THE PART NUMBER WILL MOST LIKELY NOT BE STAMPED INTO A PART.
However, some parts do have an actual part number printed on them (some electronics have part numbers inked).
Only the MoCo knows the EXACT rhyme and reasons for the part numbers that they assign to the different parts of their bikes.
We can decode some of these part numbers but in some instances we can only cypher so far.
This article is an attempt to explain some of the coding and changes to the original part numbers and upgrade numbers.

CASTING NUMBERS ARE NOT PART NUMBERS and in most cases have nothing to do with the part numbers.
However, some casting numbers have a unique number set and have been used to describe the parts in which they have been cast into.
Click Here to reference “HD Part Numbers Explained” in the Sportsterpedia.

If there is a change to the part number, it is assumed that it is a different part altogether and will not fit your motorcycle (part number is different, right?).
The truth is there is no way of knowing for sure just going by the part number.
The forgoing has been documented and links to pics and other documentation are provided where applicable and will be in blue lettering.
However, some examples listed are generic for clarity.
Also in the examples, specific year codes are shown but hardly any of the forgoing only pertains to specific year models.
Most examples can easily apply to Sportsters from 1957-present if applied to different parts/part numbers.

The length of the part numbers are determined by the MoCo and some are within reason and some are not even close to reasoning out.

Generally, HD hardware typically has between 3 and 5 digits (i.e. 511 cotter pin or 3455 bolt or 7855W washer).
But there are bolts/nuts with a 5 digit number and a dash number.
And hardware part numbers sometimes have a letter added on the end indicating a length, material, vendor change from the original version.
Reference Evo Fastener Page

Most parts other than hardware have a 5 digit number followed by a dash and year code.
Newer versions of that part may have a letter added on the end. And some parts have 2 letters on the end (19“ front fender 59007-73TB).

• Original versus Updated part numbers.
  THE PARTS OR PART NUMBERS IN ANY VERSION PARTS CATALOG IS NOT MEANT TO BE TAKEN AS ORIGINAL PARTS OR PART NUMBERS.
  The catalogs simply list what parts were available for sale or replacement at the time the book was written.
  Each parts catalog (for public use) has a part number written on it (front cover, left side or sometimes first few pages) and each catalog is dated.
  Most original parts catalogs will have 5 numbers and a dash code at the end of the part number with 2 numbers and NO letter (i.e. xxxxx-95).
  The next revision of that parts catalog will be (xxxxx-95A) then (xxxxx-95B) and so on, including a new calendar date (year it was written).
  As time goes by, original parts are obsoleted and replaced with updated parts.
  The parts catalogs are also updated to reflect the current changes at that moment in time.
  Retail stores (online or physical outlets) will usually follow suit and only list the “current” part number.
  If you have an original parts catalog for your year model and try to order that part, you may very well be told it doesn't exist but “here is the part number they show”.
  And if you want to know the original part number, you have to check the original parts book (one without a letter on the end) to be as sure as possible.
  Reference Factory Parts Catalogs
  • Example: Flywheel assembly (23905-00) is listed in 2000-2003 original version parts catalogs.
    The 2003 catalog went through a (-A) and a (-B) version. Both the (-03 and -03A) catalogs list the flywheel assembly as (23905-00).
    However, the (-03B) catalog written in 2018 lists the flywheel assembly as (23905-00A). These two flywheels look worlds apart from each other.
    And if you didn't originally have the -00 flywheels, you won't know that the -00A wheels were replacement for the originals.
• Some parts will interchange even though the part number doesn't match your motorcycle.
  Subtle differences may include color, texture, or construction material changes.
  • Example: All rigid Evo Sportster rocker boxes will bolt to all rigid Evo Sportsters.
    The color or texture may change (natural, silver, chromed or blackened) but they all have the same function.
    Some of these have different prefix and suffix numbers or a combination of old and new numbers.
    And 04-up the lower box is the same as the rigid lower box with extra threaded holes added to mount the plastic breather valve assembly.
    Slight changes sometimes just get a letter added on the end or an entire new part number.
    Reference Evo Rocker Box Page
• Some part# changes are due to defects found in certain parts.
  These changes may keep the same first 5 numbers and simply change the ending two numbers or add a letter to the end of the existing numbers.
  Or the entire part number may change.
  • Example: Company A makes continuous defective master cylinders.
    The Moco may have the vendor scrap the old part and place a marking on the new improved part (with a letter added on the dash number).
    Thus, there will a part number change differentiating (to the MoCo) where this part in their hand was made.
    And the only change being the part number in the catalog and maybe a casting number/letter/symbol change.
    Some of these type changes also included colored markings / dots etc. to help the dealerships in selecting the correct parts.
    Since some of these changes were not external but rather internal (bore sizes for instance).
  • Example 2: A recall was issued for certain 78-E79 models regarding master cylinders that were fitted at the factory with a .015” over-bore.
    The master cylinder part number (43306-78) didn't change but ones with the correct bore size were retrofitted to the affected bikes.
    Reference TSB 0736 PDF
    Another problem cropped up with the -78 master with a possible tear or crack in the metal between the master cylinder outlet and the fluid reservoir.
    A new version master was issued for L79 models (45011-77)
    Reference TSB 0747 PDF
• Some part# changes are simply due to a change in vendor / supplier.
  These may be the country of origin (where HD had the part made) or related to a different company that the MoCo used to make the part.
  These changes may keep the same first 5 numbers and simply change the ending two numbers or add a letter to the end of the existing numbers.
  Or the entire part number may change.
  Sometimes part# changes are due to procurement problems with the factory's suppliers also.
  And sometimes a vendor change means that the part changes entirely but not always.
  • Example 1: Timken swingarm inner race bearing (47082-81, made in Poland) is listed for 1986-1999 Sportsters.
    Timken inner race bearing (48367-98, made in USA) is listed for 2000-2003 Sportsters.
    They are both LM11749 bearings with the same dimensions and will actually fit 86-03 Sportster swingarms.
    Reference Evo Swingarm Page
  • Example 2: Clutch friction disc (37931-84) for BTs was obsoleted due to a procurement problem with the HD supplier and superseded by part# (37931-84A).
    The new part# does not appear in the 87-88 1340 parts catalog.
    But was listed in the Replacement Parts section of a P&A Order Book and was scheduled for an October 15, 1987 introduction anyway.
    Reference PAB-367 PDF
  • Example 3: Due to a change in vendor, rear black 16“ spoke cast wheel assembly for 1986 FL models will no longer be offered.
    It will be replaced by part number (43299-87) rear 10 spoke cast wheel assembly.
    Reference PAB 329 PDF
• Drastic changes (those where this part no longer fits previous models) usually include changes in the construction or function of a part.
  These may or may not be associated with a change in prefix numbers and/or dash numbering.
• The MoCo is notorious for changing a part but not changing the part number.
  Various parts have changed, some drastically, but the part number stays the same.
  • Example 1: The oil pump changed in 2007 with MUCH taller and wider gerotors which included a bigger bore in the pump body.
    The part number did not change (2000-2006 oil pump, 26204-91A) (2007-up oil pump, 26204-91A).
    The internals in these two pumps WILL NOT INTERCHANGE.
    Reference Comparing the 03-06 and the 07 Style Oil Pumps
  • Example 2: The 1957-1985 center motor mount bracket had 3 changes during those years all having the same part number.
    The 57-E71 bracket (16250-57) has 3 round holes.
    The L1971-76 version (16250-57) has 2 slotted holes and 1 round hole in the center and can be used on 57-76 motors.
    The 77-85 version (16250-57) doesn't tie back to the frame and Can't Be Used On 57-76 Motors.
    Reference IH Motor Mount Page
  • Example 3: 2000-2003 crankshafts (23905-00) were only sold as an assembly (you couldn't buy individual parts for it).
    However, the rods in the 2003 assemblies are reportedly thinner than in the 00-02 assembly and carry -02 casting numbers. ¹⁾.
    Reference Evo Flywheel section
• Many replacement “assemblies” are built with newer parts and the assembly fits older models but the individual parts won't interchange with older ones.
  The parts in these type assemblies are generally not sold individually (don't want anybody mixing parts that might ruin the assembly).
  Some of the parts in the new assembly may actually be the same as the older parts.
  But if one or more of the newer parts used are not compatible with older ones, individual parts are not listed for the assembly.
  • Example: 1977-E1983 oil pumps had 3 part number changes in that year span but all the parts would still interchange between them.
    However, in L1983 the MoCo built a new oil pump assembly (26197-83) with the same parts except the gerotors were cut different on their OD.
    Due to that change, the older gerotor sets could be used (77-85) but the L83-85 gerotors will bind/break in a 77-E83 pump body.
    So up into further years, the older parts were still sold. But the parts in the newer assembly were not sold individually.
    It wasn't until the MoCo obsoleted the older parts that individual parts could be bought for the L83-85 oil pump.
    Reference Late 1983 oil pump assembly design modification
• Individual parts sold now may only be sold later in a bundle (with the multiple parts under 1 part number).
  There are many parts that were once sold individually that was later sold instead with the hardware or other pieces needed for fitment.
  This is said to be done to simplify inventories at both the dealer and factory level.
  The number for the original part may not change but the other items will now be packaged with it.
  Sometimes a letter will be added to the suffix to indicate the parts bundle, and others, the part number changes entirely.
• If multiple parts are under 1 part number, the number may change if any of the parts included change.
  Bolts or washers may prove to be weak and need to be changed to a different size or material.
  With the main part staying the same, a new part number may be issued for the assembly coming with the upgraded bolts.
  Other times, it's not nuts and bolts combined but things like cams and cam cover combinations.
  The cover fits multiple year models but the cams or pinion gear or both are not compatible with older models.
• Some pieces of a “kit” may fit multiple year models but have a more narrow year model listing due to 1 or more parts in the kit.
  • Example: The pinion shaft on 1986-1987 motors is splined and the pinion shaft on 1988-up motors is not.
    P&A came out with a chrome cam cover kit in 1988 to fit 86-87 motors but it would not fit 88-up motors.
    The reason given was that the included pinion gear would not work on 88-up.
    But the cover itself was said to fit 88-up by purchasing a 88-up pinion gear (with color matched to the one from the kit).
    Reference PAB 384 PDF

Most HD part numbers generally consist of 5 numbers followed by a dash and 2 extra numbers (xxxxx-xx).
We'll call them prefix and suffix numbers for clarity here (prefix xxxxx) and (suffix -xx).
There are exceptions to that however especially with some fasteners having just 3-5 numbers alone without a dash number.

• Some of the same (specific use) parts have the same prefix numbers even if the dash number on the end changes.
  • Example (hypothetical) : Brake linkage (12345-57) used from 1957-1975 changed to (12345-76) and was used from 1976 to 1983.
    The first 5 numbers stay the same although the new suffix is (-76) indicating that something has changed regarding the brake linkage in 1976.
    The new part may or may not fit previous versions although the first 5 numbers are the same.
    (in this example, the prefix numbers only indicate the part is in the brake linkage family of products).
• Some of the same (specific use) parts have a completely different prefix number with the same dash number on the end.
  The same part is used with finish or color changes for different models.
  • Example: The upper front motor mount on rigid Evo Sportsters has the same exact dimensions from 1986-2003.
    However, part# (16222-86) has a clear metal finish and part# (16214-86) has a black finish.
    Clear metal was used for 86-03 883s (except 883R) and the black mount was used for all 1200s (and 883Rs).
    Reference Evo Motor Mount Page
• Those 5 numbers may stay the same with only the suffix numbers changing or they may change completely with or without a new dash number on the end.
  So there is a pattern to the first 5 numbers but the reasoning for the changes hasn't been documented by the MoCo (in public anyway).
  And the part number with the prefix change may fit your bike or it may not.

The original suffix (or dash number) is usually a dash followed by a 2 digit number on the end of the full part number sequence.
(although not all part numbers have the dash numbers)

• The original (2 digit) suffix number USUALLY the year that the part was first used in HD production (on SOME MODEL of HD).
  However, this is not always the case. There are -xx numbers that were not used until the following year.
  And it's not necessarily that the “first year code” was used on the Sportster, even though it may now be used on a Sportster model.
  It might have originally been used on an FXR or even FLH model when it was first assigned a number.
  
  • Example 1: A (-90) part number would not have been used in 1984. The -90 suffix indicates first use in 1990.
  • Example 2: A (-95) suffix may have been first used at the start of the 1996 XL year models.
    But it may have been first used on 1995 BT models.
• Many newer parts carry the older year dash code.
  If the part has never changed since it's original conception, the part number may stay the same.
  Some part numbers do stay the same even through material and vendor changes.
  Some parts carry older year dash codes but with a letter added at the end.
  • Example 1: L1984-2003 lower front motor mount part numbers, L(16212-84) R(16210-84A).
    16210-84 was the original right plate but it was replaced with -84A in 1987.
    Both mounts have (-84) in the year code but are factory mounted up thru 2003.
  • Example 2: Rigid Evo Sportster lower rocker boxes have the same part number (17532-86A) from 1989-1994.
    That part number was updated in 2004 to (17532-86E) and will retro to 1988 motors.
    But the -86E will have a threaded hole (for 2004-up applications) that will not be used on 88-03 motors.
• Updates to the original part number usually show up as a letter attached to the end of the dash number (-00A).
  Then future updates will usually change the “A” to a “B” and so on.
  The newer dash lettered part will usually replace the original part number (but not always without additional parts replaced).
  But then sometimes the entire part number changes and it still retros to previous year models.
  • Example: The 91-03 shifter detent plate was part# (33656-90) which was replaced with part# (33656-90A).
    The (-90) had rounded corners separating the different gear locations and the (-90A) has sharper corners (more positive engagement).
    However, both part numbers will work on 91-03 Sportsters without modifications.
    Reference Associated Transmission Wear
• Not all letters in the suffix pertain to upgrades.
  Letters on the end may or may not correspond to an upgrade of the original part.
  So a (-A) or (-B) version of the original part may or may not fit your bike (think master cylinder kits for different bores).
  Letter successions for upgraded parts sometimes will simply move to the next letter (first revision = A, second revision = B and etc).
  Sometimes the whole part number changes.
  There are also part numbers with 2 letters added on the end (i.e. HW on fasteners).
  But there are other letters further down the alphabet that breaks that succession rule (denoting part types / intentions)
  For instance, some racing parts will have (-R) in the parts number suffix or (-RA or -RB).
  Novelty items had a (-V) or (-Z) ending suffix in the part number.
• Updated suffix letters may only be compatible on a later model Sportster.
  (with the previous lettered dash# still available for earlier models or obsoleted and not available at all even though the new lettered part is not retrofittable).
  
• Some original suffix numbers used on Sportsters start out with an added update letter attached to the end.
  Whether that fact is due to an “oh pookie” moment at the factory or the part originally began life designed for a different model HD is unclear.
  But there are, for argument sake, (-86A) numbers that are original numbers in the -86 parts catalog.