That Hotchkiss Engine

Last month we referred to the vee-twin Hotchkiss engine, in association with some comments about air-cooling. It was an engine destined to give good service in BSA cars made by the great Birmingham Small Arms company, first in the BSA Ten light car which was made from 1921 to 1925, one of which the Threlfalls have campaigned successfully in VSCC events in recent times, and later in the BSA front-drive three-wheelers made from 1929 to 1936 (until a water-cooled four-cylinder was substituted) and also as a four-wheeled edition of the three-wheeler, although the latter must by now be a very rare animal indeed.

As with other armament manufacturers, BSA bad surplus production capacity when there was no major war in progress, and this was turned to making car engines: William Morris was glad to avail himself of the four-cylinder type for his Morris cars. Until, that is, he bought the Hotchkiss company, after which BSA obtained the rights to the, air-cooled vee-twin engine. The earlier engines were made at the Coventry Hotchkiss factory used after Germany had overrun Paris to ensure that munitions would still be available; the company was to a degree British-controlled. It seems that Hotchkiss may have intended to make a light car of its own. The vee-twin engine was tested in an old Morris chassis and tried out, or perhaps demonstrated would be more correct, in the 1921 MCC London-Edinburgh trial, disguised as an “Experimental” and driven by G S Bullock, who gained a silver medal. This may have been when BSA took note, or became more interested in using the Hotchkiss engine. I believe it was somewhat modified for its later adaptation to the Hulse-designed BSA three-wheelers, and made first in the Birmingham works of the Small Arms concern.

This well-made air-cooled vee-twin Hotchkiss engine was certainly well established in the first quarter of 1921. Apart from the award mentioned above, an experimental chassis registered as the “Blank” (HP 1787) had been run for 11,000 miles, and in the course of this testing had appeared in the 1920 MCC land’s End trial, driver AC Hardy, result a gold medal. The design of the engine was simple yet advanced. The cast iron cylinders were angled at 90deg, the bore and stoke being 804 x 85mm (1080cc). This gave a taxable rating of 9,9hp, although the “Experimental” was declared as a 12hp car, presumably as a disguise. The aluminium pistons had H-section conrods with roller-bearing big-ends, and the crankshaft ran on ball bearings. The overhead valves were operated by enclosed push-rods and rockers encased in cast-iron covers, the camshaft being central. Later modifications included a built-up crankshaft, the gear-type oil pump transferred to the bottom of the sump, and the exhaust pipes curved to merge into a single pipe constituting a hot-spot for the inlet manifold.

On the assumption, perhaps, that twin-cylinder engines can be hard to start, the BSA starting handle was geared up 2-to-1 via the distribution gearing, and a half-compression device fitted. In standard guise this little engine produced 17.5bhp at 2500rpm but it could be tuned to give just over 20bhp at 2600rpm by increasing the compression ratio from 4.45:1 to 5.0:1 , and installing a different camshaft. BMEP was notable, being from 93 to 115lb/sq in. With flywheel the engine weighed 190Ib, or 11 lb/hp in tuned form, with the thick cylinder walls and pistons and valve port finning used to ensure efficient cooling under road conditions. I wonder if Tom Threlfall was able to obtain one of the “racing” camshafts? As no doubt was Capt Brittain for his trials BSA which lapped Brooklands at over 63mph. WB