hoefi

In my case, at 11,000 miles, two exhaust guides were way out of spec. The failed cylinder had a guide that was 400% of the wear limit set by GM. The second guide was 200% over the limit. I sent the good valve out to check for run out and crack testing, both came out negative. The valve was perfect, but the exhaust guides were way out. All the rest of the exhaust guides were within limit, but just barely. Intake guides are averaging about .001" tighter than the exhausts'. There is no way an ower, or driver, can induce this kind of selective wear in two guides out of sixteen.

The hollow exhaust valve stem is very thin walled. I am guessing around 0.050" wall thickness or possibly less (couldn't measure mine because the end was all bashed up). If the valve was bent hitting the piston, it would form a very localized sharp kink at the bottom of the guide (this is where maximum bending moment occurs). One would think the breakage (immediately or eventually) would take place at this localized kink. However, my valve broke at the junction where the solid head connects to the hollow stem. I think OP's mode of failure is very similar. When an intake valve bends, it is quite different. Because the intake stem is solid, the bend radius is much larger and there is no localized kink. I observed these in my failed motor.

I won't be surprised that some owners who were experiencing high oil consumption (we had some reporting that) had worn guides also. I am also pretty certain that my engine, as well as some others, that had dropped valves were caused by the worn guides. I know a lot of the GM fanboys here are quick to accuse the drivers missing a shift everytime when a valve dropped.

hoefi

I believe the guide to stem clearance is the same for intake and exhaust. The stem diameter is also the same. Wear limit is also the same.

I have studied the specs many many times lately.

hoefi

I believe GM should have used roller tipped rockers. I also believe GM knows about the potential problem but refused to act on it. It's alway a matter of how many percent of the owners will push the motor hard enough to cause failure or premature wear. If GM is convinced that the percentage is not high enough, they will accept the cheaper detail. The front brake heat melting the ball joint boots is a classic example. I can not believe GM doesn't know that problem either. An engine like the LS7 would have had thousands of engine dyno hours on it. Under full load, full RPM test runs, GM would have known the guides would wear. They (not necessary their engineering department) just chose to believe that most owners would not run their engines hard enough to justify a proper, but more expensive, solution.

NORTY

Do you attribute any of the wear to your oil selection, or just to the quality of materials/engineering used in the valve train?

mirage2991

1) rocker ratio is changed on the side of the pushrod, and not the valve, so there would not be a geometry problem, unless the pushrod length is wrong, but, I would highly doubt that. You would have to test it on the valve and see where the rocker tip is on, but even a little off isn't going to give you 400% wear over normal

2) in regards to GM not choosing a better guide, that has nothing to do with the wear. If the guides we have in our heads are properly located, they aren't going to wear 400% over the normal wear. In addition, fixing an improper guide installation, at the time of the engine development is a mater of changing the data entered in the machine that instals the guides in the head, or the CNC that is drilling the guide's hole in the head. Again, there's no cost of doing that

I would think this problem is either related to:
a) QC on the head, meaning, the intended valve guide location is correct, but, due to machinery tolerances it is actually off
b) QC at the guide level: the supplier of them may have had a bad batch unknown to him, a bad batch that would wear faster, yet be installed in the head correctly

KLLRVET

Pretty sad, Ford had an issue with the Mustang, and here is what they did:

1999 Mustang Cobra Horsepower Snafu
The 1999 Ford Mustang SVT Cobra came with impressive innovations to please Mustang fans and car experts alike. However, all was not right with the Cobra.

Motor Trend noted that despite smaller tires and a solid axle, the GT's slalom and skidpad numbers were surprisingly close "at 66.8 mph and 0.86g, respectively [vs.] 67.8 mph and 0.88g. Viewed in this way, the SVT super pony seems hardly worth the extra $7000" -- a starting tab of $27,470 for the coupe, $31,470 for the ragtop.

Car and Driver praised the IRS for erasing 125 pounds of unsprung weight, even though it was 80 pounds heavier than the solid-axle assembly. On the other hand, curb weight was down by a worthwhile 110 pounds, and it was split more evenly front to rear. With that, C/D's Barry Winfield judged the '99 Cobra "more supple and thus more readable in corners. The rear end is less susceptible to bump-steer…off-center steering response is better, and [the] handling is more neutral at the limit."


Cobra's twincam V-8 claimed 320 horses for '99, but manufacturing glitches
forced a recall to liberate the whole herd.



But straightline performance was a puzzle. "We expected to hit 60 mph in about five seconds flat," Winfield said, "but 5.5 was the best we could do -- 0.1-second slower than the previous model. Top speed was also down, from 153 to 149 mph...all of which confirms that our low-mileage prototype test car wasn't making a full head of steam."

Sure enough, a manufacturing glitch had left Cobra intake runners and some exhaust components with internal aluminum residue or "flash" that upset air flow and kept more than 30 horses from showing up.

After fielding a few dozen owner complaints, mostly from drag racers, Ford recalled all 1999 SVT Cobras on the ground to replace the manifold or ream out the existing one. Ford also charged nothing to replace mufflers (found to be too restrictive), recalibrate the engine computer, and substitute a more durable accessory-belt tensioner. A decal was affixed in the engine bay to certify the work once it was done.


The manufacturing glitches that forced Ford to recall '99 Cobras was a major
setback for the Special Vehicle Team, which would wait until 2001 to release
another SVT Cobra model.



Though Cobra owners didn't seem to mind the recall or its inconvenience, the episode was a black eye for Ford and SVT, enough that they decided not to do a 2000 Cobra. As the SVT website advised at the time, fixing the '99s had top priority.

"Rather than rushing to produce a limited number of 2000 models -- and risking production/manufacturing issues by hurrying -- we're choosing to focus our efforts on the timely production of the ['01 versions]."

Cobra sales were also down but not mortally wounded by the AWOL-horsepower flap, and Cobra convertibles outsold coupes for the first time (4055 vs. 4040 units).

mirage2991

There's always another thing: cars dropping valve may have been racing road course events with the wrong oil, and the top end would probably be the first thing to show sign of accelerated wear (higher exh temps, with to thin of an oil).
What's the possibilities of that?

KLLRVET

I was using oil that meets or exceeds GM specs

KLLRVET

If it were the quality of oil, dont you think there would be wear in to the intake guides too?

BlaznZ06

According to some on this forum you modified your car when you went with Amsoil.

I agree with you. My stock '06 motor blew with Amsoil in it and GM replaced the engine. The oil wasn't the problem.

There is a mild defect in the design.

LFZ

This is true. Your engine looks a lot like a Gen 1 small block 383 I had that popped on me. It turned out to be a piston ring clearance issue. I know these are different engines, but still a pushrod motor that was high revving with a high lift cam just like the LS7, therefore it could have been anything. I'm not saying one should rule out the weak valve possiblity, but don't rule out anything else.

Sorry about your misfortune and keep us updated.

1.8t

Can any other vendors comment on the guide issue?

BgBoost1

I am not trying to start a war here just trying to further my knowledge.

So you are saying that in your experience and studies you have found that the Valve Guide to Valve Tolerance (Service Range) issued by GM is the same on the Ti Intake valve with Cold Fuel and air running over it, as the Sodium Filled 2pc Exhaust Valve that is operating at a much much higher temperature?

Painrace

I don't find anything in your post that I disagree with. Worn guides, for any reason, will cause valve failures eventually. I have seen race engines where the valves were so lose it was all but impossible to figure out why they did not fail yet the motor ran fine and was just down for a rebuild. Taping the valves will also cause guide wear. We will probably never know.

Jim

KLLRVET

I spoke to a local dealer today, and their service manager told me they are aware of a valve guide wear issue. They do keep it somewhat guarded, but the problem is known.

hoefi

Yes, that is correct. GM specified the same service limit for both the intake and exhaust stem to guide clearance. The following numbers are listed in their factory shop manual (page 9-1102, in volume 2 of 3):

1) Valve stem diameter (intake and exhaust) = 0.313-0.314"
2) Stem to guide clearance (production), intake = 0.001-0.0024"
3) Stem to guide clearance (production), exhaust = 0.001-0.0026"
4) Stem to guide clearance (service), intake & exhaust = 0.0037"

1.8t

Gentlemen,

I think we are starting to see a pattern here. I sent Richard @ WCCH(http://www.proheads.com) a PM over on LS1tech asking about the LS7 exhaust valve guide issue. They offer a stainless steel upgrade for LS7 heads and are a extremely well respected/knowledgable vendor, seeing more LSX heads than any of us could shake a stick at. They are also the vendor that Jason from Katech used to port his LS7 heads in his latest round of modifications.

Below are his comments:

obzidian

Great find.

You think you can ask him if one was to leave the Na filled valves and ONLY replace the guides to bronze, would that by itself help in slowing down or eliminating the guide wear (due to heat, potentially) and extending the life of the valvetrain while using the sodium filled valves?

thanks.

360jeepboy

That is indeed a great find!! It is good to see some respectable shops noting the same problems we see!

That is the type of theory I like to see! One that doesn't just blame driver error!

hoefi

Exactly what I observed in my failed engine. Signs of material transfer on the valve stems (both the broken one and the good one) were mainly located in the lower 1" portion. Even GM's own outside engineer reported this observation on the broken valve stem, but he said it was not an indication of anything serious. Didn't even bother to check the guide to see how severe was the wear inside the bore.

Once the clearance gets beyond a few thou, the ability to transfer heat goes down substantially, then the problem just feeds itself.