Automotive Engine Valve Failure Analysis

VALVE METALLURGY AND MANUFACTURING

VALVE FAILURE ANALYSIS: READING VALVE FAILURE MARKS

by TED TUNNECLIFF
Chief Engineer Aftermarket (Ret.)

Brief Introduction by Henry D. Manley III

Ted Tunnecliff is the most knowledgeable man I have ever met in the field of engine valve metallurgy and manufacturing. Much of what I know about engine valves Ted generously taught me.

About seven years ago Ted was asked by Eaton Corporation to write a series of articles about valves. The most interesting of these fifty some articles to me was the group dedicated to failure analysis. How better to help Manley customers save their engines from catastrophic failures than to understand what can cause a valve to fail and then pass along that information.

I am delighted and proud to reprint, with Ted's and Eaton's permission, these articles in the hope that through increased knowledge racers and enthusiasts can long enjoy their modified engines trouble-free.

Reprinted from Eaton Corporation's The Valve Event
Eaton Valve Engineering Notes and Topics
May 1995 - Issue 10 - Volume 3

VALVE FAILURE ANALYSIS:
READING VALVE FAILURE MARKS

BACKGROUND

This will be the final article in the series on valve failure analyses. In this issue we're going to discuss the various types of tattle-tale marks that can be seen on a valve and that can provide clues as to what may have caused a failure.

Not only do the failed surfaces themselves give us information on possible causes, but every place the valves touch another part during operation has the potential for showing us a cause of failure. That means the valve tip, keeper groove area, stem and seat face may have something to show us if we look carefully. One of the most important things that should be done in any valve failure analysis is to look in detail at every area of the valve before drawing conclusions.

TIP CONTACT MARKS

Most valves rotate at some time during their operation. Some rotate almost continuously and some sporadically. Many may not rotate at all or they may oscillate rather than rotate. We won't get into all the reasons they do or don't, but we do want to know if they did. That fact could be important in an analysis of the failure.

If you look closely at the valve tip you can usually see a contact pattern produced by the rocker pad as it moves back and forth across the tip during operation. A valve that has rotated well during operation will display a multiple rocker pattern and or concentric rings. A contact pattern on a valve that did not rotate but did oscillate is sometimes called a "bow tie" because of its resemblance to a bow tie. The type of pattern that is produced on the valve tip if the rocker pad and tip are not square with each other or if the pad and tip are not properly aligned is off to one side. Referred to as "side loading".

The implications are that poor rotation can cause seat leakage and guttering or excessive rotation can add to a seat a face wear problem. Misalignment can aggravate guide wear and possibly induce valve head fractures.

KEY CONTACT MARKS

Most valve keys are made of a strip of steel which has been formed to a cross section that will fit into the valve keeper groove. This strip is then rolled up and cut off to form a single key. The rolling-up process does not usually produce a perfect arc. If examined very carefully that arc looks more like a series of short, straight lines. Because of this the key, when installed on the valve, will touch at only certain high points - typically at only two. As long as these marks show no indication of circumferential motion, this should be considered a normal condition. However, if the contact marks have lateral lines through them, that would indicate that the keys were moving around the valve stem. Remember that the valve, keys and the spring retainer should be moving together as a single unit so that there should be no relative motion between these components as they move up and down with the valve. If there is circumferential movement of the keys, it means that the valve gear has been separating. That, in turn, means that the valve has not been following the cam contour and that its velocity as it seats could be much higher than designed. High seating velocity means high stress and the possibility of fractures of the valve head, head/stem blend or keeper groove areas. All that from just looking at the key contact marks. Neat, huh!

STEM CONTACT MARKS

As a valve moves up and down in the guide it will lean a little ( the stem to guide clearance ) one way or the other. As it leans, this causes a slightly heavier load on one side of the guide at the tip and on the opposite side at the bottom. If the valve is rotating as well as moving up and down, that slight sideways load will produce a burnishing of the stem all the way around it at both ends of the guide travel ( the valve lift ). The burnishing should be considered a normal condition as long as there is no indication of guide material pick up. If a lot of guide wear has taken place, that wear is usually at the hot end of the guide but not always. It depends to a large degree on what caused the wear. For example, if it was caused by distortion of the guide itself, it may wear primarily in one plane. That is, at one side at the guide bottom and the opposite side at the top. You could expect to see that type of pattern on the stem of a valve if it had not been rotating. Adhesive wear called "guide metal pick up" or "galling" occurs if there is not adequate lubrication, too tight clearance between the stem and the guide or a poor quality cast iron used in the guide.

SEAT FACE CONTACT MARKS

When a valve is not seating tightly due to a build up of combustion deposits guttering ( or burning or leakage ) takes place. Such deposits can chip away leaving a channel for exhaust gas leakage followed by the formation of corrosive gutters.

Indentations may be discovered after a valve is carefully cleaned of deposits filling pits making them look like corrosion pits. This is indentive wear which can be caused by combustion deposits embedded into the valve face while on the seat. This is a perfect example of why high temperature hardness is an important characteristic in valve alloys. The higher the valve material hardness, the more it will resist such indentations.