Sachs is a type of moped with origins from Germany.( see also Hercules ) The full name of the company that produced Sachs engines is ZF Sachs. ZF Sachs also creates 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 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 ppularity in export markets the label Sachs was 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 extremely 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 must be scrapped. 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 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.
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 but rarely.
The type of transmission fluid 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.
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. For example, the 504 engine is the smaller counterpart to the larger, more common 505 engine.
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".
Z is a letter representing the type of cylinder head on the engine. A,B,C, and D are the varieties of cylinder heads. A and C are identical with 1.8 Horsepower of a displacement of 47cc. The B cylinder also has the displacement of 47cc but because of limitations in certain states, limitations were created through porting and carburation. As a result, the B cylinder has only 1.5 horsepower. The D cylinder is much more powerful than either A, B, or C with 2.7 horsepower. Despite being 49cc, this increase in horsepower is accomplished through much wider ports. As a result, the D cylinder is highly coveted by Sachs moped owners. The D cylinder also uses a different piston with a shorter skirt.
It is physically impossible to mount a D cylinder onto a B engine block without modification due to stud placement as well as locations of holes on the head. This may be circumvented by widening certain holes but is not recommended as a loss of compression/stability may result.
504 vs. 505 engine
The difference between the most popular engine types, the 504 and the 505 are major. Despite using the same cylinder head, shaft, magneto, and clutch assembly, the gearing is different as the smaller 504 does not have the pedal shaft running through the engine as the 505 does. It is also very rare to see a D cylinder on a 504 but it does exist.
For more information on the differences between engine types, please refer to this  page. It also covers Sachs engines used on earlier mopeds.
One of the most striking features of Sachs mopeds are the "coaster brakes" or "pedal-back brakes". 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 is somewhere around 44 teeth.