Sachs was one of the biggest built-in - engines producer of the world with headquarter and production sites in Germany.( see also Hercules ) The full name of the company that produced Sachs engines is ZF Sachs. ZF Sachs also created motorcycles, motorized bicycles and ATVs.
Much like the Minarelli engine, Sachs engines were put onto a variety of other companies' moped frames including (but not limited to) Grycner, Columbia, Foxi, General, Hercules ,KTM, Moppet, Sparta, MBK as well as other smaller moped varieties. Unlike Minarelli, ZF Sachs also created their own brand of mopeds with the Sachs name as well as Hercules. The original manufacturers name is Hercules , due to lack of popularity in export markets the label Sachs or DKW were chosen. For example, the Balboa is very similar to the Hercules but with slight trim changes.
50x series Engines
The Sachs 50x series engine is known for it's high rate of compression, ease of use, stability, as well as lack of case gasket. High compression is achieved though the unorthadox use of a combined cylinder and head. Refered to as a "jug", this can be a mixed blessing as it is difficult to clean carbon deposits at the top of the jug. Also, if a spark plug hole's threads are damaged and rendered unusable, the entire cylinder may be scrapped, or have the hole heilicoiled. There are benefits to the engine with a relatively efficient clutch assembly that can be easily changed. The engine case is also split horizontally leading to no use for a case gasket as the transmission fluid sits below the seam of the engine case. Also, interchangable parts, cases, gears are used between manual and automatic models.
The Sachs 50x series engine were available with 1 gear automatic transmission ( mainly cheap mopeds and export ) and 2 gears as well as 3 gears manual transmission. The engine could be ordered with incorporated pedal start, Kick - start, handlebar gear-lever as well as foot - gear lever. Kickstarted 3 gear engines are rare to find as production numbers were significant lower than the popular 2 gear engines.
The Sachs 50x series automatic engine also features a cable that engages not only the clutch but also a decompression valve. This decompression valve can be bypassed by fabricating a new cable. Doing this can make the moped harder to start but takes the weak decompression valve out of the equation. The valve itself can be replaced by a bolt of the same size (M8).
Electronically, there were many variations of stator plate/magneto combinations. The most used solution was a Bosch variant of the 3, 4, or 5 wire type. Motoplat was also used for the stator plate/magneto combination on later produced engines until production was stopped.
The type of transmission fluid for the automatic engines used should be Type F automatic, filled to the bottom of the plastic screw on the transmission side of the engine. This screw is easily stripped or ruined but replacements are cheap, metal variants available. For all manual transmissions use SAE80 gearbox oil.
The cylinder bore was usually a stock 37.98mm.
Nomenclature 50x series
The Sachs engine has it's own nomenclature that goes as follows:
50X / Y Z
X is a number representing the model number of the engine.
1: The 501 was an early, inclined cylinder engine with a kickstart and foot shift. It was available in 3 and 4 speed models. 2: The 502 was the early single-speed automatic, available in an A and B configuration. 3: The 503 is a very rare single-speed automatic, airfan cooled and mainly produced for the French and Swiss market. 4: The 504 was available in a single speed automatic. They only have 2 motor mounts, and do not have pedals attached to the motor. A 504 and 505 are not interchangeable in frames without frame modification. 5: The 505 is the most common Sachs engine in the US. It has the pedal crank built into the case with a freewheel, connected directly to the drive gears. All US 505's. The 505 is also available in 2-speed and 3-speed manual shift. In later German models even a kickstart was applied instead of the crankshaft. 6: The 506 was a late model 3 and 4 speed, inclined cylinder engine, europe only.
Y is a number representing the amount of gears that the transmission contained. In the United States, manual transmission engines were not allowed legally so, occasionally this number was omitted. Automatics are refered to as "1". In europe 2, 3 and 4 speed manual engines were sold in a variety of applications.
Z is a letter representing the performance-rated configuration of the engine.
Some Sachs engines came with these identifiers, some did not. Most sachs engines came with an A and B configuration, denoting the fast (A) and restricted (B) models. Some engines used different cylinders, some different carbs, different head pipe diameters, some a combination.
US distributed 505's used the following nomenclature to describe the engine configuration: A,B,C, and D were the configuration designations. A has 1.8 Horsepower and a 12 mm square bing, it features a tapered cylinder. The B cylinder is similar to the A in appearance but has 1.5 horsepower due to the 10mm bing carb and more restrictive porting, and a smaller head pipe. The C cylinder was square, like the D cylinder, but produced 1.8 hp. The A, B and C varieties all use the same 42 mm crankshaft to achieve displacement of 47.6 cc, and a compression ratio of 8:1. The D cylinder can be easily identified by its larger, squared-off cooling fins. It is much more powerful with 2.7 horsepower. The D cylinder comes on a crankcase with a heavier drive gear and 44mm stroke crankshaft, boosting the displacement from 47.6cc to 49.9cc and delivering a compression ratio of 10:1. This, combined with significantly larger ports and a shorter piston skirt, results in the significant horsepower increase. Some cylinders also use a slightly different bolt pattern between the base mounting holes, so they cannot be easily interchanged.
US Distribution 504's used A and B to characterize them as 12 mm carb (1.5 hp) or 10 mm carb (1.1 hp). the 504 C is the 504 motor with the square jug. The C also came with the 12mm carb, and larger headpipe.
504 vs. 505 engine
The difference between the most popular engine types, the 504 and the 505 is the 504 does not have the pedal shaft running through the engine as the 505 does. The flywheel on the 505 is larger than the flywheel on the 504. Crank shafts cannot be swapped between the two. The 505 has a coaster brake like rear brake. Inside the engine case there is a sprag set up that allows you to backpedal to use the rear brake. They do use the same cylinder head, motor mounts, exhaust and clutch assembly. For more information on the differences between engine types, you could refer to:  
One of the most striking features of Sachs mopeds are the "coaster-brakes" or "pedal-back brakes" on mopeds with 505 motors. This feature means that the rear brake is engaged when the pedals are rotated backwards allowing for sudden stops. This also makes the left-hand lever not engage the rear brake cable as usual but the decompression/clutch cable.
US models have tires with 17 inches around and usually 2.25 to 2.5 inches wide. From Sachs/Hercules, Mag wheels as well as spoked wheels were available. The stock rear sprocket for US mopeds is somewhere around 44 teeth. European versions like the Optima 50 with 3HP engine have a stock 32 teeth rear chainwheel. The front sprocket is usually an 11 tooth. Anything larger requires cutting of the transmission side of the case. This can be done carefully because the case is fairly thick.