Technical information

1) General data
2) M24 Cams

1938 BSA M24 Engine

1938 JM24 Cylinder Head

1) General data

Engine: Single port, 496cc OHV single cylinder.
               Bore 82mm, Stroke 94mm.
               Compression ratio 7.75:1
                     (Option of 12.5:1 with High Compression Piston)
               Power output 29-30bhp @ 5,800rpm
               Alloy cylinder head with cast-in bronze thread inserts, and
               screwed-in steel valve seats.
               Alloy cylinder barrel with pressed-in steel liner
Valves: Inlet: length 4.21875in Head Dia 1.65625in Stem Dia 0.34375in
               Exh: length 4.21875in Head Dia 1.59375in Stem Dia 0.375in

Ignition: Lucas MO1 Magdyno with manual advance.

                Ignition timing 7/16in BTDC on full advance.

Carburettor: 1938: Amal 10TT36 1 & 5/32” bore, Main Jet 370,
                                   Throttle valve 5, needle groove 4
                       1939: Amal 10TT38 1 & 5/32” bore, Main Jet 350,
                                   Throttlevalve 6, needle groove 4

Transmission:

Gearbox: 4-speed, enclosed footchange, external clutch release lever

                 Magnesium Alloy (Elektron) gearbox cases for 1938 only

                 Close-ratio gears for 1939, with option of standard wider ratios
                 Ratios: 1938 & 1939 standard:
                               1^st14.3:1 2^nd9.9:1 3^rd 6.3:1 4^th   4.8:1
                              1939 Close ratio:
                               1^st 11.85:1    2^nd 8.3:1 3^rd 5.25:1   4^th 4.8:1

Clutch: 6-spring multiplate with 43 tooth chain wheel,
              driven by ½ x .305in single row primary chain from 21 tooth
              cush-drive engine sprocket. 18 tooth gearbox sprocket,
              driving 42 tooth rear wheel sprocket via 5/8 x 1/4in. final drive chain.

Frame: Reynold 531 high-tensile tube. Single front down tube, rigid rear,
              BSA girder front forks.

Wheels: 20in WM1front, (21in WM1 on Competition Model), 19in WM2 rear.

Tyres Front 3.00 x 20in,   Rear 3.25 x 19 (1938), 3.50 x 19 (1939)

Brakes: 7in Single LS drum brakes front & rear.

Petrol Tank Capacity: 3 Gal. (1938) 3.5 Gal. (1939)

Oil Tank Capacity: 4 pints (1938)   5 pints (1939)

Wheelbase: 54in

Saddle Height: 28.5in

Ground clearance: 4 .625in

Dry weight: 345lb 1938 & 1939

2) BSA M24 Cams
1938
inlet cam is numbered 66-2082, exhaust 66-2084. The number is stamped on the outer end of the shaft.
The inlet cam is the same for M24 & M23, but the exhaust cam is unique to the M24.
38 BSA cams differ from later ones as the cam & shaft are one piece, and the pinion is keyed onto the shaft.
The cam shaft rotates on inner bushes in the crankcase, and outer bushes in the timing cover. 1938 cams are not interchangeable with later ones.
1939
1939 onwards, cam & pinion are one piece, rotating on a separate shaft pressed into the crankcase.
Inlet & exhaust cams are interchangeable, the part number being 65-2420. The same cam is used in all 1939 BSA M and B series engines, except B21 & B23 Standard, and the M22.
65-2420 continued in use for IN & EX in the WDM20, and 1946-59 post war M20,M21, B31 & B33.
The post-war Gold Stars from 1949 used a variety of different cam profiles, details of which can be found in “The Gold Star Book” published by Bruce Main-Smith in 1974. These will all fit 1939 M24 engines, and a suitable combination can produce a worthwhile improvement in performance.