Short frame motors are 1957-1966 XL / XLH and 1958-1969 XLC / XLCH.
Long frame motors are 1966-up XLH and 1970-up XLCH.

All sportsters 1957-1966 and 1967-1969 XLCH were virtually identical in design. So most parts thru that period do interchange seamlessly.
The differences that do exist are easy to deal your way around.
The XLForum seems to have adopted the term 'short frame' to describe the 1st generation bikes.
(because their frames were shorter than the 2nd generation “long frame” bikes that came into being to hold all the electric start stuff in 1967)
These motors are also known as “no hump” motors as the left cases were not cast to accept a starter (kick only cases).
1967-1969 XLH was not 1st generation design though many of these second generation parts will retro-fit to the 1st generation design bikes.
Some don't make that an easy task as they are similar but not the same. ¹⁾
1967-up XLH and 1970-up XLCH motors are also known as “hump” motors as the left case is cast with a “hump” over the tranny to mount a starter.

In 1967, to power the starter, a giant battery was required.
To make room for this, the rear frame down tubes were offset to the rear with a “dogleg” and now the frame got longer than the original.
The original is a “short frame” and the doglegged version is a “long frame”.
See the long frame (dogleg) in the pic below. Note the position of the down tubes to the motor mount surface. ²⁾

Swapping motors and frames:
1952-1976 motors will be a direct bolt-on to 1952-1976 frames (may have to deal with or add bracketry for oil tanks and battery trays depending…)
1977-1985 motors will be a direct bolt-on to 1977-1985 frames (must use the motor mounts that fit the frame year).

All 1957-1966 XL / XLCH and 1967-1969 XLCH motors are basically the same (short frame) and will be a direct swap within those year (kick only) frames.
But the 1967-1969 XLH are different (long frame “hump”) motors. From 1967-1969, there were two sportsters; Short XLCH and Long XLH.
All 1970-up bikes used the long frame.
1967-1976 electric start “hump” engines will bolt directly into the early kick only frames.
What won't bolt on from there is the matching e-start accessories (oil tank or battery box). ³⁾
The e-starter is well forward of the seat tubes. You may have to do a slight amount of clearancing for the aluminum starter housing.
Or just run kick only and make it real simple.
If swapping early motors into 1977-newer frames, just remember to use the front and rear mounts for the year the frame is. ⁴⁾
Short frame motors will bolt into long frames but the dog leg seat posts will show on the later frames. ⁵⁾

Swapping case halves:
You can physically bolt any 1952-1976 case halves together (need to remove center dowel pin in certain combos). ⁶⁾
That does not mean that the cylinder spigots and crankshaft will exactly line up between the two halves.
Further machining will most likely need to be done to pair the two halves (look for a good machinist).

Click Here to view the Ironhead Motor Mount page in the Sportsterpedia.

All bikes, 1966 and earlier, use the 4 hole mount and the cases also have 4 top holes and 3 studs for the rear mount.
All 1966 and earlier bikes were kick start only. One mount and one case configuration covered all models.
In 1967, the first electric XLH came out and to fit the starter, the cases and the mount were changed from 4 bolt to 2 bolt / 3 studs on XLH only.
The XLH battery box / oil bracket was bolted in the vacated area.
XLCH still used the old 4 bolt cases and mount through 1969.
Then there were two mounts and two case sets, one each for 1967-1976 XLH and 1967-1969 XLCH.
In 1970, the old 4 bolt cases/mounts were dropped completely and the XLCH was redesigned.
It was now based on the electric start bike, with the starter removed and a kicker fitted instead.
This is how the 1970-1976 XLCH is built (as a stripped down XLH rather than the old traditional kick only design). ⁷⁾

In 1952, the K model was introduced. This was a brand new design for HD.
Between 1952 and 1976, that basic bike slowly evolved.
It was built on the same “mechanical DNA” and lots of stuff directly interchanges between a 1952 and 1976.
In 1977, the basic design changed. Fitting a 1977 engine into a 1976 frame requires mods to the frame.
The F model flatheads (side valve in factory speak), from 1952 to 1956, the bike changed each year.
But the changes were evolutionary rather than revolutionary. You could pretty easily put 1956 stuff on your 1952 and vice versa.

In 1957, the OHV XL came along. This new engine was fitted to the same basic frame. It was still the same bike for the most part.
You could put the OHV engine in the flattie frame and vice versa. Same in 1958. Basically one bike so far.

In 1958, the factory took that basic bike and introduced a new model along side the XLH, the XLCH.
The XLCH is the same motor and frame and everything bolted on is just stripped down but still the same bike.
These two models, linked at the hip, were produced until 1966.
1957-1966 (all) and 1957-1969 (XLCH) are kick start only.

The left picture below is a 1964 XLCH left case. It can be used on any short bike.
Notice the 4 blue and 3 yellow circles drawn on the picture.
On 1958-1969 XLC / XLCH, these are all tapped holes for the primary cover.
On 1957-1966 XL / XLH, the holes at the yellow circles are not drilled or tapped and the holes at the blue circles are not tapped but have 3/16“ dia roll pins. ⁸⁾
The right picture below is of a 1960 XLH left case.
In this case you don't have all the screw holes needed for the tin primary cover used on 59-69 CH.
You'd need to use the 3 hole sportster cover or pull the pins and drill and tap to convert to tin cover.

Click on a pic to enlarge:

1957-1969 XLCH (short frame) primary case 9)	1957-1966 XLH (short frame) primary case 10)

1957-1966 (All) and 1957-1969 (XLCH) (short frame) Cases. Quick glance visuals. 11)

Then it happened.
In 1967, a mutation in the DNA resulted in the 1st change away from the original design.
Electric start was offered on XLH. A “hump” was added to the left case casting above the tranny.
It was decided to put the electric starting components behind the rear cylinder mounted to the crankcase above the new case “hump”.
New (XLH only) “hump cases” with “long frames” were made to accept the new starter.
The 1967-1969 XLCH was unaffected until it also succumbed in 1970.
1967-1969 (XLH) and 1970-1976 (all) are electric and / or kick start. ¹²⁾
You may see many XLHs that added the Option Kicker Kit. ¹³⁾

On long frames, mechanically speaking, 1967-1969 (XLH) and 1970-1971 (all) cases interchange. ¹⁴⁾
Cosmetically speaking, only 1967-1969 XLH cases are identical.

1967-1969 XLH (long frame) Primary Case 15)

1972-1976 cases are machined for 1000cc cylinders.
There was a late 1973 cylinder and head change but the crankcase was left unchanged ¹⁶⁾.
All 1000cc cylinders will fit into all 1000cc cases. You just have to use either 72-E73 heads AND cylinders or L73-up heads AND cylinders.
1972 right case has the hole for the speedometer drive gear that runs off the countershaft low gear in the transmission.
1973-up right case does not have this hole and the speedometer drive unit was moved to the front axle. First year for front disc brakes also.

1975 XLH 1000 cases 17)

Cases were redesigned in 1977 to include a mini oil sump and a new gerotor type oil pump mounting boss.
1997-up frames got a divot on the right side tube to allow removal of the oil pump without removing the engine.
Crankcase breathing was dramatically redesigned. The oil trough between the crankcase and gearcase was deleted.
The crankcase now breathes thru holes drilled in the wall between the crankcase and gearcase and the timed breather was deleted.
A vertical oil scavenge passage is drilled in the sump wall connecting the crankcase to the oil pump return port.

• 1957-1969, The engine serial number pad is a rectangle casting on the left case just above the timing hole plug.
  The serial number is stamped into the pad there.
• 1970-1971, the serial number pad from previous is still on the left side case but the serial number was moved to the right case in 1970.
  So the left side pad is blank.
  The new serial number (now called VIN) and it's required casting pad is on the right side case.
  1970-E1971: VIN is between #3 & #4 tappet blocks with stars on each side of the VIN. ¹⁸⁾
  L1971: VIN is between #2 & #3 tappet blocks with stars on each side of the VIN. ¹⁹⁾
  • Here are a few comments on the left side blank pad from piniongear of the XLForum:
    The reason for this is because Harley had no reason to change something until the change was important enough to do some retooling.
    If they were to eliminate the raised rectangle in 1970 when the number switched over to the right hand side, they would have to do retooling on the dies.
    That would be a waste of good money just to eliminate the raised rectangle. So the cheapest way was to simply not stamp the number.
    Even this required some retooling but it was not like doing die work.
    Keep in mind that Harley never changed anything until it was absolutely necessary.
    Just look at how many parts still carry the old 1952 suffix (xxxxx-52) on the part numbers from the old K Model. ²⁰⁾
• 1972-1985, there is no longer a number pad cast into the left case.
  The VIN pad and number is on the right case between #2 & #3 tappet blocks with stars on each side of the VIN. ²¹⁾

1957-1969 Sportster Serial# / Vin Pad Location 22)	1970-E1971 Sportster Serial/VIN Pad Location 23)	L1971-1985 Sportster Serial / VIN Pad Location 24)

1957-1971 left cases (above pic) have 4 horizontal ribs under the serial number pad.
1972-1985 left cases have 5 horizontal ribs in that same location (below pic).
An extra rib was added in place of the number pad in 1972 when they stopped casting the number pad on the left case.

²⁵⁾

Click Here to see the Engine Case Number page in the Sportsterpedia.

You will not find any part numbers on the cases that match the parts listings in the parts catalogs.
Part numbers were never cast or stamped into the cases. Each case half has many numbers on it but none of them show up in the parts books.
There were some parts that did have part numbers on them either stamped or on a sticker label.
Some electrical parts had stickers with the part number on them but no part numbers on cases.

The parts catalog listings are for logistics in which the MoCo stocks parts, sells them to dealerships and customers all under one Part Number.
Basically the part number was created to move the product, not to build the product.
Moving the product can mean, to the builders at the factory, mechanics at the dealership or to the customer.
Part numbers are used in manuals and instructions .
Part numbers are assigned after the parts are built (more accurate to be said the part will be listed as XXXX part number when it gets built).

Each case half has a casting number, not to be confused with a part number. See below.
1957-1975 case halves have their own part numbers with the set also having a part number.
That's because you could buy a full case set or you could buy a left or right case half.
1976 and up cases were only sold as a set so there are no individual part numbers for left or right case halves, only a part number for the set.

Casting numbers on the left and right cases look like (are coded like) part numbers but they are not.
They were put there when the case half was created (cast) and represent a certain design.
Unlike part numbers, casting numbers pertain to the building of the part. One could also call them design numbers.
Many different year motors may carry the same casting numbers but there will still be differences that are important to know.
When the main design changed, so did the casting numbers, but not until then. However, the part number may change several times.

Example: 1977-1980 Sportster cases carried the same left casting number (24534-75) and the same right casting number (24558-75).
The case design was the same but within these years, there were 4 different part number changes.
Different machining (kicker or no, oil galley slot or hole etc) took place that made a new part number.
So even though the main design and casting numbers are the same, there are still different features between them.
These different features may or may not pose a problem when mixing cases or ordering parts and gaskets.

You should find, somewhere on each case half, a set of numbers representing the day and the year that part was cast.
In the early years, usually there will be 4 numbers (2 for the month followed by 2 for the year, last two digits).
In later years, a slash was added between the month and year. Example: (12 64 is December 1964), (11/75 is December, 1975)
When reading this number code, keep in mind that the months for new models range from late June to early July.
A case that was cast in January of 1964 was most likely made for 1964 models.
A case that was cast in December of 1964 was most likely made for 1965 models.

Looking over many casting date codes and cross referencing that with known VINs can give a clue to how close these events were to each other.
They should be within a month or two apart (the exact day of machining can't be deciphered from these numbers).
The casting date shows what month but not what day and the serial/VIN doesn't have anything to do with dates.
So that process is not very useable on 1957-1974 models without further information.
On 1975-1985 models, matching the date code and crankcase numbers is easier since the crankcase number has the Julian date code system.
That gets you a certain day and month the case was machined then of course the casting date will be a month or two before that.

• The 1975 XLCH left case below has a casting date of 11 74 (November, 1974) which does put it in the 1975 model season.
  Also, the crankcase number is 775 351 102 which marks it to be used on a 1975 model, machined on December 17, 1974.
  Those two sets of numbers match within reason (cast in November 1974, machined December 17, 1974).
  The finished bike could have been originally sold between December and the following January of 1975.
  (if using just these numbers alone for identification)

²⁶⁾

Belly numbers don't have anything to do with issuing a VIN and they will not tell you specific model info (XLH or XLCH).
Engine serial / VIN numbers are added much later after assembly. ²⁷⁾

Belly numbers tell you that a particular set of cases was paired up and finish machined together (line boring, cylinder base surfacing etc).
The belly numbers will tell the year model that the cases are intended to be used on.
And also either a sequential number (57-74) or Julian date and sequential number (75-85).
Each case half should have the same exact belly numbers.

• On 1957-1974 motors, the belly number is on the outside of the motor (bottom of each case half under the flywheels).
• On 1975-1985 motors, the MoCo began using a Julian Day Code system for belly numbers.
  Click Here to view the Julian Calendar page in the Sportsterpedia for full year calendars.
  The belly numbers on the right case are on the outside and the belly numbers on the left case are on the inside of the motor.
  Right case still has the numbers on the bottom under the wheels.
  Left case has the numbers on the outer primary wall (primary cover removed).
  (technically, the left crankcase numbers really aren't belly numbers anymore)

1957-1974 crankcase / belly numbers 28)	1975-1985 crankcase / belly number locations 29)

In addition to belly numbers, there is a second set of numbers on each case half that should match.
For lack of public documentation, we'll call these Case Match Numbers and they represent the fact that those two case halves were mated as a factory pair.
These will most likely be a letter(s) and numbers but the exact number set should be on the outside of each case half opposite one another.
The factory belly numbers should be the same and the case match numbers should also be the same.
There does not seem to be any connection between the case match numbers and belly numbers (no date coding visible on case match numbers).

• On 1975-E1984 and earlier, they'll be under the generator.
  
• On L1984-1985, they'll be under the oil filter mount (same location, different part above the numbers).
  

The exact location/spacing varies from case to case. Some will be close to the middle seam, some will be farther toward the ends of each case.
1976 XLH 30)	1981 Sportster 31)	L1984 XLS32)

These numbers pertain to the whole bike, not just the engine. Coding for these numbers changed over the years.
Click Here to see the VIN page in the Sportsterpedia for specific information on serial/Vin numbers.
An XLCH may have many different parts, over the whole bike, than an XLH, including the engine.
The serial/VIN number was (most likely) the last number to be stamped onto the engine case from the factory.
1957-1969 Sportsters are titled to the engine only so the motor serial number is your proof of ownership of the entire bike.
1970-up Sportsters are titled to the frame only so the motor VIN number has nothing to do with ownership of the bike.
However, having the motor VIN matching the frame VIN is a plus for those who want factory original parts.
And it helps toward the value (or haggle factor) on resell but the VIN is not the only factor in whether you have an original engine or bike.

• 1957-1971 Sportster motors are 883cc (respectfully labeled “900”)
  • The side of each cylinder base is slightly milled at the intake tappet and is barely noticeable.
• 1972-1985 Sportster motors are 1000cc.
  • The side of each cylinder base is milled heavily at the intake tappet and is very noticeable.
  • The cylinder studs have a different spacing than 900 cases.
  • The cylinder spigot hole in the cases is wider to accommodate 1000 cylinders.

1957-1971 side of 900 cylinder base 33)	1972-1985 side of 1000 cylinder base 34)

• 1957-1966 XLH and 1957-1969 XLCH have three oil tank fittings on top of the right case behind the rear cylinder.
  • Oil feed from the tank, oil return to the tank and oil tank vent fittings.
    There are galleys built into the case that run internally (feed and return go to oil pump respectively and tank vent goes into cam chest).
• 1967-1976 XLH and 1970-1976 XLCH have only one oil tank fitting on top of the right case behind the rear cylinder.
  • Oil return fitting to the tank.
    Oil feed runs from the tank to a fitting on the back of the oil pump.
    Oil tank vent runs from the tank to the 90° fitting on the back of the cam cover.
    Oil return still runs internally to the oil pump.
• 1977-1985 XLH/XLCH/XLS/XLX motors do not have any oil tank fittings on the case or the respective oil galleys as previous years.
  • Oil tank feed and return both run to the oil pump externally.
    Oil tank vent runs from the tank to the 90° fitting on the back of the cam cover into the cam chest.

1967-1976 XLH, 1970-1976 XLCH Oil Fitting 35)