Sunday, December 26, 2010

Don't Forget to Inspect the Rocker Arm

Pitted and misaligned Continental Rocker Arm Face. Notice how contact is at edge - misalignment  results in rapid valve guide wear

When you purchase replacement cylinders the rocker arms are not included. Often they are placed aside until the new cylinders arrive and then installed. The rocker arm pictured above may wear out the valve guide very quickly. We have seen instances of 50-100 hours until the guide is worn sufficiently to cause valve leakage! If you then send out the cylinder for repair and then place the rocker arm back onto the cylinder you will repeat the problem. Here is what the rocker arm is telling you:

Notice the pitting on the top edge of the rocker face in this picture? The rocker arm face is tilted in reference to the top of the valve stem. The face should lie flat if everything is in alignment. By "everything" I mean the following:
  • Rocker shaft boss
  • Rocker arm bushing
  • Rocker face
  • Valve guide boss
  • Valve guide
  • Valve seat boss
  • Valve seat face
Quite a list isn't it. If any one of these are mis-aligned then the face doesn't sit flat onto the top of the valve stem. Here's the quick tip -- After installing the rocker arm carefully look at how the rocker face is sitting on the valve tip. If it is resting flat then you know that all the items on the list above are in alignment. This is a quick, easy, inexpensive, and informative inspection -- the kind I like!

Rocker arm should be flat against valve tip.

What if it isn't flat? Then you have a problem;  The valve guides will wear prematurely and the problem can be any of the above. The first thing to check is to see if the rocker face has been re-ground, "refaced". Refacing is an awful thing to do as it is often done by hand, and it ruins the geometry of the face. Even if alignment is retained the cycloid curve is often flattened and this causes the pressure on the valve tip to move off-center causing the rocker arm to push the valve stem into the guide.

Normal rocker arm face. Notice wide horizontal contact
The rocker face above shows normal contact. Rocker arm is resting flat on valve tip by evidence of the wide contact pattern. Vertical contact pattern shows that the slightly rounded face (cycloid curve) is approximately correct. 

You can swap a suspect rocker arm with one that is resting flat from another cylinder to check if the problem is the rocker arm or the cylinder. If the rocker arm is bad, replace it.

Another tip is to inspect the rocker arm socket for wear. This must form an oil seal with the push rod ball. It is  good practice to make sure each push rod goes back into the same rocker arm. The wear surfaces know one another and are compatible. If you mix them up then the surfaces are strangers and must "wear in" and form compatible surfaces.
Inspect rocker arm socket and push rod ball

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