Technical information

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1) General data 
2) M24 Cams
3) Wiring diagrams
4) JM24 Engine rebuild 

  M2438engine1.jpg           M2438engine3.jpg
  1938 BSA JM24 Engine


                              1939 KM24 Engine in showroom condition 

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
 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

: Lucas MO1 Magdyno with manual advance.

                Ignition timing 7/16in BTDC on full advance.

: 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 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. No 
              sidecar lugs. 
              BSA girder front forks, also in 531 tube.

Wheels:  20in WM1front, (21in WM1 on Competition Model), 19in WM3 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)

: 54in

Saddle Height
: 28.5in

Ground clearance
: 4 .625in   

Dry weight
: 345lb 1938 & 1939         

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 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 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.  

 3) Wiring diagrams:

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. 

click on pic for full size
Wiring diagram with electrics in headlamp panel
Wiring diagram for electrics in tank panel

4) JM24 331 Engine rebuild 2017-18

We start with a complete set of cases, barrel, head and rocker box in need of cleaning, polishing and some fin repairs. General condition is good for an 80 year old engine. By the end of the rebuild, it will look like the engine at the top of this page.

 DSCF2198.JPG                 DSCF2199.JPG


Below are pictured various engine parts as supplied, including a complete M20 crankshaft, which will provide flywheels and drive side main shaft. The timing side mainshaft will be replaced with the correct short type for 1937/38 engines, and the conrod replaced by a polished BSA B31 item on a new big end bearing. 

click on photo for full size view
Engine Parts as supplied
M20 Crankshaft for modification

The crankcases and rockerbox have been vapour blasted to reveal any defects before reassembly. The rockerbox is in good order and all component parts are shown ready for reassembly.
The crankcases are in excellent condition, but the drive side case has had a weld repair to the main bearing housing and will be machined back to size. 
See photos below.

click for full size picture
Rockerbox and components

Clean crankcases in good condition


weld repair to crankcase

Crankcase components

Oil pump

Timing side mainshaft


Crankcase components are now ready for reassembly. The crankshaft has a short timing side mainshaft fitted, with a new big end bearing and polished conrod. The drive side main bearing housing has been machined where it was welded, and main bearings have been cleaned checked and reinstalled. The spacers between the inner and outer drive side bearings were missing, and new ones have been made to position the inner roller bearing correctly, so that the centre of the big end and conrod lines up with the centre of the crankcase mouth and barrel. Replace the 4 long cylinder head bolts in the crankcase top face with a smear of Loctite on the threads. 

The oil pump has been stripped cleaned and overhauled to rotate smoothly when reassembled. When installing the oil pump, use a new gasket and drill the 2 bolt heads and wire-lock them. If the bolts are overtightened, they can distort the oil pump body and prevent smooth rotation.
When reassembling the crankcases, a light smear of silicone sealant on the mating faces will prevent oil leakage. Before pressing the crankcase together, dont forget to slip the magneto straps over the 2 long studs in the timing side case!  Ensure that the crankshaft rotates freely after the fixing bolts have been tightened, before starting top-end reassembly.