These photos are from a pump that had a loose valve deck bolt make its way into the rotating area of the pump.

One of the most critical areas in a Stokes® Vacuum pump when it comes to preventative maintenance are the clapper or poppet valves.  These valve assemblies are located in the oil reservoir under the aluminum oil baffle.  In the 412 pump there are four valve assemblies and each assembly contains two valves.  Each assembly is held to the block with six bolts.  During routine preventative maintenance these bolts are removed in order to replace the springs and clappers/poppets.  Below are some tips to ensure that when the rebuilt valve assemblies are installed the bolts are properly tightened.

  1. Inspect the bolt.
    • Look for signs of thread stretch.  This is common in older pumps, the bolts that were used when these pumps were built are not of good quality.  It is common to have the head break off  during reinstallation.
    • Make sure the head of the bolt is in good shape.  Having a rounded off head or socket head cap screw stuck in the pump is a real inconvenience.
    • If in doubt replace it.
  2. Clean out the tapped holes.
    • In our maintenance kits we provide a disposable pipette that works great for sucking the oil out of the bottom of the tapped holes.  Some times it is even helpful rinse the hole out with a small amount of solvent.
    • Believe it or not, oil left in these threaded holes can give you the impression a bolt is tight when it is not.  When tightening the bolt hydraulic pressure can build and stop the bolt.  After some time the pressure will release and the bolt will not be tight as you thought.
  3. Triple check each bolt.
    • When tightening the valve assembly hold down bolts it is good practice to go over each bolt three times.  The first time around we tighten each bolt with a 3/8″ air ratchet, using a criss-cross pattern of course. Next give each bolt a twist by hand with a standard 3/8″ socket handle and appropriate socket.  Finally start at the beginning and check each one by hand again.