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

[73shark]

The machining error was that some heads had guide and seat center lines which were non concentric.

That'll do it. Thanks, that saved reading the 23 pages of that other thread to find it.

 

[megavettes]

If the valvetrain isn't making noise, don't be afraid to drive the car while you wait to have the guide clearance tested.

The guy who told you that all 06-11 LS7 engines will eventually have a problem is vastly distorting the issue, perhaps in the interest of selling you a head repair before you even decide you need it.

First assess wether or not the problem even exists in your engine. Once you do that, then decide on a repair.

Thanks Hib. I'll certainly take your advice and check it first.

 

[kpic]

The heads were fully CNC'ed there was no "reaming".

The machining error was that some heads had guide and seat center lines which were non concentric.

Hib,
As I said, reaming is an old term. Today all we care about is holding the required value and proving capability. Years ago, production drawings would say drill to X and ream; that is no longer true. Reaming is a process and Design could give a hoot if the bore is reamed or cut with a single point tool. Surface finish is another story.

Cpk is the measure of process capability and requires the process to be centered.
If anyone is interested: Process Capability (Cp, Cpk) and Process Performance (Pp, Ppk) – What is the Difference?

With a valve guide concentricity means the OD is centered within an allowable error to the ID.

So what is concentricity: GD&T Concentricity Definition | eMachineShop

What the picture means is the ø25 ±0.05 with be centered on Datum A within a diametral zone of 0.05. No matter if the bore is ø25.05 or ø24.95 there is no bonus tolerance which means no movement based on size deviation.

This is bonus tolerance: Untitled Page

As checking concentricity requires a median of points which means a gage doesn't work. Short form, it is expensive to check.Usually, automotive is more prone to use position tolerance; aerospace uses concentricity.

If you take two pieces of round stock and drill a hole in both. If both bores are .0015" larger than the mating piece; no matter if it is centered or off center from the OD there will be no difference in wiggle. That is just the nature of fits. There is no difference between .0015" of clearance and .0015" of clearance. Wiggle would be induced by a size, taper or other factors, but not a concentricity error

Sorry, it might be some placebo floating around but, the concentricity boat don't float. Can anyone guess what I do 8 hours a day..

 

[LLC5]

Hib,
As I said, reaming is an old term. Today all we care about is holding the required value and proving capability. Years ago, production drawings would say drill to X and ream; that is no longer true. Reaming is a process and Design could give a hoot if the bore is reamed or cut with a single point tool. Surface finish is another story.

Cpk is the measure of process capability and requires the process to be centered.
If anyone is interested: Process Capability (Cp, Cpk) and Process Performance (Pp, Ppk) – What is the Difference?

With a valve guide concentricity means the OD is centered within an allowable error to the ID.

So what is concentricity: GD&T Concentricity Definition | eMachineShop

What the picture means is the ø25 ±0.05 with be centered on Datum A within a diametral zone of 0.05. No matter if the bore is ø25.05 or ø24.95 there is no bonus tolerance which means no movement based on size deviation.

This is bonus tolerance: Untitled Page

As checking concentricity requires a median of points which means a gage doesn't work. Short form, it is expensive to check.Usually, automotive is more prone to use position tolerance; aerospace uses concentricity.

If you take two pieces of round stock and drill a hole in both. If both bores are .0015" larger than the mating piece; no matter if it is centered or off center from the OD there will be no difference in wiggle. That is just the nature of fits. There is no difference between .0015" of clearance and .0015" of clearance. Wiggle would be induced by a size, taper or other factors, but not a concentricity error

Sorry, it might be some placebo floating around but, the concentricity boat don't float. Can anyone guess what I do 8 hours a day..

If I had only on guess, it would be Boeing Engineer.

 

[kpic]

Dinner is over..

The heads were fully CNC'ed there was no "reaming".

The machining error was that some heads had guide and seat center lines which were non concentric.

As a CNC can do rough bores, finish bores and even the fancy cylinder bore crosshatch finish; if needed a ream can be mounted in a tool holder. When this is done, the ream removes an extremely small amount of material; however, it does yield a very nice surface finish.

Typically a CNC (computer numeric control) or a computer controlled machining center has a limited number of tool holders. To reduce tolerance and tool holders usage; profile tools are used. A profile tool is a combination tool. The slang name for best example is a Parker tool which cuts every feature of an O-ring port in one operation.

Where I used to work; the guide bore and the valve seat bore were cut in one operation by one tool. Profile tools which cut multiple features in one operation significantly reduce tolerance as a profile tool cuts a bunch of features in one operation and is built to tool maker's tolerances; not production tolerances.

The seats were pressed and cut using the guide bore as the locator. It depends on the OEM if the seats are cut; they can easily be already cut and simply pressed in.
Potentially, offsetting the seat bore to the guide bore could cause wiggle after XXXX cycles by wearing the guide bore. However, just as easy, it would cause the valve to bind or gall due to induced angularity. The next valve opening cycle would cause a catastrophic failure.

As we don't have access to GM's FMEA (Failure Mode Effects Analysis) we will never know the real reason or reasons. Point is the boat still don't float.

 

[kpic]

If I had only on guess, it would be Boeing Engineer.

LOL, no Sir. I'm a hill billy power train designer. Automotive is and has been my love and I was fortunate to work in vertically integrated businesses. Where I saw the processes instead of offshore.
Except for the last 10 years; Class 8 engines (I6 and V8), transmissions, differentials and transfer cases. I've been in Design Engineering since 1970.

After I retire, I'll own up to where I work now. I enjoy where I am now because most of the staff enjoys high performance vehicles, classic cars, muscle cars, auto crossing and more. They understand why I added a centrifugal to my Corvette.

 

[Hib Halverson]

(sigh)

All this bandwidth expended on "concentricity".

Here's the 411:

If the centerline of the guide bore and the CL of the valve seat are not concentric, as the valve hits the seat, it's pushed sideways. That wears the guide. How quickly the guide wears depends on how great the non-concentricity.

In an LS7 head, concentricity is very simple to measure with a tool like this.

I have measured numerous LS7 valve seats with that tool and found up to .005-in non-concentricity on new and used cylinder heads. The factory limit is .002. Someone really good with a Serdi should be able to hold it under .001-in.

The other way to measure concentricity, and how GM Powertrain does it is with a CMM...clearly a bit more accuracy is involved with that but, in the field, the a seat runout gauge is sufficient.

Merry Christmas, everyone

 

[Hib Halverson]

Thanks. So the 10K miles is not hard data but as GM states "where this condition has been observed, it has occurred early in the vehicle life". The type of miles (tracking vs. casual city / hwy) would likely impact that as well.

General Motors has never stated that LS7 owners should have engines inspected for valve-stem-to-guide clearance at 10,000 miles. The Corvette Action Center, based on our research into the LS7 valve guide problem, makes that suggestion. if you are a C6 Z06 owner concerned about guide wear, but your engine is not demonstrating any signs of the problem, if the engine has 10,000 miles or more on it; do a wiggle test. If the engine passes, it does not have the problem and, as long as the engine's valvetrain is not modified and the engine is not subjected to frequent severe duty cycles, it likely will never experience premature valve guide wear. If the engine flunks, have it repaired.

In fact, there have been a few cases of excessive guide wear, even valve failures due to guide wear, in engines with far less than 10,000 miles on them. Some of those cases may have demonstrated obvious symptoms, such as excessive valve noise, high oil consumption or the MIL on due to misfire. Even though the symptoms might preclude some from measuring all 16 valves' stem clearance, proper diagnosis is going to require wiggle testing several of them.

An engine can have excessive guide wear and not demonstrate any of the above symptoms. Its those engines that the "wiggle test at 10,000 miles" is intended to detect.

As far as the "2006 to 2013 LS-7's", the post quoting GM appears to state that a limited number of '08, 09, ’10 and ’11's only may be affected.

CAC's research indicates that GM understated the amount of model years in which the problem can occur.

This guide wear problem is not widespread–there are many LS7s with a lot more than 10,000 miles on them which have no worn guides–but it is a documented problem which has affected some engines and it's affected some outside the model year range GM has stated publicly. We proved that with during the research for the revision of the CAC's main LS7 article.

If it occurs, in its early stages, it will not cause any of the symptoms discussed in the Jan. 2013 Information Service Bulletin covering the problem. Again, our recommendation to wiggle test at 10,000 miles is directed at those who are concerned about the problem but have an LS7 that's not showing those symptoms, i.e.: no excessive valve noise, no excessive oil use or no MIL. Obviously, if your engine currently demonstrates those symptoms, you need to get the engine repaired rather than just checking valve stem clearances.

Happy New Year, all!

 

[c4c5specialist]

This is now where I get involved because now you are talking complete speculation!

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.

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

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.

ACCURATE DIAGNOSTIC PROCEDURE, NO MATTER ON WHAT ENGINE MUST BE FOLLOWED. Mileage varied between 11,200 and 166,341 on the LS7 I have served. 3 had valve guide issues.

Assumption is a luxury that professional GM service technicians NEVER HAVE.

3 out of over 50, hm, not too bad of an average.

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.

Allthebest, Paul

 

[dougelam]

This is now where I get involved because now you are talking complete speculation!

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.

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

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.

ACCURATE DIAGNOSTIC PROCEDURE, NO MATTER ON WHAT ENGINE MUST BE FOLLOWED. Mileage varied between 11,200 and 166,341 on the LS7 I have served. 3 had valve guide issues.

Assumption is a luxury that professional GM service technicians NEVER HAVE.

3 out of over 50, hm, not too bad of an average.

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.

Allthebest, Paul

Well put Paul
The 3.6L in our Enclave has a known problem with water pumps, should I remove it and wiggle the impeller to see if it is going to fail someday?

If there is a symptom check it out
If not don't worry about you will only lose sleep over spilled milk

 

[Hib Halverson]

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.

 

[kpic]

(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. :eyerole

 

[Hib Halverson]

(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.

 

[c4c5specialist]

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

 

[kpic]

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.

 

[Hib Halverson]

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.