Technical: Serial and Other Numbers
Scott Lange with help from Jerry Raino and Dave Hennessey


Serial and Other Numbers

Identifying your K Model or Sportster requires looking at a lot of numbers to determine what year the motorcycle is, and whether many of the parts are original to the bike. The following descriptions of "numbers" applies specifically to Ks and Sportsters, and in general to all Harley-Davidsons up to 1969.

A Word About Years

There are two kinds of years we'll be dealing with here. First, there's the calendar year, which runs from January 1 to December 31. Then there's the automotive model year, which runs from September 1 to August 31. Since the beginning of time, car, truck, and motorcycle manufacturers have introduced their shiny new models after summer's over, and people are heading back to work and school. 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. 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.

When we speak of a change occurring during early/mid/late year, we are referring to the model year. In some cases, it is possible to narrow it down further than that, but in most cases it isn't. If we really have no idea, we will use “sometime during the year”. Other books and websites often use “mid-year” to mean “sometime during the year”, but we'll try to reserve the term “mid-year” to really mean “middle”.

Motor Seral Number (“VIN”)

Almost every vehicle ever made has a manufacturer's serial number located somewhere on it. There was no industry standard, so each manufacturer made up their own numbering scheme, and affixed it to the vehicle wherever it wanted. In 1981, the U.S. Department of Transportation mandated a standard 17-digit Vehicle Identification Number (VIN) scheme, and for motorcycles, mandated the VIN be affixed to the frame. But before that…

From the early days, up to and including the 1969 model year, Harley-Davidson stamped the serial number of a motorcycle into the left engine case. The frame did not have a serial number. The serial number followed the code:

YY – a two character (numeric) model year, last two digits 62
MMMM – one-to-four character (alphabetic) model designation XLCH
SSSSS – a four or five character (numeric) production sequence number 2373

The production sequence number gets complicated.

Before 1960, the sequence number started with either 1000 or 1001 - NOT 0001. Although the common wisdom has long been 1001, there have been a few documented cases of a serial number 1000 uncovered. We will use 1000 through the remainder of this article.

The sequence number runs with the basic motorcycle model, not the different variants. So the first few 1959 Sportsters might have been numbered 59 XLH 1000, then 59 XLCH 1001, then 59 XLH 1002, then 59 XLCH 1003…

From 1960 through 1969, Harley started an unusual odd/even scheme. In years ending with an even number (example: 1960), the sequence numbers ran 2000-2999, then skipped to 4000-4999, then 6000-6999, then 8000-8999, then 10000-10999, then 12000-12999. In years ending with an odd number (example: 1961), the first digit of the production number was always odd. So it ran 1000-1999, then skipped to 3000-3999, then 5000-5999, etc.

Why? One theory is that this was an anti-VIN-forgery scheme. 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.

The font used for the VINs varied over the years. Here are some photographs of serial numbers.

When Was My Bike Produced?

The serial number was applied late in production, close to the factory door. Historian Jerry Hatfield researched monthly production figures at the Harley-Davidson archives, but found they varied "all over the place". Normal production scheduling required things to be made in batches. Sales might not have matched the factory's predictions, so they might have had to catch-up by increasing production of one model while slowing production of another. Harley's book, The Legend Begins, provides annual production figures. Dividing the total production by 12 months can give you a best-guess as to what month your bike was produced, but this will not be truly accurate.

Note: the 1952 K model was late to market. It was was unveiled for the first time to the dealers in Milwaukee at their November 1951 convention.

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 same sequence number was stamped into each crankcase half. The numbers should match each other, but they will NOT match the serial number of the engine. The code is:

MYY SSSSS 761 3334
M – one-character (numeric) the model designation (See below) 7
YY – two character (numeric) model year, last two digits (See below) 61
SSSSS – a four or five character (numeric) mating sequence number 3334

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. This is usually the same as the model year of the bike. However, if you have a very low motor serial number, your belly numbers may have a year code one year below the model year. In some years, the factory mated too many case sets, and then used them up early the following year.

An oddity is that all 1952 K models have 552-XXXX, and all 1957 Sportsters have 757-XXXX belly numbers.

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 un-coded VIN, but this is not always the case. We are actively researching this area. The important thing is that the belly numbers match, and the year code is plausible.

Here are some photographs of belly numbers.

Frame Date Codes

Most K and Sportster frames have a date code stamped into the frame. It is on the right side, in the area where the rear gas tank bolt goes through the frame – for the large tanks. On Ks and XLHs, it is under the right rear tank ear, so you can't see it with the tank on. On XLCH with the small tank, it can be seen with the tank on.

M Y H 1
M – one-character (alphabetic) month code (See below) H
Y – one-character (numeric) year code, last digit 1

The month code is as follows: A=January, B=February, C=March, D=April, E=May, F=June, G=July, H=August, I=September, J=October, K=November, L=December

The year code is the last digit of the calendar year the frame was produced. Frames were typically produced a month or two before they were used. So a low VIN number bike will have a date code from the previous year. 1952 K model frames do not have date codes, and only the last few months of 1953 had them. See the Technical article on Frames for more information.

Here are some photographs of frame date codes.

Part Numbers

Every Harley-Davidson part has a part number assigned to it. Part Numbers are shown in the Spare Parts Catalog, but only in rare instances do these ever appear on the actual parts, and then they are typically painted, to help the retail dealer quickly identify them in his spare parts stock. Some wheels, fenders, and chainguards have the part number painted in an inconspicuous place.

BBBBB-YYS 34961-57
BBBBB – five character (numeric) base part number 34951
YY – two character (numeric) year of first use 57
S – optional one character (alphabetic) Suffix n/a

The "year of first use" typically, but not always, is the first year the part was used. However, in some instances, particularly with the first year of a new model, YY may be one year prior.

The Suffix indicates a minor revision. Parts with a Suffix letter replace, and will physically fit on models back to the YY, but they are different than the non-suffix parts, sometimes in a very minor way, but sometimes in a major way. The first revision gets the letter "A". Subsequent revisions get "B", "C", etc.

The suffix can also be "R", indicating a Racing part.

Casting Numbers

Most of the cast aluminum and forged steel parts have a raised “casting number” cast/forged into them. This number has the same format XXXXX-YY as a part number, but you will never see this part number in a Spare Parts Catalog. The reason - it is an factory internal use only part number. Why would there be such an animal?

Harley outsourced aluminum castings and steel forgings. Harley did extensive machining and fabrication, but the castings and forgings required specialized manufacturing capabilities that were cheaper to buy than to build. For inventory reasons, Harley had the outside supplier cast/forge an internal use only part number into the “raw material” part.

Most castings required machining before becoming an actual part. The raw material Footrest support (left), which has casting number 50950-52, was drilled and tapped to become part number 50951-52.

Many "parts" are built from "sub-parts", so the casting number does not identify the part. A K-model Saddle bar (“seat tee”) is a good example. The 1952-1953 Saddle bar (Part# 51902-52) uses the outsourced front section (casting #51910-52) with an in-house made rear section. The 1954-later Saddle bar (Part# 51902-54) uses the same front section, but with a different rear section. To confuse matters, around 1959, the front section changed slightly (now casting# 51901-52A), but the Saddle bar's Part# 51902-54 did not change.

So casting numbers do not identify parts. But they can be important clues to the version, or production date, of a part.

Casting Date Codes

Some parts, particularly the cast engine crankcases, covers, and cylinders, have date codes cast into them. These are usually the month and year, but their use, and format, are inconsistent. There are also many miscellaneous casting marks, which might indicate the foundry, or some production sequence.

Here are some photographs of engine casting date codes..

Cylinder Date Codes

The cylinders were date code marked at the base to denote the casting date of the cylinder. There is a capital letter (“I” omitted) which identifies the month of the casting. Then there is a small plate fastened with screws. On the plate are two groups of digits separated by a dash. The first group of one or two digits is the day of the casting and the second group is the last digit of the year.

Cylinders cast in 1957 to 1961 have the part number cast on both sides of the cylinder base and 62 and later have it cast on the spark plug side only. Early 1957 cylinders have the date code on the spark plug side and later 57 cylinders have them on the push rod side. The earlier cylinders are a harder casting material than those with the date codes on the push rod side. The cylinder date codes can precede the bike manufacturing date by anywhere from two to eight months.

K/KH Left Engine Cases - The Letter D

K or KH left engine cases with the letter “D” stamped near the serial number denotes that the case was a replacement case (made in 1957 or later, or perhaps refurbished existing stock) that was redesigned to use the Sportster 34844-57 transmission access cover. The original K/KH case and access cover used cone-shaped dowels. The replacement “D” case has straight dowels, and requires that the -57 access cover be used with it.