1938 BSA JM24 Engine

1. General Data           
2.  M24 Cams       
3. Wiring diagrams
1) General data

:  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
                                       Exhaust: 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, Throttle valve 6, needle groove 4

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  
                1938 & 1939 standard:    1st   14.3:1      2nd   9.9:1      3rd    6.3:1       4th   4.8:1                      
                1939 Close ratio:               1st  11.85:1    2nd   8.3:1     3rd  5.25:1      4th  4.8:1
Clutch: 6-spring wet multiplate with 43 tooth chain wheel, driven by ½ x .305in single row primary chain from 21 tooth 4-lobe 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. No sidecar lugs.
               BSA single spring girder front forks, also in 531 tube.

Wheels:  20in WM1 front, (21in WM1 on Competition Model), 19in WM3 rear.

3.00 x 20in  Front      3.25 x 19in Rear 1938     3.50 x 19in Rear 1939

:  7in Single Leading Shoe drum brakes front & rear.

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

Oil tank capacity:        
4 pints (1938)   5 pints (1939)

54 inch       Saddle Height: 28.5 inch    Ground clearance:     4.625 inch     Dry Weight:     345lb 

2) BSA M24 Cams
inlet cam is numbered 66-2082, exhaust 66-2084. The number is stamped on the outer end of the shaft.
The inlet cam 66-2082 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.
cam & pinion are one piece, and bushed to rotate on a separate shaft pressed into the crankcase. Cams and timing pinion are held in place by a steel outrigger plate mounted on the crankcase beneath the timing cover.
Inlet & exhaust cams are interchangeable, the part number being 65-2420. The same cams are 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.  

Wiring Diagrams:
The wiring layout differs between 1938 and 1939 models in that lighting switch and ammeter are located in the headlamp in all JM24 bikes and KM24 competition models, and in the petrol tank panel in standard KM24 models. Lucas also switched to colour coded wiring in 1939, earlier models having all black wiring with coloured tracer rings on the end of each wire. 

1938 Wiring Diagram
1939 Wiring Diagram