European Motorcycles

Motorcycle Flat Twins

Horizontally Opposed Motorcycle engines of 1916

The HO flat twin engine was designed and patented by Karl Benz in 1896, who named it the Kontra engine. The flat twin, or boxer, has both conrods on the same big-end journal. A flat four has two big-end journals and probably the best known example is the air-cooled Volkswagen. The design is used in a great number of light aircraft, beginning in 1909 and continuing today with Lycoming, Rotax and others.

HO Twin

Horizontally Opposed Twin

Examples include BMW, Zundapp and Douglas HO Twins have conrods running on a common crank, with one on the exhaust stroke and the other on inlet.


The Motor Cycle November 9th, 1916.

Some Observations and Comparisons in Design of the Various Specimens of Horizontally-opposed Twins fitted to Motor Cycles.

THE horizontal twin seems to be coming so much to the fore of late {vide the issues of The Motor Cycle for October 5th and 12th) that some observations and comparisons of the better-known makes will not be out of place. Two-stroke horizontal twins are not included in the .present article, as they were separately dealt with on Feb. 17th, 1 916; moreover, the flat twin is in its nature more suited to the four-stroke cycle, for this type of engine gives excellent torque, having one impulse in every revolution, and exceptionally good balance. In the case of the two-stroke, either both cylinders must fire together, necessitating a special magneto, or the balance must be sacrificed by utilising a single-throw crank instead of the 180° cranks invariably used, so making the pistons move together instead of in opposite directions for the sake of obtaining the more frequent firing impulses which constitute one of the advantages of a two-stroke.

There is, of course, no novelty at the present day about the horizontal motor cycle engine. The Holden motor cycle, one of the earliest designs, had an engine of this type, the connecting rods of which drove the small rear wheel direct, i.e., without gearing. Later came the Fée, in 1905, afterwards called the Fairy, which was the forerunner of the Douglas, the machine to which the largest amount of credit for popularising this type must be given.

We have already mentioned the excellent balance of the horizontally-opposed engine. Readers who are interested in this subject are referred. to three articles on "The Mechanics of Balancing," which appeared in The Motor Cycle on May 4th, May 25th, and July 6th ; the installment referring more particularly to the flat twin will be found in our issue for July 6th. It will be sufficient to remark here that the primary balance is well nigh perfect, but that a small couple exists in engines as usually constructed. This couple can be eliminated by fitting a three-throw crank to avoid off-setting the cylinders, but this adds complication. We may remark, however, that the less the cylinders are off-set, the smaller will the couple be.

We will now deal briefly with some of the more popular flat twins in alphabetical order.

A.B.C. 3½ h.p., 70x64 mm., 194 c.c.

The A.B.C. engine, the design of Mr. Q. W. Bradshaw, bristles with novelties and ingenious points. The cylinders are steel turned out of the solid, with the fins placed circumferentially ; the cylinder heads are cast iron. Contrary to usual practice, the exhaust valve is placed overhead instead of the inlet, the objects of this being to improve the cooling of the exhaust valve and to remove the exhaust port further from the cylinder, and thus give it a better opportunity of dissipating its heat into the atmosphere and the cylinder less cause for warping. Provided that' this engine is properly lubricated (and lubrication is carried out by a mechanical pump) it is impossible to make it konk. Aluminium pistons are sometimes used. A Claudel-Hobson carburetter is fitted as standard, and the air entering this is effectively warmed by a pipe from the exhaust. The crank is in one piece, and yet the big ends are not split, for they consist of a specially designed roller bearing ; these are threaded over the crank and the rollers placed in position afterwards.

A.B.C. 3½ h.p., 60x44 mm., 249 c.c.

This is the smallest flat twin at present on the road, and weighs 14 lb. exclusive of the flywheel, magneto, and carburetter, which bring the complete weight to 27 lb. The general design is very similar to that of the 3½ h.p. A. B.C., and the exhaust valves are placed overhead as in the larger engine, but the cylinder head differs slightly, and the exhaust pipes are taken from the bottom of the exhaust ports instead of from the side. The pistons are of aluminium alloy. On a low gear of 11 to 1 it is claimed that this engine can attain a speed of 37 m.p.h., which means 5,500 r.p.m. The lubrication is by means of a mechanical pump. This little engine has been made principally for Government purposes, but it has been tested in a motor cycle with excellent results, and will be marketed after the war.

Bradbury 3½ h.p., 68x68.7 mm., 499 c.c.

The cylinder heads of the Bradbury are detachable, which enables the whole of the cylindrical part of the cylinder, including the horizontal cooling flanges, to be machined. The cylinder is then nickel plated to prevent rust. This gives it a very distinctive appearance. The ' valves are side by side and operated by adjustable tappets. A B. and B. carburetter is employed, and Best and Lloyd semi-automatic lubrication. The Bradbury flat twin was introduced just prior to the war, and only a limited number are in the hands of motor cyclists.

Brough 3½ h.p.. 70x64.5 mm., 497 c.c.

Two models of the 3½ h.p. Brough engine are manufactured, one being the racing engine, which is fitted with steel pistons and racing cams, but in other respects resembles the standard model. Both inlet and exhaust valves are placed in the head and operated by outside tappets and rockers. The valves are of large size with detachable ports the exhaust port being provided with cooling flanges. The two-throw crankshaft is provided with balance webs, and carries a steel flywheel. The engine is secured in the frame by three bolts, and is easily removable, but the cylinders can be taken off without disturbing the engine when desired. Lubrication is by Best and Lloyd drip feed, and the oil is forced under high pressure to the big end bearing ; the carburetter is an Amac.

Brough 6 h.p.. 70x90 mm., 692 c.c.

Unlike the 3½ h.p. engine of the same make, the larger Brough has side by side valves arranged above the cylinders - in fact, the cylinder is very similar in design to that of the 6 h.p. V twin previously made by this firm, from which it may be gathered that the cooling flanges are vertical. Roller bearings are used throughout, and not for the crankshaft only, the result being a very smooth-running engine. Like its smaller brother, it is possessed of unusually silent valve gear. The 6 h.p. Brough is a war-time innovation, and has been designed to meet the demands of those who desire an efficient double-purpose mount. After the war the 6 h.p. Brough is bound to attract.

Douglas 2¾ h.p., 60.5x60 mm., 345 c.c.

The smaller Douglas has undoubtedly been the pioneer of "flat twins," and, has been so successful for years that very few alterations have been required. Mechanically-operated inlet valves were fitted in 191 2. The valves are now side by side and placed in a slanting position at the side of the engine and operated by adjustable tappets. The cooling flanges are cast longitudinally on the cylinders, and the crankshaft is balanced. One of the difficulties which have to be overcome in engines of this type is the lubrication of the front cylinder ; in this engine, it is accomplished partly by splash and partly by a small duct which is drilled in the crank case and cylinder base ; this is situated just below the oil lead from the drip feed, and carries the oil straight to the front piston. A Douglas or Amac carburetter is usually fitted ; the latter is provided with a jacket which can be heated by the exhaust gases.

Douglas 4 h.p., 72x68 mm., 544 c.c.

The larger Douglas model differs in several points of design from its smaller brother, the most noticeable being the mechanical lubrication system in which oil is carried in a sump below the crank case and supplied by a pump to the bearings and cylinders. This pump is provided with a window, through which the level of the oil may be, readily ascertained. The valve ports are of streamline form internally, which, of course, allows the gas to enter and escape with the utmost freedom, and the valves are placed side by side above the cylinders. The carburetter is a jacketed Amac heated by exhaust gases which are taken by a pipe from a point near the exhaust port and carried by another pipe to the silencer.

Humber 6 h.p., 78x78 mm., 746 c.c.

Water-cooling is a prominent feature of the Humber sidecar engine. For the sake of obtaining large bearings for the big ends and central bearings on the gudgeon pins the cylinders are placed rather more out of line than is usual. Another point of interest is the method of attachment of valves. These are side by side and carried in detachable pockets, which, when in position, are held firmly by a forked yoke, and a single bolt placed between the two valves. The advantage of being able to remove the seating and valves complete for cleaning and grinding purposes is very obvious. The carburetter is the Claudel-Hobson automatic.

Humber 3½ h.p., 68x68.75 mm., 497 c.c.

We have described this entirely new engine fully so recently (October 12th) that a detailed account will be unnecessary here, and a few notes will suffice. No specimens are yet in public hands, though the model has been selected by one of the Allied Governments. Like its big brother, the 31/2 h.p. type has valves (inclined at an angle from the crank case to the cylinder heads) which are removable with their seatings, and large adjustable tappets are used. The starting handle operates through the camshaft. A Longuemare-Hardy carburetter is employed. The radiating fins are horizontal and very finely cast. The cool running of this engine can be ascribed very largely to the absence of valve caps which harbour heat.

Matchless 6 h.p., 70x95 mm., 257 c.c.

This engine is the very latest addition to the ranks of flat twins. It is to appear on the 1917 model Matchless, but at present no illustrations are available. The length of the stroke of this engine is noteworthy (the usual flat twin is not far removed from the square). The lubrication of the engine is by an automatic forced system.

Indian 21/4 h.p., 50.8x63.5 mm., 257 c.c.

Like the 31/2 h.p. Humber, the little Indian, introduced for 1917 and representing the only flat twin at present on the American market, has been dealt with in our pages within the last few weeks (October 5th). Up to the outbreak of war there were not many makers of the "flat twins" in this country, and they are even rarer in the States. In this case the valves are sloping, as in the 4 h.p. Douglas and Humber models, and neatly covered to exclude the dirt. With the object of attaining equal lubrication in both cylinders, an oil duct is drilled from the crank case to the front cylinder.

Montgomery 6 h.p., 75x78 mm., 688 c.c. The Montgomery was produced essentially as an engine for a sidecar outfit. The valves are placed over the cylinders, and are side by side. The cooling flanges are horizontal and nicely shaped. The split big ends are of rather unusual construction, being divided only on one side. The crankshaft and camshaft both run on Skefko ball bearings, a belt pulley of large dimensions taking the drive from the latter. The pistons are fitted with two rings, and the gudgeon pins are held in position by brass end cages. Lubrication is by splash.

Williamson 8 h.p., 85x85 mm., 964 c.c

The last engine we have to describe is the largest flat twin at present on the motor cycle market. The Williamson engine is made by Douglas Bros., but it differs from the smaller Douglas engines in the fact that it is water-cooled, though air-cooled Williamsons have also been supplied. The valves are side by side, and placed at the side of the cylinder in a horizontal position. Either a B. and B. or Amac carburetter can be fitted.

The Motor Cycle, November 9th, 1916. pp401-403

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