BMW R42

Make Model	BMW R 42
Year	1926 - 28
Engine	Four stroke, two cylinder horizontally opposed Boxer, pushrod operated, SV
Capacity	494 cc / 30.1 cu in.
Bore x Stroke	68 x 68 mm
Compression Ratio	4.9:1
Cooling System	Air cooled
Induction	1 x BMW Spezial 2 valve, ∅22 mm
Ignition	Bosch magneto ignition
Starting	Kick
Clutch	Single plate saucer spring, dry
Max Power	8.9kW / 12 hp @ 3400 rpm
Transmission	3 Speed
Final Drive	Shaft
Gear Ratio	1st 2.50 / 2nd 1.50 / 3rd 1.07:1
Frame	Twin loop steel tubular frame
Front Suspension	Plate spring, 5 laminae
Rear Suspension	Rigid, Spring loaded saddle
Front Brakes	∅150mm Drum
Rear Brakes	External shoe brake at the gearing on Cardan shaft
Front Tyre	27 x 3.5" low pressure or 26 x 3.0" high pressure
Rear Tyre	27 x 3.5" low pressure or 26 x 3.0" high pressure
Wet-Weight	126 kg / 277 lbs
Fuel Capacity	14 liters / 3.6 US gal

n 1926, the R32 was joined by a sibling, the R37. The R37 shared a chassis with the R32. but it was considered an aggressive, spotting motorcycle and was powered by an overhead-valve (OHV) engine (versus the R32*s side-valve arrangement). The OHV M2B36 engine offered a nice increase in performance; it could produce 16 horsepower, compared with the R32's 8.5. The improved airflow to the cylinder heads enabled the engine to run a higher compression ratio and at a higher rpm range.
Lubrication of an OHV cylinder head proved to be a challenge, but in typical fashion BNWs engineers developed some unique solutions. Early BMW OHV engines incorporated three separate oil sumps.

The first was located in the main engine case and operated by a high pressure oil pump, which lubricated the crank, cam, lifters, and gear train. The roller bearing rocker arms were lubricated by their own separate oil sump in each cylinder head.
Once the valve cover was installed, they were filled with 250 milliLitresof oil, and the valves were now able to get adequate splash to lubricate the bearings in the cylinder heads.
By the end of 1926, the R42 replaced the R32. The R42 was powered By an updated side-valve engine that produced 12 horsepower.

This engine, the M43a, retained the "square- 68-miltimeter bore and stroke measurements, but new alloy cylinder heads and an improved carburetor increased performance. The R42 retained a three-speed transmission, while engineers improved the model's braking abilities with an expanding-shoe front brake and a rear brake that applied its braking force to the driveshaft. A new frame cradled the engine in a lower position and further back, thus dealing a lower center of gravity. With these engine and chassis improvements, the R42 proved much more capable than the R32, especially when mounted with a sidecar.

The R47 was introduced in 1927, and it succeeded the R37 as the performance model in the BMW lineup. At a price of OH 1,850, the R47 was considerably more expensive than an R42 (ON 1.510), but priced far below the outgoing R37 (DM 2,900). The R*7 shared the same chassis as the R42, but its engine produced 18 horsepower and could propel the R67 to a top speed of 68 miles per hour.

BMW R 42