Wednesday, July 04, 2012

Valve Leakage - serial offender

"I pulled the cylinder and tried to lap the valve. The valve only showed contact to the seat at 12 and 6 a´clock positions in a range of maybe 10 degrees of the circumference. Valve guide was perfect.So I shipped it to the engine shop and they performed a valve job. New guide & reaming, grinding valve and seat and lapping.Cylinder back on the engine, everything was fine, performed as his brothers.Now it is 75 Hrs since that valve job and guess what ?  Number 3 is slightly starting hissing on the intake valve again !"

 By repairing the damage but not correcting the problem, your #3 cylinder is behaving just as expected.  What caused your #3 intake valve to start leaking after 75 hours? Until you can answer this question you cannot solve the problem.

1. Fix the damage
2. Identify the problem
3. Correct the problem

You only did step 1. now you have to start the process over. 

First thing to check is the easy stuff; dry tappet clearance and making sure the correct parts were used and the basic repair looks satisfactory. Next, and most important is to check alignment: Rocker arm boss, rocker arm, intake valve boss, intake valve ID, valve seat boss, valve seat face. The most common cause of serial valve problem is poor valve train alignment.. Use the two checks below to check for proper valve train alignment:

Rocker Arm Check
Rocker arm faces are abused and often ground misaligned. You purchased new cylinders but where the old rocker arms ever inspected for alignment and the proper face curvature (cycloid curve)? Probably not as most shops just "rebush and reface" without ever checking alignment and curvature.

Inline image 1
Rocker Arm

The rocker arm face must sit flat (parallel) onto the valve tip. If the face of the rocker arm is parallel to the valve tip then you have established that the rocker boss bushings, rocker arm bushings, rocker face, valve guide boss and valve guide ID are in proper alignment.. This is one of my favorite checks because it is easy to do, quick, and takes no special tools. If the face is tilted as it contacts the valve stem it will wear out the valve guide very quickly (< 50 hours).  Position yourself parallel to the valve tip with a good light and look at how the rocker face contacts the valve tip. It should be resting flat --its ok that it is not centered onto the face -- just make sure the surfaces are parallel.  I have written a more detailed article called  Don't Forget to Inspect the Rocker Arm

Valve Seat Check

This check requires that the valve be removed from the cylinder but once that is done it's also a visual only check that doesn't require any special tools. 
If the valve seat alignment is off then the valve cannot center itself onto the seat and this side-loads the valve guide causing rapid wear. Once the seat alignment is off then most cylinder/engine shops do not have the jigs and tools to move the hole back into the proper position (concentric to the guide axis). Seat alignment is checked by visually inspecting the top of the valve seat. The width of the flat (unground) area should be the same throughout the entire circumference.

Inline image 2
Axis of seat and guide not concentric (seat and guide should have the same axis)

Notice in the picture above that the flat area (black lines) has unequal width. It takes some special tools to move these holes so that they have the same axis (concentricity).

I have written a more detailed article called Bad Valve Seat Job

I've done all that but it still goes bad after 75 hours

This sometimes happens, however, you have verified that the problem is not due to faulty cylinder manufacturer or repair and you can move on to more esoteric (expensive) causes. From a cylinder shop perspective I would replace the cylinder for two reasons:
1. Blaming the pilot or airplane for poor cylinder performance is a hard sell. Replacing the cylinder removes my workmanship from the aircraft. I have capped my future expense.
2. By replacing the cylinder with one that I haven't worked on prvides two outcomes; either the problem goes away in which case we have solved the problem without further expense or fuss, or it doesn't solve the problem but provides convincing evidence that the problem isn't due to the cylinder and is no longer my (read cylinder shop's) responsibility.

Other Observations

  • Compression leakage "60 over 80" is not the standard used for Continental engines with valve leakage. You can verify this by reading Continental's service instructions on how to perform a leakage "compression" check. Once a valve is leaking gas it only get worse with time -- never better.
  • "Staking" and "lapping" a valve to fix a leak is rarely successful.

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