J.A.P engine coding


Until the period immediately after the First World War, JAP engines had been recorded on a numerical basis, it is safe to assume that any number prior to 52,000 dates from pre-1915. From 1920 onwards the problems of identifying and dating engines are much simplified, due to the fact that the company introduced a coded engine number sequence. By stamping symbols and numbers on top of the timing chest cover or on the top of the crankcase adjacent to it, it became possible not only to determine the year of manufacture but also to identify the type of engine, its basic specification and in some cases, whether it had been modified or produced with a specific use in mind. The way in which the engine number code can be interpreted is as follows:


1. The first letter before the oblique mark identifies the cubic capacity of the cylinder, or of each cylinder if the engine is a twin.

2. The remainder of the letters before the oblique mark identify the engine type.

3. The first letter after the oblique mark identifies the year of manufacture.

4. The serial number of the engine follows immediately after the letter identifying the year of manufacture.

5. Additional letters after another oblique stamped immediately after the engine serial number, identify any deviations from standard specification. An asterisk after the serial number, indicates the engine incorporates parts which deviate from the standard specification.


To provide more detail on the above, the following is a list of letters and numbers, with an explanation of their meaning:-


First letter (cylinder capacity)

A = 300cc side valve, 70 x 78mm bore and stroke

B = 250cc side valve, 64.5 x 76mm bore and stroke

D = 650cc side valve

E = 375cc side valve, 74 x 85mm bore and stroke

F = 300cc side valve, 70 x 76mm bore and stroke

G = 350cc side valve, 70 x 88mm bore and stroke

I = 350cc side valve, 70 x 9Omm bore and stroke

J = 500cc side valve, 80 x 99mm bore and stroke

K = 500cc side valve, 85.7 x 85mm bore and stroke

L = 550cc side valve, 85.7 x 95mm bore and stroke

M = 375cc side valve, 70 x 97mm bore and stroke

N = 200cc side valve, 55 x 83mm bore and stroke

P = 250cc side valve, 62.5 x 8Omm bore and stroke

S = 35Occ side valve, 74 x 5Omm bore and stroke

U = 600cc side valve, 85.7 x 104mm bore and stroke

V = 175cc side valve, 60 x 62mm bore and stroke

Z = 175cc side valve, 55 x 73mm bore and stroke



Remaining letters before oblique

T = Twin cylinder

0 = Overhead valves (cancels reference to side valve in preceding letter)

W = Water cooled

Z = Dry sump lubrication

S = Short stroke sports engine (or special)

C = Sports engine

R = Racing engine

Y = Twin exhaust port cylinder head

Often, these letters are used in combinations of twos or threes.



Letters after first oblique (1st September previous year - 31st August)

P = 1920

N = 1921

E = 1922

U = 1923

M = 1924

A = 1925

T = 1926

I = 1927

C = 1928

S = 1929

W = 1930

H = 1931

Y = 1932

Z = 1933

D = 1934

R = 1935

V = 1936

F = 1937

O = 1938

G = 1939

These letters were later reused in the same sequence to cover the years 1940 to 1959 inclusive, taking into account the war years during which machines for civilian use were no longer manufactured. It is worth noting that the JAP production year ran from 1st September of the year previous to that identified by the dating letter, and ended on 31st August of the year indicated.



Letters after the engine serial number and second oblique (Prior to 1926)

A = High lift cam

B = Modified gear cover and exhaust valve lifter

C = Crankcase with inclined magneto platform

D = Cast iron, flat-top piston

E = Cast iron, dished-top piston

F = Aluminum alloy piston with domed top

G = Ball bearing gear spindle

H = Roller bearing pulley spindle

J = Roller-type cam levers and exhaust valve lifter

K = Special chain line requirement for pulley spindle

L = Outside flywheel with screw-on sprocket, for two-strokes

M = Crankcase without oil box

N = Magneto sprocket for type NA magneto of ML manufacture (two-strokes)

O = Cylinder No. 2 type

P = Cylinder No. 4 type

R = Cylinder No. 3 type

S = Double row roller big-end bearing (250cc and 350ce ohv racing engines) *

T = New Type crankcase and rotary valve breather (500cc and 600cc side valve)

U = Modifications for new tappet centres (350cc side valve roadster)

V = Reduced centre connecting rods (low compression engine)

Y = 1924 pattern crankcase converted to rotary valve breather

Note: On later Auto-trucks 'S' refers to made to Lister's specification


The above coding relates only to engines made during the period 1921-1925. After this period, the letter 'S' denotes the engine was made to the customer's specification. The * denotes an engine that had been specially prepared for record attempts, or specially tuned to meet a rider's requirements. Inevitably, there are exceptions to this coding and also the use of certain letters for which an explanation is no longer available. In consequence, this guide is by no means infallible.