The Big Twin Oil Pump 1936 to 1999; both Harley-Davidson and S&S

ALTHOUGH VASTLY different to look at than the early Harley cast-iron oil pumps, the S&S high performance oil pump—to suit 1936 through to 1999 big twin engines—is fundamentally the same pump fitted to all single-cam, big twin Harleys through to 1999. This clever design combines both supply (feed) and return (scavenge) gear-sets on a common shaft driven via a worm gear on the crankshaft.

Harley has continually improved the oil pump over the years to keep pace with the development of their engines. The basic layout for the pump originated on the first OHV engine in 1936, the Knucklehead. Four versions of this oil pump were used to take us through to the end of 1967. The oil pump shaft diameter remained unchanged. The gears have also kept their original diameter and number of teeth. This basic design has remained with the pump all the way through Harley production as well as S&S production.

The length of the shaft was increased to accommodate the extra width gears introduced in 1968 in conjunction with the aluminium body.

The earlier pumps used a centrifugal bypass valve to deal with excess oil at low engine speeds, and as rpm’s increased, extra oil became available to supply demand. This system was changed in late 1950 and the centrifugal bypass valve was eliminated and replaced with a regulating valve.

The first oil pumps manufactured by S&S used a die-cast aluminium body and cover and were similar to the 1968 and later Harley pumps. These pumps could be installed on any 1936 and up engine as long as some minor modifications were made to some right-hand crankcases.

By the time S&S became involved in oil pumps in the 1980’s, Harley had already increased the size of both the feed and return gears increasing the volume of the pump by 33 percent. This coincided with the introduction of the aluminium body in 1968.

In 1973 Harley further increased both the pressure and volume of the pump by 25 percent with a change of drive gears stepping up the speed of the pump to 4:1 compared to the earlier 5:1 reduction. The 1981 Harley-Davidson pump is what S&S based their first models on.

Harley enlarged the stud pattern for their oil pumps in 1992 and these pumps continued in production through 1999.

Optional drive gears to run S&S oil pumps on pre-1973 engines at max. speed. Notice that S&S offer a splined version of the pinion shaft drive gear to suit the ’36—’54 engines.

Due to quality control issues with the castings, S&S started to use billet aluminium for their next generation of oil pumps. Internal dimensions of the pump and the performance were unchanged but with the .300” shorter pressure relief tower and the fully machined and polished finish, the pump took on a high quality custom look. This pump also is available in the 1992 and up version with the larger stud pattern.

Final versions of the S&S oil pumps were machined with this relief channel between gear pockets to reduce oil drag between drive and driven gears.

One of the many features of the S&S billet pumps, particularly with the cover, is the versatility of routing the oil hoses. They can be run to the top of the cover as they would be on a stock engine or they can be run to the bottom of the cover for an uncluttered, custom look.

To complete their family of oil pumps, S&S brought out their HVHP (High Volume, High Pressure) pumps for their large capacity and higher performance engines. These pumps were originally designed for the 4 1/8” bore engines fitted with the piston cooling jets. A new gear profile with fewer but larger teeth provides increased oil volume for both supply and return sides of the pump.

Comparison of the larger HVHP teeth to standard teeth of the oil pump gears.

Redgrave Motorcycles are Australian Distributors of S&S Products and stock the entire range of S&S pumps as well as an extensive line-up of spare parts to suit. To compliment their range of oil pumps, Redgraves also stock a very extensive range of Colony items to not only help with the installation of pumps but to replace difficult to find missing parts, repair sections for cast iron pumps, to restore any model H-D pump to concours standard, and to dress up show bikes.

Hex head plugs to replace slotted stock fittings for check valve and relief valve plugs can be removed with a ring spanner.

Chromed mounting hardware for 1992 and up Big Twins (all models available).

Various authentic plugs in correct finish for concours standard restorations.

Stock application, oil pump mounting kit to suit all 1936 to 1967 Big Twins.

For any enquiries regarding oil pumps, give Redgrave Motorcycles a call; they would be only too happy to discuss all the parts and options available. Redgrave Motorcycles: 02-9484-9900.