Tight Piston Repair

Low product flow with high feed pressure is commonly caused by clogged up membranes and/or prefilters.  However, some 700 and 1000 model watermakers may exhibit these symptoms due to “tight pistons”. The pistons and/or the piston bushings, can swell slightly especially in very warm waters which will cause the piston assembly to expand inside the cylinder, slowing the Clark Pump and causing the feed pressures to rise and production to fall. In most cases the system will also be erratic, operating normally for a time and then shutting down on “System Stalled” or “High Pressure” and/or showing varying system pressures.   

Before attempting a repair, please refer to Tight Piston Check-Up for some brief tests to run to confirm the issue. 


If you have identified that you have tight pistons, the below method is useful for fixing out in the field. 


You will need to remove the cylinders to access the pistons and gently shave down each bushing about 10 times on each edge. The below video is helpful for easily removing cylinders. Once you have the cylinder off there are two ways you can try to get the piston assemblies out. Clark Pump Valve Port Video (for assistance accessing pistons)


  1. Cover up one of the two small holes on the bottom of the cylinder with a finger and blow compressed air or pressurized water through the other hole. This will push the piston out of the bore (watch out or have the heel of your hand here to catch the piston so it does not shoot into the ground)
    1. This technique works much better when the cylinder assembly is cold (as close to freezing as possible without freezing the water in the cylinder)
  2. Take the cylinder apart and push the piston out of the bottom of the cylinder. There is a chapter in the user manual about high-pressure fittings that will be helpful for disassembling the compression lock fittings.


Now you can shave down the bushings. We recommend that you shave down your bushings rather than replace them as it is a very easy procedure and will lower the chances that the problem happens again.



Here is a detailed video of the process:

Tight Piston Repair Video


Below is a photo of the pistons in the Clark Pump with relevant part numbers included:


The outer bushings are used as a water-bearing surface. These are what we have seen expand in warm waters, you can shave them down since they are non-sealing. Once you get the piston out of the cylinder remove them and put them back in the Clark Pump.

If the pistons/bushings have expanded, you will notice that the pistons now move within the Clark Pump much easier after removing some material from the bushing. You should be able to slide it deeper into the cylinder with your forefinger alone. When you shave down the piston bushings, you really can’t shave too much as it is relatively thin to begin with. When you put the shaved bushings on the piston back in the cylinder it should take an even amount of force to slide the piston assembly deeper into the cylinder.