• LS7 Valve Guide Wear Issue
  • LS7 Valve Guide Wear Issue
  • LS7 Valve Guide Wear Issue
  • LS7 Valve Guide Wear Issue

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  1. #1
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    Default LS7 Valve Guide Wear Issue

    Last week I was in Michigan for a day of meetings with various engineers at the General Motors Powertrain Division. Two of the meetings were about GM's two new premium V6 engines, the LGW and the LGX. The third concerned the LS7 valve guide wear problem.

    Present for the LS7 meeting on the afternoon of 24 March at GM Powertrain Headquarters in Pontiac, were: Jordan Lee, Chief Engineer and Program Manger for the Small-Block V8, John Rydzewski, Assistant Chief Engineer for Small-Block V8 Passenger Car Engines, Chris Cogan, Cylinder Head Design Release Engineer for the LT1, LT4 and LS7 and Yoon Lee, LS7 Design System Engineer. Also present was Tom Read, Director of Communications for GM Powertrain. In attendance for part of this meeting were representatives from GMPT's Inspection Department, from Zeiss U.S. and the GM entity which complies service information.


    This meeting included a visit to the Inspection Department at GM Powertrain HQ where I observed a Coordinate Measuring Machine (CMM) session with the passenger side cylinder head which was removed from my engine in July of 2014 during a warranty replacement. A CMM captures measurements used to create an extremely precise, three-dimensional, digital model of an object, such as an LS7 head. The GMPT Inspection Department uses Zeiss "Prismo Navigator" CMMs which are accurate to two microns over a distance of 300-mm (.00008-in over one foot).


    What I learned from LS7 Team at Powertrain will be incorporated into revisions to my series of LS7 articles which are posted here on the Corvette Action Center. These revisions will take some time to produce. There is no posting date for them at this time.


    There are some open issues remaining after this meeting and they will be explored via a follow-up exchange of emails I will be having with Tom Read and the LS7 Team in the next week or so.


    At this point, I can reveal some news items which came out of that meeting. I'll cover them briefly, here, and will expand upon them, later, in my revisions to the LS7 article series.


    1) "Wiggle Testing" at best is inaccurate and in many cases is completely unreliable. Observing one of my heads being measured by one of GM's Zeiss CMMs proved to me conclusively that even the complicated and careful procedure I covered in my Wiggle Test article produces data which is inaccurate and inconsistent such that, unless the clearance measured is significantly greater than the Service Limit of .0037-in., the measurements are useless for determining if a head needs repair or replacement due to valve guide wear.


    2) It is possible that heads which had Wiggle Test results of more than .0024 (intake) or .0026 (exhaust) but less than .007-in. stem-to-guide clearances had actual clearances below GM's .0037-in Service Limit, regardless of how the Wiggle Test was done. That possibility becomes greater as Wiggle Test measurements get closer to .0037-in. Once they approach .005-in., guides are likely in spec even though they Wiggle Test as bad.


    3) Some, but not all, heads which failed "Wiggle Tests" and were repaired or replaced, either under warranty or not, actually did not have faulty valve guides and did not need replacement.


    4) Wiggle Testing is "out" at GM. In early March, GM released to its dealers an update to ESI mandating a new procedure for measuring stem-to-guide clearance for warranty purposes in all high-performance engines. It requires a hole gauge to measure guides and a micrometer to measure valve stems or a valve guide bore gauge, such as a Sunnun P310, and must be done with the heads removed and disassembled.


    5) The demise of Wiggle Testing as a way to determine if guides are worn was a result of the LS7 Engineering Team's review of the CAC's LS7 article series, three CMM inspections of the heads removed from my engine in July of 2014 along with the Team's need for more accurate information from the field about warranty replacements of LS7 heads. Additionally, the LS7 Team's review of selected content on the CAC and other forums may have influenced the decision.


    6) According to Jordan Lee, the "machining error" stated on the CF by Chevrolet Customer Service in October of 2012, was a failure of the supplier to properly deploy statistical process controls and, as a result, the diameters of valve guides in some, but not all, heads made during that period were machined too large.


    7) The "suspect period" for this machining error, previously stated by Chevrolet Customer Service to be 2008 to Feb 2011, is not correct. According to Chris Cogan, and confirmed by Jordan Lee, the suspect period was July, 2008 to March, 2009.


    8) Only LS7 heads are manufactured by Linamar. LS9 cylinder heads were never manufactured by Linamar. LS9 heads were made in GM's engine plant in Silao, Mexico. I am partially responsible for that long-standing piece of misinformation. I apologize for any confusion it has caused.


    9) The LS7 is currently manufactured at the Performance Build Center in Bowling Green and will remain so until the 5th Gen Camaro Z28 goes out of production.


    I may post additional information concerning my 24 March visit to GM Powertrain as conditions warrant.


    Thanks to the LS7 Engineering Team along with Tom Read, GMPT Director of Communications, for the time and resources they devoted to my visit with them in Michigan last week. I'd also like to thank the LS7 Team for their willingness to show me all the information they had available at the time of the meeting and their willingness to consider sharing additional information going forward. Finally, I appreciate the LS7 Team's interest in working with me to get as much information on LS7 valve guides as possible into the public domain.


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  2. #2
    Technical Advisor c4c5specialist's Avatar
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    I credit your honest revision and constant research on this subject matter.

    And I do applaud your ability to keep an open mind.

    Allthebest, Paul
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    The accuracy of a CMM depends on the model. Using Zeiss as the example, the Gage Lab has an Accura and production (manufacturing) has a Contura. I believe Zeiss offers one less accurate, the Spectrum. Do not kid ourselves, we are talking .0000x or less in mm for the worst.
    The difference between a Accura and a Contura is a few additional zeros to the right of the decimal in metric. I'm sorta surprised GM expressed values in inches as their designs are full metric.

    As we discussed in an earlier thread, a wiggle test is inaccurate as the top of the valve could be supported by two lines (circles) of contact a few millimeters apart, whereas the bottom free to vibrate which enlarges the clearance. A valve vibrates.

    Probably not too clear, but it would require 4-5 slides to explain it at work.

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    Quote Originally Posted by kpic View Post
    The accuracy of a CMM depends on the model. Using Zeiss as the example, the Gage Lab has an Accura and production (manufacturing) has a Contura. I believe Zeiss offers one less accurate, the Spectrum. Do not kid ourselves, we are talking .0000x or less in mm for the worst.
    The difference between a Accura and a Contura is a few additional zeros to the right of the decimal in metric. I'm sorta surprised GM expressed values in inches as their designs are full metric. (snip)
    GM expressed all values metric, where I thought it would be easier for most CAC members to understand, I converted it to English. The CMM I watched do an LS7 head was a Zeiss Prismo. GMPT's Inspection Department is big enough that Zeiss has a resident engineer there. He was present during this session and said the Prismo in which that head was installed was accurate to 2 microns/300-mm.
    Last edited by Hib Halverson; 04-05-15 at 07:28 PM. Reason: edited content

  5. #5
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    "....the suspect period was July, 2008 to March, 2009."

    Then why reported low mileage failures since then.....

    Mike

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    The corrected "suspect period" listed on the OP is for heads machined with oversized guides.

    No suspect period has been released for the much-discussed problem with valve seat run-out (lack of concentricity).

  7. #7
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    Thanks for the clarification Hib.
    What a great car to have this machining cloud hanging over our collective cylinder heads.

    Mike

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    "5) The demise of Wiggle Testing as a way to determine if guides are worn was a result of the LS7 Engineering Team's review of the CAC's LS7 article series, three CMM inspections of the heads removed from my engine in July of 2014 along with the Team's need for more accurate information from the field about warranty replacements of LS7 heads. Additionally, the LS7 Team's review of selected content on the CAC and other forums may have influenced the decision."




    Does this paragraph state that GM at one time used the "wiggle test" as a verification for warranty?

    If so, do you know when it started and ended? I wasn't aware that GM ever used any type of wiggle test for warranty claims, but maybe they did......
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  9. #9
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    LET US BE VERY CLEAR.

    In no document, anywhere in either printed manuals or on SI, did GM ever state to do a wiggle test on the car, EVER.

    You were instructed to remove the cylinder heads, disassemble the heads and perform a hole gauge to valve stem measurement comparison.

    The stem to guide clearance of .0037 maximum was the specification. Now, I do more than the GM procedure.

    Clean valve stem and guide correctly, which means no brake cleaner due to the titanium intake valves.

    I take my guide measurement at top, middle and bottom of the length of the valve guide. Then I micrometer measure the valve stem at the same 3 places on the stem. Therein is my stem to guide clearance, as GM has always stated to perform.

    From my days working as an ASE Master engine machinist, I then would do the following to accurately determine other potential problems.

    I mount the dial indicator on the cylinder head and take a radial measurement and and axial measurement of the stem to guide at FULL LIFT. This was NOT specified by GM but it was something that I would do. This is because full lift is where maximum load between valve stem and guide would be.

    I did this because of the offset of the rocker arms and the angle of attack between arms and valve stems, which could in theory, lead to abnormal wear patterns.

    Allthebest, Paul
    Last edited by c4c5specialist; 04-09-15 at 07:40 AM.
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  10. #10
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    Quote Originally Posted by c4c5specialist View Post
    LET US BE VERY CLEAR.

    In no document, anywhere in either printed manuals or on SI, did GM ever state to do a wiggle test on the car, EVER.

    You were instructed to remove the cylinder heads, disassemble the heads and perform a hole gauge to valve stem measurement comparison.(snip)
    I agree. We need to be very clear on this issue.

    From the late 1960s until about the middle of March 2015, the factory Service Manual and ESI both mandated the so-called "wiggle test" to measure valve stem-to-guide clearance in cylinder heads of Gen 1/2/3/4/5 Small Block V8 engines as well as many other GM engine families.

    "c4c5Specialist" is correct in saying that the "factory method" never, ever mandated doing a "wiggle test" on an engine in the car. The instructions called for cylinder head removal and at least partial disassembly.

    In articles posted elsewhere on the CAC I have advocated "wiggle testing" an engine IN the car in some cases, two of which are:
    1) With an out-of-warranty engine, where a DIY user wants to assess valve guide condition without having to do the work necessary to pull the heads. Contrary to the belief of some, you can get a reasonably accurate measure of stem-to-guide clearance if, and only if, rather than a dial indicator, you use the "test indicator method" discussed in the CAC's article about wiggle testing elsewhere on the site.
    2) With an engine covered under warranty, but one for which a dealer has refused warranty coverage because the engine is not making excessive valve noise, not exhibiting high oil use or not having its MIL on, AND a DIY user wants to gather some evidence to support his/her risking the labor cost to pay GM up front to remove and do an official check of the clearance. Again, you must use the "test indicator method" rather than a dial indicator to achieve a high enough level of accuracy such that the data is useful.

    About mid-March this year, GM changed it's Service Information via an update to ESI to mandate the method "c4c5Specialist" discusses above, i.e.: measuring valve stem diameter with a micrometer then transferring guide diameter to a small hole gauge then measuring that gauge with a micrometer.

    Obviously, the even more accurate methods–using a small hole bore gauge to measure guide diameter directly or a Sunnen P-310 to directly measure the clearance are also acceptable for warranty purposes.

    It, also, goes without saying that the additional procedures c4c5Specialist performs when he inspects a set of heads for guide wear do even more to accurately assess the problem.

    Finally...it should be noted that one of the several reasons GM made this change was research presented here on the CAC comparing results gained from use of a dial indicator in a "wiggle test" to that gained from measurements taken with more accurate equipment such as a test indicator or a bore gauge, such as a a "Fowler Holemike" or a Sunnen P-310. GM's validation of the comparison with a coordinate measuring machine (CMM) was another reason GM made the change.
    Last edited by Hib Halverson; 04-13-15 at 08:02 AM. Reason: added content
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  11. #11
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    "From the late 1960s until about the middle of March 2015, the factory Service Manual and ESI both mandated the so-called "wiggle test" to measure valve stem-to-guide clearance in cylinder heads of Gen 1/2/3/4/5 Small Block V8 engines as well as many other GM engine families.

    "c4c5Specialist" is correct in saying that the "factory method" never, ever mandated doing a "wiggle test" on an engine in the car. The instructions called for cylinder head removal and at least partial disassembly."




    So are you saying that the "wiggle" test was in the factory service manual as a valid test for valve guide wear until March 2015 for warranty purposes, and that after March 2015 it is no longer a valid warranty test?

    Your second quoted paragraph above seems to contradict this. Which is correct?
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    Quote Originally Posted by LLC5 View Post
    "From the late 1960s until about the middle of March 2015, the factory Service Manual and ESI both mandated the so-called "wiggle test" to measure valve stem-to-guide clearance in cylinder heads of Gen 1/2/3/4/5 Small Block V8 engines as well as many other GM engine families.

    "c4c5Specialist" is correct in saying that the "factory method" never, ever mandated doing a "wiggle test" on an engine in the car. The instructions called for cylinder head removal and at least partial disassembly."

    So are you saying that the "wiggle" test was in the factory service manual as a valid test for valve guide wear until March 2015 for warranty purposes, and that after March 2015 it is no longer a valid warranty test?

    Your second quoted paragraph above seems to contradict this. Which is correct?
    I know the two posts are confusing.

    "c4c5Specialist" was reminding all of us that GM Service information never sanctioned "wiggle testing" engines in vehicles for warranty purposes. When it was the approved method for warranty purposes, GM required it be done with the heads removed.

    I was reminding everyone that the procedures discussed my writing about the "wiggle test" being done on heads on engines IN VEHICLES were never valid for warranty purposes.

    At present, the factory method for measuring guide clearance for warranty purposes is the new, "mic and hole gauge" method discussed in ESI.

    Sorry for the confusion.

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    Thanks to those who went over and above to announce out the correct way to determine the viability and reliability of the GM lS7 engine.
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    I just had my oil and filter changed this week (and had the low beam recall taken care of). I sent a sample of the drain oil to a analysis lab in CA for evaluation. I want to see what what's going on inside my engine without removal and disassembly of the heads. I figure this will be a good way to see if there's too much TI or SS in the oil. And it'll also allow me to see how accurate the oil life monitoring system really is. I had 6954 miles and the life monitor was at 24%. If anyone would like a copy of this report, feel free to ask. Or, I could post it here.
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    LS7 Valve Guide Wear Issue Tuna's Avatar
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    Quote Originally Posted by Iroc2Vette View Post
    Or, I could post it here.
    Post it here. I've got 26K on my LS7 and the oil has been changed 3 times. Maybe I ought to have my oil tested by a lab to see how it's doing.
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