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How to Date a J.A.P Engine

JAP Engine Numbers

JAP Engine Numbers

What age is your Iron Horse? A question we get asked a lot and have struggled to find a definitive answer to. The following guide will help give you at least a year of manufacture.
and here's what those numbers mean(below)...

Deciphering your Horse engine number centres on the second digit. This is your year of manufacture and the letters run in twenty year cycles, remembering that JAP years actually start in the September of the previous year. In the example above the year is 1948 but theoretically it could have been made in September 1947 but not after September 1948.

The first digit is the model number, in this case the Horse used the Mk 5 engine and is so denoted here. Then comes the year, then the engine number (which we have no details of, it is just a factory record), and lastly the modifications. These are what the factory called deviations from the standard specifications and could denote cam spec, piston type etc. We have the codes for motorbikes but unfortunately the Horse engines were designated as industrial motors and the codes we have don't apply.

Code Year of Manufacture
P 1920 1940
N 1921 1941
E 1922 1942
U 1923 1943
M 1924 1944
A 1925 1945
T 1926 1946
I 1927 1947
C 1928 1948
S 1929 1949
W 1930 1950
H 1931 1951
Y 1932 1952
Z 1933 1953
D 1934 1954
R 1935 1955
V 1936 1956
F 1937 1957
O 1938 1958
G 1939 1959