[Z06] Basic causes of valve guide wear
01-24-2015, 01:44 PM
#1
Safety Car
Thread Starter
Basic causes of valve guide wear
I thought it might be helpful to some, to list the governing causes of guide wear with a brief description of each. The intent being, individuals apply logic to determine what to look for, should they find guide clearance to be excessive. These causes of guide wear have been discussed at length, but sporadically in various threads, to later get lost within pages of disagreement. This thread is to compile the base causes in one location for easy reference. I had not intended to get into the details and/or the merits of differing alloys, coatings, type of component, or weights. While those are all important aspects that can contribute to wear, they are not base causes.
The sources for these causes are from the following books: The Internal Combustion Engine in Theory and Practice V1/V2, second edition, Charles Fayette Taylor; Internal Combustion Engine Fundamentals, John B. Heywood; and Introduction to Engine Valvetrains, Yasu ****. These are text books used in engineering classes, including M.I.T. I have spent many hours reading through the respective valvetrain component related sections, as well as failure analysis sections. I wanted to compile a list that was not based on opinion or claim, but supported by internal combustion engine theory and design, from respected SME’s that could be referenced for failure analysis by anyone with the tools and measuring equipment necessary to take pertinent measurements.
Keep in mind that many things can contribute to, or lead to a cause, but that does not make them the base cause. Also remember the rule of causality; an event (cause) has a relation to another event (effect).
For example: Valve guide I.D. finish. Rough guide I.D. finish will cause valve stem scuffing, which will then cause guide wear, so that returns to number 7 as a cause for guide wear....
I was able to identify 7 basic causes of valve guide wear and/or scuffing in the aforementioned books. Again, there are other things that can affect guide wear, but they ultimately lead to these seven base causes:
1) Insufficient lubrication
2) Debris or contaminant between the valve stem and guide
3) Misalignment of the valve seat insert to the guide and/or guide insert
4) Excessive concentricity of the guide ID to valve seat ID
5) Excessive valve or valve seat run out
6) Excessive side loading to the valve stem tip
7) Improper or poor valve stem finish (roughness)
I will attempt to expend on each in layman’s terms. Recommended tolerances noted, are from the text books. They are for reference only.
1) Insufficient lubrication will result in direct contact between wear parts, and in this case, between the valve guide and the valve stem. To prevent this, the engine builder needs to ensure that sufficient lubrication is allowed to enter the guide, but not too much. This is a delicate balance. The methods to do this vary. Recommended metering rate is between 0.01 – 0.1 cc/10 hours. Over the years, tighter emissions controls have mandated a reduction in valve lubrication, with an emphasis on the exhaust valves, as oil is directly carried away with the exhaust gas stream. Oil entrainment into the intake stream is burnt during the combustion process. Not a good thing, but nonetheless, it has a lesser impact on exhaust emissions than the exhaust valve.
2) Contamination should be self-explanatory. If debris becomes entrained with the oil, the oil film is interrupted creating localized rub points.
3) Misalignment of the valve seat insert to the guide insert is when the guide insert bore (I.D.) is not perpendicular to the valve seat insert at a true 90 deg angle. This will put the valve stem into a binding situation as the valve goes full closed onto the seat. This binding action will interrupt the oil film and the stem will rub on the guide.
4) Guide to seat concentricity is most easily described by visualizing two perfect circles. One circle is considerably smaller than the other. The smaller circle would represent the valve guide, whereas the larger represents the valve seat. When the smaller circle (guide) is positioned within the larger circle (seat) and the distance between the smaller circle’s outer diameter and the larger circle’s inner diameter - are exact at all points, they are concentric to each other. If you move either circle in any direction, while the other circle’s position remains fixed, they are no longer concentric. Recommended maximum allowable tolerance is .002” per inch of valve O.D. (Ideally, .001” or less is preferred, but that is difficult to achieve as diameter exceeds 2”.) If concentricity is in excess of the recommended value, valve stem binding will occur. This binding action will interrupt the oil film and the stem will rub on the guide.
5) There are two different, but related types of run out. One is in relation to the valve head to the valve stem. The other is “roundness” of either the valve seat or valve head. A “run out” measurement is used for both, which is generally expressed as TIR (total indicated reading). Recommended Max runout is +/- .03mm. When the valve head is not concentric to the valve stem, binding action will occur in the guide as the valve goes closed. When the seat or valve head is not “round”, or is elliptical in shape, non-uniform seat contact occurs, resulting in valve stem binding. These binding actions will interrupt the oil film and the stem will rub on the guide.
6) Valve tip side loading is probably the most widely known cause, and it is easy to visualize. It is easy to visualize what will happen if you were to impart a force to the tip of the valve stem as the valve reciprocates in the guide. The stem then, once again, interrupts the oil film and rubs the guide. What an engine builder needs to do to prevent side loading, is to first check rocker to valve tip contact, through the entire valve reciprocating cycle, and make adjustments to the rocker pivot height, or valve stem height (or combination of both) to correct this. The methods of correction vary with each engine, and unfortunately, GM LS7 heads do not have provisions for this.
7) Stem finish is self-explanatory. The valve stem should be free of rough spots and micro-polished. If it is not, it will act as a file within the guide. Roughness of the stem should be between (Ra) .1 um and .45 um – (Ra = roughness / um = micrometer).
I’ve been in mechanical and maintenance related trades for roughly thirty years. I have often found that the broke widget gets blamed for breaking, without any regimented analysis of that failure. I have learned to not jump to conclusions or make assumptions. When troubleshooting a mechanical failure, I tend to go back to the design basics, theory of operation, and work my way out from there. There have been instances where the component failed due to being defective in some way, but that has been the exception, and the norm.
I have also learned to not fix something that doesn’t need fixing. If it isn’t broke, don’t fix it. If there is a need or desire to improve it, you better have a complete understanding of what it does, how it does it, and why it was designed the way it was, FIRST, and then make informed changes.
If further explanation is needed, just let me know, but please be specific.
If you feel I have omitted a base cause, let me know, but please site your reference and use logic and / or theory to support your position.
NO BICKERING PLEASE. I will be leaning on the “report post” button if that occurs.
The sources for these causes are from the following books: The Internal Combustion Engine in Theory and Practice V1/V2, second edition, Charles Fayette Taylor; Internal Combustion Engine Fundamentals, John B. Heywood; and Introduction to Engine Valvetrains, Yasu ****. These are text books used in engineering classes, including M.I.T. I have spent many hours reading through the respective valvetrain component related sections, as well as failure analysis sections. I wanted to compile a list that was not based on opinion or claim, but supported by internal combustion engine theory and design, from respected SME’s that could be referenced for failure analysis by anyone with the tools and measuring equipment necessary to take pertinent measurements.
Keep in mind that many things can contribute to, or lead to a cause, but that does not make them the base cause. Also remember the rule of causality; an event (cause) has a relation to another event (effect).
For example: Valve guide I.D. finish. Rough guide I.D. finish will cause valve stem scuffing, which will then cause guide wear, so that returns to number 7 as a cause for guide wear....
I was able to identify 7 basic causes of valve guide wear and/or scuffing in the aforementioned books. Again, there are other things that can affect guide wear, but they ultimately lead to these seven base causes:
1) Insufficient lubrication
2) Debris or contaminant between the valve stem and guide
3) Misalignment of the valve seat insert to the guide and/or guide insert
4) Excessive concentricity of the guide ID to valve seat ID
5) Excessive valve or valve seat run out
6) Excessive side loading to the valve stem tip
7) Improper or poor valve stem finish (roughness)
I will attempt to expend on each in layman’s terms. Recommended tolerances noted, are from the text books. They are for reference only.
1) Insufficient lubrication will result in direct contact between wear parts, and in this case, between the valve guide and the valve stem. To prevent this, the engine builder needs to ensure that sufficient lubrication is allowed to enter the guide, but not too much. This is a delicate balance. The methods to do this vary. Recommended metering rate is between 0.01 – 0.1 cc/10 hours. Over the years, tighter emissions controls have mandated a reduction in valve lubrication, with an emphasis on the exhaust valves, as oil is directly carried away with the exhaust gas stream. Oil entrainment into the intake stream is burnt during the combustion process. Not a good thing, but nonetheless, it has a lesser impact on exhaust emissions than the exhaust valve.
2) Contamination should be self-explanatory. If debris becomes entrained with the oil, the oil film is interrupted creating localized rub points.
3) Misalignment of the valve seat insert to the guide insert is when the guide insert bore (I.D.) is not perpendicular to the valve seat insert at a true 90 deg angle. This will put the valve stem into a binding situation as the valve goes full closed onto the seat. This binding action will interrupt the oil film and the stem will rub on the guide.
4) Guide to seat concentricity is most easily described by visualizing two perfect circles. One circle is considerably smaller than the other. The smaller circle would represent the valve guide, whereas the larger represents the valve seat. When the smaller circle (guide) is positioned within the larger circle (seat) and the distance between the smaller circle’s outer diameter and the larger circle’s inner diameter - are exact at all points, they are concentric to each other. If you move either circle in any direction, while the other circle’s position remains fixed, they are no longer concentric. Recommended maximum allowable tolerance is .002” per inch of valve O.D. (Ideally, .001” or less is preferred, but that is difficult to achieve as diameter exceeds 2”.) If concentricity is in excess of the recommended value, valve stem binding will occur. This binding action will interrupt the oil film and the stem will rub on the guide.
5) There are two different, but related types of run out. One is in relation to the valve head to the valve stem. The other is “roundness” of either the valve seat or valve head. A “run out” measurement is used for both, which is generally expressed as TIR (total indicated reading). Recommended Max runout is +/- .03mm. When the valve head is not concentric to the valve stem, binding action will occur in the guide as the valve goes closed. When the seat or valve head is not “round”, or is elliptical in shape, non-uniform seat contact occurs, resulting in valve stem binding. These binding actions will interrupt the oil film and the stem will rub on the guide.
6) Valve tip side loading is probably the most widely known cause, and it is easy to visualize. It is easy to visualize what will happen if you were to impart a force to the tip of the valve stem as the valve reciprocates in the guide. The stem then, once again, interrupts the oil film and rubs the guide. What an engine builder needs to do to prevent side loading, is to first check rocker to valve tip contact, through the entire valve reciprocating cycle, and make adjustments to the rocker pivot height, or valve stem height (or combination of both) to correct this. The methods of correction vary with each engine, and unfortunately, GM LS7 heads do not have provisions for this.
7) Stem finish is self-explanatory. The valve stem should be free of rough spots and micro-polished. If it is not, it will act as a file within the guide. Roughness of the stem should be between (Ra) .1 um and .45 um – (Ra = roughness / um = micrometer).
I’ve been in mechanical and maintenance related trades for roughly thirty years. I have often found that the broke widget gets blamed for breaking, without any regimented analysis of that failure. I have learned to not jump to conclusions or make assumptions. When troubleshooting a mechanical failure, I tend to go back to the design basics, theory of operation, and work my way out from there. There have been instances where the component failed due to being defective in some way, but that has been the exception, and the norm.
I have also learned to not fix something that doesn’t need fixing. If it isn’t broke, don’t fix it. If there is a need or desire to improve it, you better have a complete understanding of what it does, how it does it, and why it was designed the way it was, FIRST, and then make informed changes.
If further explanation is needed, just let me know, but please be specific.
If you feel I have omitted a base cause, let me know, but please site your reference and use logic and / or theory to support your position.
NO BICKERING PLEASE. I will be leaning on the “report post” button if that occurs.
The following 3 users liked this post by Michael_D:
01-24-2015, 02:00 PM
#3
Drifting
Good read, thank you for posting this, and I do not have anything to add, subscribed 
01-24-2015, 02:59 PM
#4
Team Owner
Good post
01-24-2015, 05:44 PM
#7
Drifting
Thanks for taking the time to put these causes into one post. Most of us don't have the patience to read through 200 pages of arguing to pick these out.
01-24-2015, 06:24 PM
#8
Burning Brakes
01-24-2015, 06:32 PM
#9
Drifting
01-24-2015, 07:05 PM
#10
Which one is responsible for LS7 exhaust guide wear? In a normal engine the valve seat is cut using the valve guide as the pilot/center. With this method concentricity with valve guide is just about guaranteed. How are the LS7 seats cut?
Last edited by juanvaldez; 01-24-2015 at 07:08 PM.
01-24-2015, 07:47 PM
#13
Safety Car
Thread Starter
It is not just the exhaust that is showing wear on LS7 cylinder heads.
Post cast machine work was done using single point 5 axis CNC. No positive registry with guide centerline using this type of machine. This does not however mean they cannot be cut to extremely exacting tolerances using CNC. But it would appear that something happened, exactly what that was, is anyone's guess.
01-25-2015, 09:56 AM
#18
Burning Brakes
If you are feeling suitably ambitious, I think pictures to illustrate things like run out would help convey the information in your post.
Additionally, I know one thing that always interested me was the shape of guide wear. We see plenty of guide wear measurements that aren't uniform along the length of the guide. Sometimes wear is excessive just at the bottom, sometimes top and bottom but not center, etc.
Great information, and an interesting read.
Additionally, I know one thing that always interested me was the shape of guide wear. We see plenty of guide wear measurements that aren't uniform along the length of the guide. Sometimes wear is excessive just at the bottom, sometimes top and bottom but not center, etc.
Great information, and an interesting read.
