73shark
Well-known member
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.
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The machining error was that some heads had guide and seat center lines which were non concentric.
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.
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..![]()
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.
If I had only on guess, it would be Boeing Engineer.![]()
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.
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.
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
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 is now where I get involved because now you are talking complete speculation!
While I can be accused of doing a mediocre job of making a point with the above, there were no assumptions involved.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.
I agree.2006-15, yes I am including LS7 in Camaro, there are over 250 different causes for misfires, MIL or other factors.
As poorly phrased as my statement was, no where does it assume or imply that all LS7s have problems with valve guidesI 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.
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.ACCURATE DIAGNOSTIC PROCEDURE, NO MATTER ON WHAT ENGINE MUST BE FOLLOWED.
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.Mileage varied between 11,200 and 166,341 on the LS7 I have served. 3 had valve guide issues.
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.Assumption is a luxury that professional GM service technicians NEVER HAVE.
Surely you jest!3 out of over 50, hm, not too bad of an average.
I agree.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.
(sigh)
All this bandwidth expended on "concentricity".
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
(snip)
Typically valve seats are cut using the guide to form the axis.
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®
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 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