Originated from an article by DirtyCory of the XLFORUM ^{1) 2)}
See the 77-85 Oil Pump section of the Sportsterpedia for pics, dims and more information.

• The information below is a compilation of individual year catalogs plus long year span catalogs.
• There were 2 long year span parts catalogs published for sale as the 78 catalog. Both were written years after 1978;
  • The -78A parts catalog was printed in April 1981 and reflects parts available at that time.
  • The -78B parts catalog (printed post IH era) was 'copyright 1989' and reflects parts available then.
    (namely parts that were available in 1985 or later for the 77-85 oil pump, disregarding some - i.e. gerotors as above)
• 1979 and later parts catalogs only reference prior fitment back to 1979.
  (although, many parts will fit 77-78 models)

There were 4 different pump assemblies used on Sportsters from 77-85.
Each one will bolt to any 77-85 engine as it's own assembly.
(and may be found at swap meets or online)
The only mounting difference being that you might have to change out or work around the fittings.
45° and 90° fittings were changed in some of the different year model pumps.

Sold for use on 77-81 engines only. Last sold in 1984.
There were 3 different versions of this pump in regards to upgrades but the body and major internals are the same.
Upgrades and parts list versions created thereof are shown below.
The assembly has a 45° feed fitting and a 90° return fitting.

• New parts / numbers from the -78 parts catalog:
  • New check valve body O-ring (12042) for the '1600' pump only.
    (to account for a possible oversized condition in the check valve bore in the 77 '1600' pump)
  • New check valve (26435-76A).
    • Both the prior and new check valves have the same lower O-ring.
• Discontinued for Parts Order:
  • Original check valve (26435-76) no longer available for parts order.
  • The 1600 pump is no longer produced.
• Notes:
  • 77 check valve body O-ring (26432-76) still available for standard oil pumps.
  • The Oil pressure switch (26554-77) was listed in the -77A parts supplement catalog.
    • However, it was listed as a L77 addition in the -78 parts supplement.
      Previous models had a pressure switch. The circumstance of this switch being a late addition is unclear.
      There was a wiring harness upgrade in 78 and oil pressure gauge available for 77-78 models.
  • Double nuts (7638) for the oil pressure switch also a late addition part #.
    (retainer for the wire to the oil light)

• Discontinued for Parts Order:
  • O-ring (12042) no longer available for parts order.
• Notes:
  • O-ring (26432-76) now the only one available.

• New parts / numbers:
  • Upgraded cover O-ring (26434-76A)
    • This O-ring replaces the original and is used in future oil pumps through 1985.
• Discontinued for Parts Order:
  • Previous cover O-ring (26434-76) no longer available for parts order.

There were no changes to the oil pump noted in the -81 parts catalog.

• New parts / numbers:
  • New oil pump assembly (26204-82) for 82 only.
    • All internal components are the same part numbers from the 77-81 except for the oil line fittings.
  • The part number changed for the 4 Allen head mounting bolts (3478) although their size didn't change.
• Discontinued for Parts Order:
  • Oil pump mounting bolts (1794).
• Notes:
  • The 77-81 pump has a 45° fitting and a 90° return fitting.
  • The 82 pump has the 45° fitting on both the feed and return outlets.
    • This coincides with the oil tank upgrade in 82 (oil line sizes did not change).
  • 77-81 oil pump (26204-75), as upgraded, still sold for 81 and prior models (due to the fittings being different in the new assembly).

• New Parts / Numbers:
  • Upgraded pump assembly (26204-82A) for 82 and up.
  • New oil pump body (26484-83).
  • New pump cover (26486-83).
  • New outer plate (26493-75A) for 83 year model only.
  • New outer plate seal (12036A).
• Discontinued for Parts Order:
  • The 82 pump assembly (26204-82) no longer available for parts order.
  • Pump cover (26486-75) no longer available for parts order.
• Notes:
  • 77-81 oil pump (26204-75), as upgraded, still sold for 81 and prior models (due to the fittings being different in the new assembly).
    • 82-E83 oil pumps have the 45° oil fittings on both the feed and return hose connections.
  • Pump body (26484-75) still available for parts order.
  • The (26486-83) replacement cover is the same as the -75 cover.
    (and also assembled with the 45° feed fitting - old part number deleted)
  • Outer plate (26493-75) still sold for 82 and prior models.

One year production assembly.
The assembly has a 45° feed fitting and a 45° return fitting.

Production assembly on E83 models only.
The assembly has a 45° feed fitting and a 45° return fitting.

L83-85 production assembly.
The assembly has a 90° feed fitting and a 90° return fitting.
This pump first appeared in the -84 parts catalog.

However, it's unclear of how many of it's upgraded parts were listed in the 84 parts catalog.

The MoCo was still producing the 77-E83 style oil pumps for 77-81 models
(-84 copyright August-83, includes the E83 gerotors which were discontinued for production models by November of 1982 per the TSB).
The new style gerotors were not in the book but the (26197-83) part number is for the L83 pump.
It's upgraded parts appear in the -85 parts catalog.

• There were 2 different sets of gerotors used in Sportster oil pumps from 77-85.
  • 77-E83; Return set (26491-75), Feed set (26492-75). Discontinued for production models by November of 1982. ³⁾
  • L83-85; Return set (26491-83), Feed set (26492-83).
• The -78B parts book lists the original gerotors.
  (even though that catalog was written in 1989 after the design change).
• Oil pump check valve:
• The oil pump check valve had 2 different part numbers.
  • The original was (26435-76) in 1977.
• The part number was upgrade (26435-76A) for L77-78 models ^{4) 5) 6)}
• Speculation could say…,
  • The original check was for the special “1600” pump (discontinued L77).
  • And the upgraded check was for installation in a newer, non publicized, pump casting not cataloged.
• Either way, the upgraded check valve was used through 1985 on IHs.

• Notes:
  • Retainer nuts for the oil pressure switch connector.
    • There was no mention of the nuts on the oil pressure switch.
    • Most likely OEM on 77 models and offered for replacement parts in 78?
    • The -78A parts catalog shows them as a L77 part. ⁷⁾
  • Gerotors.
    • Both feed and scavenge gerotors have a chamfered radius (.020“ x 45° chamfer or .020”) on each outside gearset.
    • The same radius is designed into their respective oil pump body and cover.
    • The gerotor part numbers were used from 77-E83.
    • However, there was a TSB ⁸⁾ regarding the early feed gerotors.
      • The feed gerotors fit inside the oil pump cover and should extend .001” - .005“ above the ridge.
      • This bulletin also says the later feed gerotors should extend up to .011” above the ridge.
    • It is unclear at this point what the difference would have been between the early and late feed gerotors by June of 1977.
    • However, the part number did not change.
  • The special “1600” oil pump.
    • From the Sportsterpedia history section, There was a special “1600” oil pump installed on E1000 engines. ⁹⁾
    • This special (77 only) pump is not listed as a separate item in any of the parts catalogs or supplements.
    • It's possible this was a vendor quality control issue on their casting.
    • A quantity of them had oversized oil pump body 0-rings (which are assembled in the groove in the check valve bore to seal the pump cover).
    • This oversize 0-ring (12042) must be used in place of the standard 0-ring (26432-76) on these pumps only.
    • All such special pumps are identified with the letter “0” stamped on the pump body (exact location unknown).
    • So, the “1600” oil pump (body at least) appears to be the original casting on 77 models.
    • However, it's exact origin and demise is still unclear.

Parts list from the -77A and -78 parts catalog supplements: ^{10) 11)}

Parts list from the -78 parts catalog supplement: ¹²⁾
Part # changes in the table below are highlighted.
(although HD has changed parts without changing the part numbers)

Parts list from the -79 parts catalog: ¹³⁾

Parts list from the -80 parts catalog: ¹⁴⁾
Part # changes in the table below are highlighted.

• Notes:
  • There are no changes noted in the -81 parts catalog.

Parts list from the -81 parts catalog: ¹⁵⁾

Parts list from the -82 parts catalog: ¹⁶⁾
Part # changes in the table below are highlighted.

Parts list from the -83 parts catalog: ¹⁷⁾
Part # changes in the table below are highlighted.

• New Parts / Numbers:
  • New oil pump assembly (26197-83) for L83 models (listed as a 'Pump Kit' in the -78B parts catalog). ¹⁸⁾
    • All other internals are the same as in 83 (according to the 84 catalog).
    • However, the 85 catalog and a service bulletin revealed the true internal changes to this new pump.
    • The new pump assembly will retrofit to 77-E83 ¹⁹⁾
    • These modifications limit interchangeability and affect the installation of repair parts:
    • Either early style or late style gerotor gears may be used with late style (under-cut) oil pump bodies and covers.
    • Only early style chamfered gerotor gears may be used with early style oil pump bodies and covers.
      (Use of late style gerotor gears in early style oil pump bodies and covers could cause the oil pump to bind up and possible pump failure).
• Discontinued for Parts Order:
  • Oil pump assembly (26204-82A) no longer available for parts order.
• Notes:
  • Oil pump assembly (26204-75) still sold for 81 and prior models.

Parts list from the -84 parts catalog: ²⁰⁾
Part # changes in the table below are highlighted.

• New Parts / Numbers:
  • Upgraded body O-ring (26432-76A)
  • Upgraded oil pump body (26484-75A)
  • Upgraded pump cover (26486-75A)
  • New gerotors (L83-85 for the 26197-83) available for parts order:
    • (26491-83) return, set - new part# for 82-85
    • (26492-83) feed, set - new part# for 82-85
    • The new gerotor gears are fabricated without the chamfered top and bottom on the outer diameters as was in the 77-E1983 gerotors.
• Discontinued for Parts Order:
  • Old pump (26204-75) no longer available for parts order.
  • Oil pump body (26484-83) no longer available for parts order.
  • Pump Cover (26486-83) no longer available for parts order.
  • Old gerotor set (26492-75) and (26491-75) no longer available
• Notes:
  • Oil pump assembly (26197-83) now being sold for 85 and prior year models.
    (as well as the individual upgraded parts along the way with the exception of the gerotors)

Parts list from the -85 parts catalog: ²¹⁾
Part # changes in the table below are highlighted.