The oil pump is a positive displacement unit.
If you block the discharge flow path, discharge pressure will rise and literally approaches infinity until something fails. 1)
It's non-regulated and delivers its entire volume of oil under pressure to the oil filter mount.
However, don't take the term “positive displacement pump” literally. 2)
Compared to a centrifigal pump, gear / gerotor type pumps use displacement to function.
How positive it is depends on what it's pumping.
The most 'positive' pump on our bikes may be the master cyl for the disc brakes.
You pull the lever and it pumps real good, positive, unless there is air in the system.
Then you still get the displacement but not the pressure because liquid is not compressable but air is.
Gear / gerotor type pumps work by filling the spaces between the teeth with whatever it is that is being pumped (oil in our case).
The oil in the tooth spaces gets squezed out of the spaces when the gear teeth mesh together (because a tooth is now in that space.
But there is still a little space left at full mesh.
Gearotor pumps have a smaller space at full mesh than gear pumps.
Now when the mesh breaks, a void is created. Oil gets sucked into that void.
Now what if air is in the spaces instead of oil?
It gets sqeezed out also unless there is pressure (restriction) in the curcuit its trying to flow into.
In that case, some air stays in the spaces and gets compressed in the small space remaining as the teeth mesh.
When the teeth unmesh, air expands (refilling void that should be getting oil sucked into it.
The pump can't make enough pressure to overcome the exit restriction.
Then there is no flow and no ability to reprime itself since.
(it's not making enough suction due to the expansion of the compressed air refilling the void that the oil should be filling)
Eventually the pressure in the return line gets released.
While this is happening oil is building up in the pickup sump and waiting for the return side to come back online.
The supply pump is still pumping while the return is toggling from air locked to primed and pumping.
Think about what state the return oil is in as it collects in the return pickup cavity. It's just been thru a 60 cycle blender.
Our bikes live with that every mile they travel with lots of air whipped into every drop of return oil.
Restrictions in the return side of the pump will lead to a more sustained oil level in the crankcase sump.
(large enough to reduce outflow from the engine to a volume less than the supply pump is feeding the engine)
This is a condition known as wetsumping.
Read more about wetsumping in the REF section of the Sportsterpedia.
There are 10 teeth on the drive gear at the pinion shaft and 20 teeth on the oil pump breather gear or driveshaft respectively.
This creates a 2:1 ratio of the gears (20/10 = 2:1) meaning the oil pump RPM is half the speed of the engine.
(running at 4,000 RPM, the oil pump is turning at 2,000 RPM)
See also Installing an Oil Pressure Gauge in the REF section of the Sportsterpedia.
Engine oil pressure was measured (by the MoCo) with a pressure gauge at the oil pump.
Expected oil pump pressure per FSM's:
Gauge mounted at oil pump:
As checked with hot oil and a gauge at the oil pressure switch location at the oil pump.
The oil pressure switch has to be removed for the gauge to be installed.
1957-1969: 3)
Minimum: 3-7 psi (idle, with spark retarded)
Normal riding conditions: 10-14 psi (6 psi at 20 mph)
1970-1978: 4)
Minimum: 3-7 psi (idle)
Maximum: 15 psi (60 mph in high gear)
Normal riding conditions: 4-15 psi
1979-1985: 5)
Minimum: 4-7 psi (idle)
Maximum: 10-20 psi (3500 rpm)
Normal riding conditions: 4-15 psi
Note: On a cold startup, expect pressure to reach ~60 psi 6)
Click Here to reference the Oil Tank, Lines and Routing page in the Sportsterpedia.
NOTE: Please be sure of your particular parts assembly before ordering seals.
1977-1985 Oil Pumps look pretty much identical before dis-assembly but some parts won't interchange.
Click Here for individual pics of pump assembly parts in the Sportsterpedia.
Year Model | Part# | Notes | Pics |
1952-1976 K Model, Sportster | 26256-52 | Pump to engine case mounting gasket | Pic 10) |
L1958-1976 XL, XLH, XLCH | 26227-58 | Gear shaft oil seal | Pic 11) |
1952-E1962 K Model, Sportster 1972-1976 XLH, XLCH | 26259-52 | Inner cover gasket | Pic 12) |
1952-E1962 K Model, Sportster 1972-1976 XLH, XLCH | 26258-52 | Outer cover gasket | Pic 13) |
L1962-1971 XLH, XLCH | 26259-62 | Inner cover gasket | Pic 14) |
L1962-1971 XLH, XLCH | 26258-62 | Outer cover gasket | Pic 15) |
1977 (only) Sportster | 12042 | Small o-ring around check valve body (used in 1600 pumps stamped “0”) | Pic 16) |
1977-1990 Sportster | 26495-75 | Pump to engine case mounting gasket | Pic 17) |
1977-1985 Sportster | 26432-76 26432-76A | Small o-ring around check valve body Updated part# retros to 1977 | Pic 18) |
1977-1985 Sportster | 26433-77 | Small o-ring between pump body and cover (over check valve bore) | Pic 19) |
1977-1990 Sportster | 26434-76 26434-76A | Big o-ring between pump body and cover (cover seal) Updated part# retros to 1977 | Pic 20) Pic 21) |
1977-1982 Sportster | 12036 | Outer divider plate oil seal (between divider plates). Use only with outer plate 26493-75 | Pic 22) |
1983-1990 Sportster | 12036A | Shaft oil seal between divider plates. Use only with outer plate 26493-75A | Pic 23) |
A dry pump won't pump oil.
Any time you have removed the oil pump or the removed / drained the feed hose from the oil tank, the pump needs to be primed.
1952-1966 K Model / XL / XLH 1958-1969 XLC / XLCH Oil Pump Oiling Routes 30) |
Oil Pump Assemblies (per parts catalogs) | |||
Pump # | Body Casting# | Year Model | |
26203-52C | 26215-52 | 1957-1958 | XL, XLH, XLCH |
26203-52D | 26215-52 | 1959-E1962 | XL, XLH, XLCH |
26203-52E | 26215-52 | L1962-1966 | XL, XLH, XLCH |
1967-1971 | XLCH | ||
26204-67 | 26215-52 | 1967-1971 | XLH |
1970-1971 | XLCH | ||
26204-67A | 26215-72 | 1972-1976 | XLH, XLCH |
On some pump casting bodies you may see an Allen plug in the back of the pump. 39)
Also the earlier pumps had a casting number of -52.
In 1972 and with the birth of the 1000cc engine, the pump numbers were -72.
It had larger gears and a bigger breather slot.
1952-1976 K models and Sportsters have an oil return strainer in the bottom of the gearcase.
The strainer is part# (24975-37) and it is fixed in place by a dowel (333) staked into the side of the case.
Oil collects in the gearcase, flows through the screen and down a passage to the oil pump return gears.
Sportsters have a gasket (24978-57) under the strainer.
Strainer (24975-37T) superseded the original strainer part#.
See the link in the sub documents above for 77-85 Oil Pump - Parts Lists and Upgrades.
79 Oil Pump 87) |
The gear mesh (oil pump driveshaft gear to drive gear on the pinion shaft) looks to be a worm gear configuration.
However, that is not accurate. It's actually two right handed helical gears in a 90 degree configuration (or cross axis helical gears).
The teeth on these gears stay in contact longer than straight teeth spur gears. 88)
(which allows them to transmit higher loads at higher speeds than spur gears)
The gears are carbon steel for strength with hardened teeth for increased wear resistance.
(once axial play wears the hardness, the teeth wear down rather quickly)
For the gears to mesh correctly, they must have the same pressure angle and pitch.
Helical gears can be configured to transmit motion in a straight line (or used for Sporty oil pumps at a 90° angle).
To transmit motion at a 90° angle, 2 gears are paired in the same tooth direction (right handed).
The 57-76 pinion shaft has 4 external splines and one is longer than the other three.
The 57-76 oil pump drive gear (26318-37) has 4 internal splines that match to the pinion shaft with one longer recess between them and is a slip fit over the shaft.
The drive gear slides onto the pinion shaft one way (only) with a timing mark in the front over the wider recess.
You can't install the drive gear in any 360° position where it will fit on the shaft and it's timing mark does not line up with the pinion gear timing mark.
The timing mark on the front of the drive gear isn't used to time anything but may simply be there to distinguish that to be the front of the gear.
The 1977-1987 pinion shaft has 6 external splines all being the same length.
The 1977-1987 oil pump drive gear (26318-75) has 6 internal splines that match to the pinion shaft and is a slip fit over the shaft.
The drive gear slides onto the pinion shaft with no direct radial or front to back positioning and has no timing mark.
So you can install the drive gear in any position where it will fit on the shaft (has no front or back either).
1977-1987 Sportster Oil Pump Drive Gear 92) | ||