second crankpin and is carried on into the attached piston rod.  The oil that escapes from the crankpins is flung against the cylinder surfaces, the camshaft, and the timing wheels so that all parts of the engine are amply provided with oil.

The oil that runs off the sides of the crankcase pools back into the crankcase and is used again after passing through an oil filter.  The oil level in the crankcase has to be measured from time to time with the dipstick provided on the crankcase.  The oil should reach the upper mark and must not, in any case, drop below the lower mark.  In winter, thinner oil is to be used than in summer, as with lower temperatures, the oil always gets thicker, and the engine is hard to start with oil that is too thick.  The lubrication does not have to be attended during riding. Only the following three points have to be taken care of :

1.  Before the operation of the engine, the crankcase needs to be filled with oil up to the highest mark of the control rod.

2.  The oil level needs to be checked from time to time so that it will not drop below the lowest mark on the dipstick.  If necessary, add fresh oil.

3.  After ca.  2000 km, the contaminated oil must be completely drained after removal of the filter tube screwed in under the flywheel and replaced with fresh oil.

Decompression: seen from the flywheel side of the engine, sits, on the left side under the magneto, a guide nipple provided with adjusting screws for the connection of the decompression actuator.  After unscrewing this nipple and removing the round-headed screw provided on the opposite side of the magneto, the pull rod for decompression can be moved by strong pressure with a ca.  3mm broad wire, after which the Bowden-cable can be attached in the formerly mentioned location.  The decompression is to be used if the engine is to be stopped every now and then or continuously.

B.  Set-up of the engine after prior disassembly.  After the assembly, both pistons must be set at top dead center.  In this position of the crankshaft, the gears need to be set up so that the arrows on the cam gear line up with the arrows on the crankcase.  When this is the case, the top part of the crankcase can be put back and bolted tight.  For the setup of the ignition, the piston must be set ca.  7mm before TDC so that the interruption hammer of the magneto is in the detached position.  The magneto can then be put back.

C.  Accessories

With every engine, the following tools are included :

1 special spanner for the connection of the flywheel

1 spanner for the closing nipple of the valves

1 spark plug socket

1 handle for a spark plug socket

1 valve lifter

II.  Mode of operation

The Bayern small engine works in the four-stroke regime, i.e. four-piston movements or piston strokes are necessary for a single force output.

1st stroke: Intake stroke.  The piston goes down, the flammable gas-air mixture is sucked in with the intake valve open and the exhaust valve closed

2nd stroke: Compression stroke: Piston goes up, and the mixture gets denser (compression).  Intake and exhaust valves closed.  Shortly before reaching top dead center, ignition of the compressed mixture takes place.

3rd stroke: Power stroke: second down stroke of the piston, caused by the explosion and consequent expansion of the mixture.  Both valves remain closed.

4th stroke: Exhaust stroke: the second upstroke of the piston with the open exhaust valve and closed intake valve.  The burned mixture is expelled.

This game repeats itself during every two turns of the crank, during which time both cylinders ignite once each.

III Operating conditions.

A.  Fuel and lubrication

1.  Fuel: Gasoline serves as fuel insofar as no other fuels are indicated in the acceptance certificate that is delivered with every engine.  The fuel must be filled through a filter.  No impurities should enter the fuel tank, as plugging of tubes and jets will ensue.

2.  Lubricating agent: For the lubrication of the engine, only the best acid-free engine oil must be used, and a good brand of automobile oil.  In winter, a thinner oil should be used than in summer, as the oil is thicker at lower temperatures anyway.  Like the fuel, the oil must be filled through a fine-maze sieve.  Putting the filler funnel on sandy soil must be avoided.

B.  Treatment of the engine.

1.  Filling fuel: In a motorbike, the fuel is provided to the engine from the tank serving that purpose.  The fuel must, as mentioned higher, be filled through a sieve, after which the cap is to be closed.  The fuel line, its connections, and the fuel cock are to be tested for secure fastening and fuel tightness.

2.  Filling oil: Through the filler situated on the crankcase, oil is to be filled until the highest mark on the measuring stick.  Attention must be paid that oil is only filled through a funnel with a sieve.  At temperatures under 0 degrees C (32 F), warm oil is to be filled.  Lubrication of all the working parts of the engine is automatic and does not need to be attended to.

3.  Valves: It is necessary to test the valves for tightness from time to time and, in case they are not entirely closed anymore after longer operation times, to grind them carefully.  Upon reassembly, it must be looked after that in no case grinding dust ends up in the valve chambers, as otherwise, the valves, the pistons, and the cylinders will be affected.

Updated 30 March 2023