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 also discussed.
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.

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 specific year models.
Most examples can easily apply to Sportsters from 1957-present if applied to different parts/part numbers.

• Some changes are subtle and only require a letter added to the part number.
  Subtle differences may include color, texture, or construction material changes.
  • Example: All rigid Evo Sportster rocker boxes will bolt to the heads but some are silver or chromed or blacked.
    Some of these have different prefix and suffix numbers or a combination of old and new numbers.
    And some are the same boxes with an extra hole added to them. Slight changes sometimes just get a letter added on the end.
• Some 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).
• Some changes seem to be strictly for accounting purposes within the factory and/or current parts selection at the dealerships.
  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.
  • Example: 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.
• 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.

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).
• 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 indicates 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.
• 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 1: 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.
• 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.