In many small engines, splash lubrication is used unlike in cars which always have an oil pump.

I understand that when the small engine is a vertical crankshaft one like in lawnmowers, where the cylinder is horizontal, the valve mechanism is actually at the same height as the oil pan is. So it's easy to have enough oil provided by gravity at the valve mechanism.

However, many small engines actually have a horizontal crankshaft like the engines in cars. In car engines, the valve mechanism is lubricated by oil pump feeding oil to the head via oil passageways. However, this mechanism is not possible in engines that don't have an oil pump.

So, how do small engines with a horizontal crankshaft manage to lubricate the valve mechanism?

2 Answers 2


I think I have found the answer.

The patent US4563986A describes a new way of lubricating valves of splash lubricated OHV engines.

Every patent application must describe the state of the art while at the same time describing the new invention, and the state of art is described as:

In the splash lubricated over head valve engine of the known type, lubricant is fed by means of reciprocation of push rods from the crank room to the rocker arm chamber. In this manner, the strength of feeding pressure is determined by the stroke of the push rods. Therefore, it is impossible to obtain sufficiently high feeding pressure. Moreover, the limitation in the location of the opening for spout in the rocker arm chamber results in the difficulty of the sure feeding of the lubricant to the moving parts of the valve drive mechanism.

In order to avoid these problems, a breather, which serves to dispose of blow-by gas leaked from the combustion chamber to the crankroom, is disposed in the space specially provided on the rocker arm chamber so as to induce blow-by gas into the rocker arm chamber for the separation of the oil therefrom. However, in this known manner, the breather is extruded over the head cover, so that the overall height of the engine is tall disadvantageously.

So apparently the oil travels with the push rods and blowby gases, and there is some kind of mechanism to create a pressure although a true oil pump is not present.


I've seen two types:

  1. "Underhead" valves (or L configuration) where the valves get lubricated by the same splash that lubes the piston.
  2. OHV (overhead) valves which always have an oil pump to get the oil to the valve.
  • Interesting... so do splash lubricated OHV engines actually have an oil pump? I wasn't aware of that. I have splash lubricated horizontal crankshaft OHV engine in my generator.
    – juhist
    Aug 24, 2022 at 17:55
  • I have not seen one but I suppose it's possible. The splasher might be designed such that it throws oil up into the cylinder head and lubricates the valve.
    – jwh20
    Aug 24, 2022 at 17:56

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