AJS Motorcycles

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TECHNICAL SPECIFICATIONS



AJS 18S
AJS 18S Specifications
Restoration Basics

All Technical data from the service guide AMC Heavy weight singles. Published by Bruce Main-Smith LTD



1953 AJS MODEL 18

MOTOR / GEARBOX / FRAME

Bore, mm 82.5
Stroke, mm 93
Capacity, cc 498
Compression ratio 6.0
Valve Timing All models use 0.016in tappet clearance
Inlet opens B.T.D.C. 32 degrees
Inlet closes A.B.D.C. 63 degrees
Exhaust opens B.B.D.C. 65 degrees
Exhaust closes A.T.D.C. 30 degrees
Ignition before T.D.C. full advance 1/2
Tappet clearance, engine cold All models nil clearance
Carburetter, bore size 1 3/32
Main jet 180
Slide 29/4
Needle position 2
Needle jet 29.076
Needle 29
Cylinder size 3.250in tolerance +0.0005 / -0.0005
K.L.G. sparking plug F80 iron head FE80 alloy head
Magneto contact gap 0.012in
Alternator gap 0.019 - 0.016
Gearbox lubricant 1 pint engine oil
Petrol tank capacity 24 pints
Petrol tank reserve 4 pints
Oil tank capacity 4 pints
Top of skirt piston diameter 3.2490in
Bottom of skirt piston diameter 3.2494in
Gudgeon pin size 7/8 -0.0010 \ -0.0013
Connecting rod length (centres) 6 3/8
Timing side shaft diameter 1.2275 \ 1.2300
Rocker axle bush 5/8: +0.00075 \-0.00050
Camshaft bush 1/2: +0.0005 \-0.0005
Rocker axle sleeve High limit 0.6235: Low limit 0.6230
Camshaft axle 3/8: -0.00125 \ -0.00175
Small end bush 7/8: +0.00050 \ -0.00025
Flywheel end float

Flywheel diameter

0.025 max

7 3/4 x 1.156in

Balance factor 65%
Total rotating weight 843.3gm
Reciprocating weight 673.2gm
Balance weight 1lb. 4oz.8 1/4gm 1 flywheel
* All dimension are in inches unless otherwise stated

All Technical data from the service guide AMC Heavy weight singles. Published by Bruce Main-Smith LTD

Exhaust pipe Best length open 48in
Wheel base 54in
Head angle 63 1/4 degrees
Trail 2 5/8
Valve spring free length narrow 3 1/8in
Valve spring free length wide 2in
Valve lift 5/16in
Valve seat angle 45 degrees
Push rod overall length 9 5/32
Valve guide protrusion (inlet & exhaust) 1/2in
Valve stem diameter inlet 0.3730 high limit & 0.3720 low limit
Valve stem diameter exhaust 0.3715 high limit & 0.3705 low limit
Crank pin diameter 1.20375 high limit & 1.20350 low limit
Crankpin rollers 0.250 x 0.0250 (30 off)
Connecting rod sleeve diameter 1.70400 high limit & 1.70375 low limit
Timing side bush 1.125 / 1.255
Driving side bush 1.0002 high limit & 0.9997 low limit
Gearbox B52 Burman
Engine sprocket 1st 2nd 3rd top internal ratios
15 teeth 18.69 12.32 8.96 7.0
16 teeth 17.5 11.54 8.39 6.56 1st 2.67
17 teeth 16.44 10.48 7.88 6.16 2nd 1.76
*18 teeth 15.57 10.26 7.47 5.83 3rd 1.28
19 teeth 14.6 9.6 7.0 5.49 top 1-1
20 teeth 14.01 9.24 6.72 5.25
#21 teeth 13.35 8.8 6.4 5.0
* Standard 350cc / # standard 500cc

TYRES

Front 3.25 x 19
Rear 3.50 x 19
ELECTRICS
Magneto Lucas N1 chain driven
Dynamo Lucas E3N
Regulator Lucas cvc MCR2
Horn Lucas HF1234
Battery 6 Volt Lead acid
System Positive to Earth

ENGINE NUMBERS

AJS engine numbers are easy to work out. If your engine for example is stamped 53/18/89642

this means

  • It was made in 1953
  • It is a 497cc from a model 18
  • It is engine number 89642

FRAME NUMBERS

Frame numbers are more complicated, AMC started in 1949 with frame number 1, I presume and by 1952 had reached 99999. So they started again with an A prefix. The frames were made in batches and stored ready for cycle assembly. AJS and Matchless used the same frames and it is not uncommon for a 1953 model bike to have a frame made in 1952.

APPEARANCE

AJS bikes in 1952-53 had

BLACK-Frames, Forks, Hubs, Oil tank, Toolbox, Chaincase, Chainguard, Headlamp shell and other painted items.

CHROME-Exhaust, Handlebars, Controls, Pushrod tubes, Fork caps Fork oil seal holders, Petrol cap, Oil cap,Headlight rim and Wheel rims (export Model).

ARGENISED-Wheel rims (Domestic model). NB: Powder coating with a silver/grey matt finish has given a fairly realistic finish on the wheel rims, according to a post in the Jampot forums.

POLISHED ALUMINIUM-Lower fork legs and Brake back pads.

PETROL TANK-Black with lining in gold paint, Die cast AJS Badges and Moulded knee rubbers(domestic model). Or Chrome plated with wide black panels, Die cast AJS Badges and Moulded knee rubbers (export model)

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