Greetings All,
McDougal - very nice Atcos. I have a few myself. From what I know of the post war Atcos the the oblique finned 79cc engine came into production in 1947. I have a 1947 12" which has the very first series of this engine. The model engine on your machines came into production in 1948 and finished production in 1959/60. The long tubular exhaust was a version of those used on previous models. These exhausts were used up until 1951. In 1951 the first version of the pepper pot exhaust came into production. These exhausts were forward facing (rather than to the side). It appears that in about 1952 the pepper pot exhaust took on its well known orientation.
As far as other changes in the models that help to date the 1950's Atcos there are several. The handles were solid aluminium from 1947 - 1951. In 1952 the handles were hollow aluminium. In 1954 the handles were turned oak. In 1958 they were tubular steel with plastic covers.
The clutches are the small solid steel type from 1947 - 1953 and then the more common plate type from 1954 - 1959/60. The clutches on the 1958 4 stroke machines changed slightly. Some of the final 2 stroke machines had the same clutch as the 4 stroke including the drive dogs.
The fuel tank configuration also changed. The fuel tank was supported by separate supports from 1947 - 1953. From 1954 to 1959 the tank was supported by the main handle bars. However the separate supports were retained on the 20" machines. From 1958 the more squat tank appeared on the four stroke models.
The kick starter on the 1947 model was steel, changing to aluminium in about 1948. The starter dogs were milled steel from 1947-1953 with the best known pressed steel type appearing in 1954. In about 1957 the starter changed again. The rope start appeared in 1954 and was an option on the 14" and 17" models.
The engines remained much the same through out the 1950's however the main difference was on the 20" machine. 1948 - 1953 the 20" used the 78cc oblique finned engine and from 1954 onwards the 98cc engine was used.
As far as I can ascertain the rollers were either aluminium or steel depending on the option chosen, although later models tended to be steel.
Catchers also changed over time with the very first being steel (probably a hang over from the previous model) with the aluminium ones entering into production in 1947. In 1954 the 14" and 17" machines were provided with steel catchers. The 20" machines appear to have remained aluminium the whole way through.
There are other small changes between models including changes to the chain tensioner, bed knife arrangement, minor changes to the clutch and so on.
There is one little trick on machines after 1950 - there is a series of letters and digits on the flywheel under the cover plate, the last two digits are the year. These are the date of the engine rather than the mower but if the flywheel and engine are original it will give you a good idea of when the mower was made. If you couple this information that above, you can get a fairly good idea of when the mower was made.
I hope that these details help others to date their Atcos.
