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  • Wiggle Testing: How to Perform On-Engine Test for C6 Corvette LS7 Valve Stem-Guide Clearance
  • Wiggle Testing: How to Perform On-Engine Test for C6 Corvette LS7 Valve Stem-Guide Clearance
  • Wiggle Testing: How to Perform On-Engine Test for C6 Corvette LS7 Valve Stem-Guide Clearance
  • Wiggle Testing: How to Perform On-Engine Test for C6 Corvette LS7 Valve Stem-Guide Clearance

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  1. #31
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    Quote Originally Posted by c4c5specialist View Post
    This is now where I get involved because now you are talking complete speculation!
    I welcome your participation in the discussion, however, I sense your involvement is driven by an instinct to protect Corvette Action Center members from rampant speculation by members of the media. You can rest assured, in this instance, the CAC is safe from that.

    This quote is absolutely assumptive.

    In fact, there have been some cases of premature guide wear in engines with far less than 10,000 miles on them, but those engines demonstrated symptoms, such as excessive valve noise, high oil consumption or the MIL on due to misfire. Obviously, a wiggle test in those situations is unnecessary because it will only tell you what you already know: the guides are worn.
    While I can be accused of doing a mediocre job of making a point with the above, there were no assumptions involved.

    One of the sources I used in my research was a World Class GM Service Technician, like yourself, who is a Corvette specialist at a Chevrolet dealer which sells and services a lot of Corvettes. When I interviewed him, we discussed the various LS7s with valve guide issues on which he'd worked. Another source I used was an LS7 guide wear "registry" on another web site. Those and other sources are where I learned of engines having guide wear or valvetrain-related engine failures at less than 10,000 miles.

    As for the three symptoms cited–excessive valve noise, excessive oil use or MIL on due to misfire–they come straight from a GM Information Service Bulletin issued in January of 2013. I'm sure you are familiar with the document. Also, Chevrolet Customer Service, in statements it made on various web sites in the Fall of 2012, cited some of the same symptoms in its discussion of cylinder head machining errors made by a GM supplier.

    Where I went wrong is the way I phrased that statement. I agree that it implies that with an engine having less than 10,000 miles which exhibits one of or a combination of those symptoms has valve guide wear. Further, I agree that just the presence of a symptom doesn't confirm the existence of a problem. That paragraph in the article needs rewriting and I will attend to that in the near future

    2006-15, yes I am including LS7 in Camaro, there are over 250 different causes for misfires, MIL or other factors.
    I agree.

    I have had over 50, yes 50 LS7 apart, and to make the assumption that all these engines had issues with the valve guides is false.
    As poorly phrased as my statement was, no where does it assume or imply that all LS7s have problems with valve guides

    ACCURATE DIAGNOSTIC PROCEDURE, NO MATTER ON WHAT ENGINE MUST BE FOLLOWED.
    I agree. If you have read a few of the many thousands of discussions to which I have contributed here on the CAC which cover service problems, you know that "proper diagnosis first" is a recurring theme of mine.


    Mileage varied between 11,200 and 166,341 on the LS7 I have served. 3 had valve guide issues.
    Ok. Let's discount the one with 166,341 miles as a case where excessive guide wear is likely present, but the other two–the one with 11,200 and the other with some odometer reading between those two extremes–represent 2 problems in 50 engines. GM uses problems per 1000 engines as its unit-of-measure, so, if we translate your experience, it works out to 40 problems per 1000 engines. The only public statement GM has ever made about how prevalent problems with valve guides are in LS7s is 6.5 per 1000 engines. Your number, 40-per-1000, is 616% greater. Even if we cut that in half, your number is still three times higher than GM's official figure.


    That tends to confirm one of the beliefs I developed in the process of researching the LS7 valve guide wear problem: that GM has, at best, misunderstood the magnitude of the problem and likely has publicly understated its magnitude.

    Assumption is a luxury that professional GM service technicians NEVER HAVE.
    In a perfect world, all GM service techs would understand that. Unfortunately, we don't live in a perfect world. I hate to say it, but not all GM service techs have that important principle in mind.


    3 out of over 50, hm, not too bad of an average.
    Surely you jest!

    Three out of 50...60 problems per 1000 engines?!

    That is an obscenely high level of valve guide wear trouble.

    But I will NEVER EVER EVER GUESS that an engine, LS7 or others have these issues.

    EACH engine must be measured, correctly tolerance checked on its own merits of diagnostic procedure.
    I agree.

    "C4c5Specialist", your devotion to proper diagnosis is a quality which I have always admired and respected. It comes though in virtually all the content you post here on the Corvette Action Center and CAC members are better for that.


  2. #32
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    Quote Originally Posted by Hib Halverson View Post
    (sigh)

    All this bandwidth expended on "concentricity".
    Hib,
    Wasted would have been a better choice.

    As I said earlier, aerospace uses concentricity. Automotive would use either total runout or position tolerance. As a valve seat is an angle total runout is not a choice.

    Concentricity is complex and rare as it controls opposed median points to a datum axis. Concentricity will control location and only has some effect on the form and orientation of the feature. Concentricity will not control the form of perfectly oval parts but may have an impact on irregular or "D" shaped features.
    How to Measure Concentricity Correctly
    Elsmar is a place where Measurement Lab people hang out; they measure what people like me put on production drawings.

    As words would take forever, we use symbols: ASME Y14.5M-1994 to 2009 Update Workshop

    As I designed engines for 20 years; they don't use concentricity. By the nature of the beast, it is impossible to develop a 3D axis from an angled surface. In design, a gage diameter is used on the valve and seat to define a circular line of contact.. From there the whole mechanism is set up.

    Typically valve seats are cut using the guide to form the axis.

    If one is doing one at a time: https://www.youtube.com/watch?v=RK0djZ2Ha_Y OEMs

    More modern: Cylinder Head Valve Seat & Guide Machine - SG10A if you view the images you'll see the ground pins which locate the cutter to the valve guide.

    This one is really cool: NEWEN®

    As you see, no matter if it is a set of manual Neways or a single cutter or a multistage; the cutter locates on a gage pin inserted the guide. That is how they avoid tolerance issues and insure the seat location is correct.
    So how GM screwed the pooch locating the valve seat to the valve bore is something only a GM internal employee would know. Plus as Paul said a rare failure.

    This is beginning to feel like I'm at work.
    Whoosh, and blur
    Likes dougelam liked this post

  3. #33
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    Quote Originally Posted by kpic View Post
    (snip)

    Typically valve seats are cut using the guide to form the axis.
    With LS7 and LS9, the guides were installed then both guide ID and the seats were established with a single plunge movement of the work head in a CNC machining center. I suspect that, at this point in time, all the Gen heads are done that way.

    (snip)

    More modern: Cylinder Head Valve Seat & Guide Machine - SG10A if you view the images you'll see the ground pins which locate the cutter to the valve guide.
    That's, basically the same idea as a Serdi, with which I am more familiar.

    This one is really cool: NEWEN®
    I don't want to get started on Serdi vs Newen machines. It will take this thread way off-topic.

    So how GM screwed the pooch locating the valve seat to the valve bore is something only a GM internal employee would know. Plus as Paul said a rare failure.
    I know how it was done and, yeah, the supplier responsible for LS7 head manufacturing "screwed the pooch"...big time.

  4. #34
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    I stand behind every single thing I have said.

    Anything can be picked apart on the internet, but when you work on these cars every day, serve Corvette every day, you see much more than any internet article or press release or anything else can convey.

    This is my last statement on this website about this.

    I do not agree with you.

    Paul
    GM World Class Certified Technician.
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  5. #35
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    Quote Originally Posted by c4c5specialist View Post
    I stand behind every single thing I have said.

    Anything can be picked apart on the internet, but when you work on these cars every day, serve Corvette every day, you see much more than any internet article or press release or anything else can convey.

    This is my last statement on this website about this.

    I do not agree with you.

    Paul
    From my viewpoint, there isn't enough information to do a FMEA. When one works every day in design engineering; I end up in the same place as you.
    Whoosh, and blur

  6. #36
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    As of early March the "Wiggle Test" is no longer the factory method for measuring stem-to-guide clearance for warranty purposes. GM has decided that the Wiggle Test is just to inaccurate.

    I agree to a point. I think Wiggle Testing is very difficult to do in a manner which makes it useful so I can see why GM axed it, however, for DIYs working on an out of warranty engine and looking for a way to assess guide condition without removing the heads, I think the "test indicator method" discussed in the Wiggle Test article is an acceptable procedure.

    Eventually, the Wiggle Test article is going to be rewritten to cover the new method GM wants dealers to use for stem-to-guide clearance.

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