The ‘Orbit’ Power Unit

Careful attention to detail characterises the design of the two-stroke engine here described

On the occasion of a recent business journey to the Midlands we visited the works of Orbit Motors, Ltd., of Sedgley Street, Wolverhampton, and examined the various component parts of the ‘Orbit’, 2½ hp. two-stroke, three-port, power unit there produced. The complete engine is attractive in appearance, being, as is obvious from the accompanying illustrations, well proportioned, while a feature which at once strikes the observer is the ample area of the cooling fins, especially on the cylinder head; this has an appreciable effect on the efficiency of the unit, for overheating is unknown and a minimum of plug trouble - an all too-common fault with two-stroke engines is experienced.

The Large Port Area

The "Orbit" engine has generously dimensioned ports, thus allowing the gases to gain ingress to and egress from the cylinder with the utmost dispatch. An unusually large exhaust pipe, 1½in. in diameter, is utilized, and this is so arranged as to be free from sharp bends which might restrict the flow of the outgoing burnt gases. For 1921 a new compression release valve has been evolved; this delivers the released gases into the main exhaust pipe through the medium of a steel tube, and thus no oily vapour is thrown all over the rider's legs when the valve is operated.

Bore and stroke are 68 by 72 mm. (261. 5 c. c.), and the R.A.C. rating is 2.86 hp. Both cylinder and piston are of close-grained grey iron, and are machined on up-to-date plant and finally ground to fine limits. The material used acquires a perfect surface after use and wears remarkably well. For the one-piece crankshaft, the connecting rod and flywheel, high-grade steel forgings are used. The reciprocating parts are carefully balanced, resulting in an almost entire absence of vibration at all speeds. A split type phosphor bronze big-end bearing has been adopted in order that any wear at this point may be ‘taken up’ with a minimum of trouble. The generously dimensioned main bearings are of ‘Richards’ plastic metal, and these retain crankcase compression for a long period. In order to enable the inlet port to be cleaned, the induction stump is readily detachable.

Both sides of the engine showing the large area of the cooling fins, together with the easy 'sweep' of the generously proportioned exhaust pipe.

Efficiency in Action

As one would expect after a careful examination of the unit under discussion, it pulls remarkably well on the road, and has quite a respectable turn of speed. Owing to the large fins and ports the engine keeps cool even at the highest attainable speeds, while an excellent feature is that it is not prone to four-stroking. All the proprietary units, including carburettor, magneto, and sparking plug, are of good quality, and the unit is supplied as shown in the accompanying illustrations complete with engine plates, silencer, magneto and carburettor, ready to drop into the frame. Immediate deliveries in quantities can be given, and we understand that the price is reasonable.