| Bore | Stroke | Horse-power |
| 105 mm. | 140 mm. | 100 |
| 120 mm. | 140 mm. | 135 |
| 140 mm. | 150 mm. | 150 |
| 140 mm. | 160 mm. | 160 |
The largest of these motors has recently had its horsepower increased to 176 at 1450 R. P. M. This general design of motor has been the foundation for a great many other aviation motor designs, some of which have proved very successful but none of which is equal to the original. Among the motors which follow more or less closely the scheme of design and arrangement are the Hall-Scott, the Wisconsin motor, the Renault water-cooled, the Packard, the Christofferson and the Rolls-Royce. Each of these motors show considerable variation in detail. The Rolls-Royce and Renault are the only ones who have used the steel cylinder with the steel jacket. The Wisconsin motor uses an aluminum cylinder with a hardened steel liner and cast-iron valve seats. The Christofferson has somewhat similar design to the Wisconsin with the exception that the valve seats are threaded into the aluminum jacket and the cylinder head has a blank end which is secured to the aluminum casting by means of the valve seat pieces. The Rolls-Royce motors show small differences in details of design in cylinder head and cam-shaft housing from the Mercedes on which it has taken out patents, not only abroad but in this country.
THE BENZ MOTOR
In the Kaiser prize contest for aviation motors a four-cylinder Benz motor of 130 by 180 mm. won first prize, developing 103 B. H. P. at 1290 R. P. M. The fuel consumption was 210 grams per horse-power hour. Total weight of the motor was 153 kilograms. The oil consumption was .02 of a kilogram per horse-power hour. This motor was afterward expanded into a six-cylinder design and three different sizes were built.
The accompanying table gives some of the details of weight, horse-power, etc.
| Motor type | B | FD | FF | |||
| Rated horse-power | 85 | 100 | 150 | |||
| Horse-power at 1250 r.p.m | 88 | 108 | 150 | |||
| Horse-power at 1350 r.p.m | 95 | 115 | 160 | |||
| Bore in millimeters | 106 | 116 | 130 | |||
| Stroke in millimeters | 150 | 160 | 180 | |||
| Offset of the cylinders in millimeters | 18 | 20 | 20 | |||
| Rate of gasoline consumption in grams | 240 | 230 | 225 | |||
| Oil consumption in grams per b.h.p. hour | 10 | 10 | 10 | |||
| Oil capacity in kilograms | 36 | 4 | 4 | 1⁄2 | ||
| Water capacity in litres | 5 | 1⁄2 | 7 | 1⁄2 | 9 | 1⁄2 |
| The weight with water and oil but with two magnetos, fuel feeder and air pump in kilograms | 170 | 200 | 245 | |||
| The weight of motors, including the water pump, two magnetos, double ignition, etc. | 160 | 190 | 230 | |||
| The weight of the exhaust pipe, complete in kilograms | 4 | 4 | .8 | 5 | 1⁄2 | |
| The weight of the propeller hub in kilograms. | 3 | 1⁄2 | 4 | 4 | ||
The Benz cylinder is a simple, straightforward design and a very reliable construction and not particularly difficult to manufacture. The cylinder is cast of iron without a water jacket but including 45 degrees angle elbows to the valve ports. The cylinders are machined wherever possible and at other points have been hand filed and scraped, after which a jacket, which is pressed in two halves, is gas welded by means of short pipes welded on to the jacket. The bottom and the top of the cylinders become water galleries, and by this means separate water pipes with their attendant weight and complication are eliminated. Rubber rings held in aluminum clamps serve to connect the cylinders together. The whole construction turns out very neat and light. The cylinder walls are 4 mm. or 3⁄16′′ thick and the combustion chamber is of cylindrical pancake form and is 140 mm. or 5.60 inch in diameter. The valve seats are 68 mm. in diameter and the valve port is 62 mm. in diameter.
The passage joining the port is 57 mm. in diameter. In order to insert the valves into the cylinder the valve stem is made with two diameters and the valve has to be cocked to insert it in the guide, which has a bronze bushing at its upper end to compensate for the smaller valve stem diameter. The valve stem is 14 mm. or 9⁄16′′ in diameter and is reduced at its upper portion to 91⁄2 mm. The valves are operated through a push rod and rocker arm construction, which is 7⁄16′′ and exceedingly light. Rocker arm supports are steel studs with enlarged heads to take a double row ball bearing. A roller is mounted at one end of the rocker arm to impinge on the end of the valve stem, and the rocker arm has an adjustable globe stud at the other end. The push rods are light steel tubes with a wall thickness of 0.75 mm. and have a hardened steel cup at their upper end to engage the rocker arm globe stud and a hardened steel globe at their lower end to socket in the roller plunger.
The Benz cam-shaft has a diameter of 26 mm. and is bored straight through 18 mm. and there is a spiral gear made integrally with the shaft in about the center of its length for driving the oil pump gear. The cam faces are 10 mm. wide. There is also, in addition to the intake and exhaust cams, a set of half compression cams. The shaft is moved longitudinally in its bearings by means of an eccentric to put these cams into action. At the fore end of the shaft is a driving gear flange which is very small in diameter and very thin. The flange is 68 mm. in diameter and 4 mm. thick and is tapped to take 6 mm. bolts. The total length of cam-shaft is 1038 mm., and it becomes a regular gun boring job to drill a hole of this length.
The cam-shaft gear is 140 mm. or 51⁄2 inches outside diameter. It has fifty-four teeth and the gear face is 15 mm. or 19⁄32′′. The flange and web have an average thickness of 4 mm. or 5⁄32′′ and the web is drilled full of holes interposed between the spur gear mounted on the cam-shaft and the cam-shaft gear. There is a gear which serves to drive the magnetos and tachometer, also the air pump. The shaft is made integrally with this gear and has an eccentric portion against which the air pump roll plunger impinges.