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The Oil Pump (Ölpumpe)

The oil pump is obviously an essential part of any engine but with the WW2 era Zundapp big twins, with their needle roller bearings, it is more important that the oil pump is working well and the oil is very clean. If either is not the case, you can expect your engine to suffer a catastrophic failure sooner or later that could easily destroy the entire engine block, heads, cylinders and crank.

The pump is a simple two gear compression fluid pump which works by the gears rotating counter-clockwise and compressing liquid. The teeth do not inter-mesh that well so you get some back drain. As the gears work they force the liquid through a hole, along pipes to the various exit points and to a cooling radiator that is a small tank built into the case. Quite a neat design.


The main reason the pump fails is due to wear and thus leakage of oil around the gear wheels which makes the efficiency drop off until it will no longer prime and will then stop supplying oil. You can test this by looking in the inspection hole (where the filter was once fitted on the KS600 motor) whilst turning it. Try blocking the inlet with you finger and feel the pressure (or not). The inspection hole is not there on the KS601 motor.

To test the pump, leave it fitted and turn the gear wheel. See how it feels. When turning counter-clockwise you should eventually feel some resistance - not a lot, but some (see video below). Also and more importantly, pull and push the wheel in and out on its axle, you should feel no play what-so-ever, if you do - rebuild it.

Zundapp oil pump measurement

Any questions? or something to add? comment below.

Oil pump repair

In almost all cases the oil pump can be repaired. You see many of these on ebay for sale where the pump has been replaced, but with a bit of simple engineering you can make a fully functional pump that will last. Basically you need to ensure there is no play and therefore no slippage or leakage of fluids above or below the gears.

To do this you simply need to sand the gears and or casing until they are the correct heights so that the gears rub on the copper plate with a gap of 1 thou, the no fluids will leak under them. When sanding, only sand in a circular motion and use about 800 grit oiled unless you have a lot to take off.

  • Take the pump apart (remove the little spring clip holding the main wheel on)
  • Measure the thickness of the two gear wheel and make them the same with smooth faces.
  • Measure your gasket thickness
  • Insert the wheels loosely into the pump case and measure how they are in relation to the pump gasket face
  • Adjust as per instructions below and then re-measure
  • What you are trying to do is make the gear wheels sit proud of the pump housing gasket by enough that when the gasket is fitted and the copper plate bolted down, the pump wheels do not quite touch the copper, leaving a maximum gap of 1 thou. Now this depends also on how much your gasket compresses when bolted, so trial and error is required a bit, but measuring will get you going.

    Zundapp oil pump sanding gears

    Zundapp oil pump gears

    When refitting, tighten the three bolts and turn the wheel by hand. It helps to full the pump with oil first. It should not be stiff, but have noticeable resistance. Turn it about 20 times and see if it loosens, in not loosen the bolts just a fraction. if it is still tight, sand the two gears about another 20 or 30 strokes. It is a pain this job because you have to keep installing and removing the small wire spring clip.


    Watch the video below to see the operation of the pump and how easily it will now function. Make sure you have plenty of oil in it. Also note that the old filter / inspection cover is not there on the post war KS601 motors. I intent to make use of this old port and modify the brass jet I made and solder on some fine mesh as a removable pre-filter.

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