Try the Shipping Date Guess-O-Meter Motor Belly Numbers This section is undergoing revision "Belly numbers" are stamped into the bottom of each crankcase half.
The crankcase halves were cast, mated in pairs, line-bored for the crankshaft, and then numbered to ensure they remained a mated pair.
The individual parts, particularly basic things like engines and frames, were often made a month or two ahead of time.
So that first 1960 may have parts manufactured in July or August 1959. In some cases, it is possible to narrow it down further than that, but in most cases it isn't.
The code is: The M model designation is "5" for K models and "7" for Sportsters.
The YY is the model year that the crankcase halves were mated.
The other theory is this was a nefarious scheme to make people think production was much higher than it actually was, to mask Harley's low sales figures in the 1960's. Normal production scheduling required things to be made in batches.
We will use 1000 through the remainder of this article.
This is four months – one-third of a year – ahead of the calendar year!
The first 1960 model year motorcycle that appeared on the dealer's showroom left the factory around September 1, 1959.
SSSS[S] is a four or five character (numeric) sequence number.
Usually, the SSSS of the belly number is within /- a few hundred of the SSSS of the .
In years ending with an odd number (example: 1961), the first digit of the production number was always odd. The serial number was applied late in production, close to the factory door.