This is the 1921 owners manual for the very first BMW motorcycle engine
made in 1920. It was sold to motorcycle manufacturers Victoria,
Helios, Corona, Heller and Scheid. The next year, BMW made the engine for
their own motorcycle, the R32. It is reported that the
ignition and carbs are different on the R32.
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.
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 both in 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
A thorough study of the instructions and data in this booklet is
I. The Bayern Small Engine
Cylinder : the two gray cast-iron cylinder 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 dust-free. After removal of a
knurled thumb screw, the valve stems and pressure springs are easily
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 scrape
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 very resistant.
Crankshaft : composed of special steel and fully balanced. The
fabrication of the doubly 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, 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 oil container and as 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 by the float
b and a needle valve c 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 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 which 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 i 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 places 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 a 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
temperature, 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 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
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 crank 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 set up 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.
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 sparkplug socket
1 handle for sparkplug 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
1st stroke : Intake stroke. Piston goes down, flammable gas-air
mixture is sucked in with the intake vlave open and the exhaust valve
2nd stroke : Compression stroke : Piston goes up, mixture gets more
dense (compression). Intake and exhaust valves closed. Shortly
before reaching top dead centre, ignition of the compressed mixture takes
3rd stroke : Power stroke : second down stroke of the piston, caused by
the explosion and consequent expansion of the mixture. Both valves
4th stroke : Exhaust stroke : second upstroke of the piston with opened
exhaust valve and closed intake valve. 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 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
2. Lubricating agent : For the lubrication of the engine only the
best acid-free mineral oil must be used, and namely only 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 me 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
A couple of pages are not included as they contain no
technical information of interest. I have this manual stored in
format too. These photos shown are low quality jpg images. I
also have them in higher quality .jpg.