Valve Stem Tip Spalling

[X-file]

so explain to me for my education if you will, why they ran thousands of trouble free miles with lesser materials?

It's more than possible that engine designers knew more about rocker-to-valve stem geometry in 1950 than they know now.Anything you've ever learned about roller rockers does not apply.

If the thousands of experts on the internet advising how to set up no-roller rockers are any indication,you would have to conclude that no-one knows how to do it and avoid scrub and side thrust.I actually found one person on this site who understood about the valve stem,valve tip and centre of rocker spindle forming a right angle at the instant of zero-scrub.Not bad,after 10 years or so of searching the internet.

Has anyone checked the geometry to see if the point of zero-scrub happens at a suitable valve-lift,or if it even happens at all?Somewhere below 1/2-lift would be suitable,but it depends on the rpm you plan to use.

[Kevin Johnson]

so explain to me for my education if you will, why they ran thousands of trouble free miles with lesser materials?

It's more than possible that engine designers knew more about rocker-to-valve stem geometry in 1950 than they know now.Anything you've ever learned about roller rockers does not apply.

If the thousands of experts on the internet advising how to set up no-roller rockers are any indication,you would have to conclude that no-one knows how to do it and avoid scrub and side thrust.I actually found one person on this site who understood about the valve stem,valve tip and centre of rocker spindle forming a right angle at the instant of zero-scrub.Not bad,after 10 years or so of searching the internet.

Has anyone checked the geometry to see if the point of zero-scrub happens at a suitable valve-lift,or if it even happens at all?Somewhere below 1/2-lift would be suitable,but it depends on the rpm you plan to use.

Yes, the geometry was examined.

It has been theorized that errors here in manufacturing were the root or a root cause of the valve guide wear and eventual valve head dropping.

I think a realistic appraisal of the level of engineering expertise at GM is that it is comparable to that of MIT but with the benefit of ready access to internal unpublished research.

The LT1 valvetrain was designed with the benefit of hindsight with respect to the LS7 and vastly improved computing simulations (and perhaps greater respect for fundamental issues in information theory) and has retained key elements.

I am all for empirical testing of components but at some point common sense prevails that the engineers at GM are not idiots. The geometry issue is certainly interesting but CMM comparative testing of components would have easily teased out this data.

[lorax]

so explain to me for my education if you will, why they ran thousands of trouble free miles with lesser materials?

It's more than possible that engine designers knew more about rocker-to-valve stem geometry in 1950 than they know now.Anything you've ever learned about roller rockers does not apply.

If the thousands of experts on the internet advising how to set up no-roller rockers are any indication,you would have to conclude that no-one knows how to do it and avoid scrub and side thrust.I actually found one person on this site who understood about the valve stem,valve tip and centre of rocker spindle forming a right angle at the instant of zero-scrub.Not bad,after 10 years or so of searching the internet.

Has anyone checked the geometry to see if the point of zero-scrub happens at a suitable valve-lift,or if it even happens at all?Somewhere below 1/2-lift would be suitable,but it depends on the rpm you plan to use.

I don't fully understand this post. Are you saying anything and everythng we know of roller rockers has no bearing on LS 7 rockers? How? Because they don't have a wheel on their nose? The nose wheel on an roller rocker doesn't even really roll. The body of the rocker body rotates around the nose roller much more than the roller rotates. There should virtually no rotation of the nose roller in relationship to the valve tip in a properly set up roller rocker.
The 1950 stovebolt should be infinitely harder on the rocker's shoe tip and valve tip than a roller rocker is. No computers, no million mile testing. But you are all certain the problem lies in either a bad design, or bad modern day materials and manufacturing.

[Cubic_Cleveland]

I don't understand how he thinks its "more than possible" GM engineers in 1950 knew more about rocker to valve stem geometry than they do now. Do you think there hasn't been any development done on the subject in more than 60 years? Or maybe GM forgot what they learned and wrote internal research papers on?

[clshore]

Maybe that nubbly alligator hide surface finish on the rocker tip (no matter how hard it is) has something to do with it?
I'm certain that same portion on a stovebolt rocker is at least a smooth machined radius (no matter how hard it is).

[X-file]

I don't fully understand this post. Are you saying anything and everythng we know of roller rockers has no bearing on LS 7 rockers? How? Because they don't have a wheel on their nose? The nose wheel on an roller rocker doesn't even really roll. The body of the rocker body rotates around the nose roller much more than the roller rotates. There should virtually no rotation of the nose roller in relationship to the valve tip in a properly set up roller rocker.

I'm exactly saying that 99% of what you know about roller rockers can't be applied to a solid rocker with no roller or swivel-foot.You'll have some serious scrubbing and side-thrust if you try to use it on a non-roller rocker.Yes,everything has been forgotten,if it was ever understood in the first place.

"The roller on a roller rocker doesn't even roll",but the radius on a non-roller rocker MUST roll across the valve tip.If it doesn't roll,it skids,wears and creates side-thrust.It requires different geometry to avoid skidding.The bigger the radius,the more it must roll,for a given angle of swing.Very few people understood it before roller rockers were fashionable,and even less people understand it now.It's explained here,if you read toward the bottom of the page.viewtopic.php?f=1&t=20333&hilit=geometry&start=30

[novadude]

For all you guys comparing the LS7 to the stovebolt and 69 Z/28....

Have you considered that these engines had NO valve lift and NO spring load? The 30/30 cam was like .450 net lift after lash, and 200 lb open load on the springs. Less geometry issues when you have a fixed pivot rocker and you aren't moving the valve tip much due to low lift.

Older engines were far more forgiving than modern high lift deals. Just my opinion.

[airflowdevelop]

Where's RWtech when you need him?

[lorax]

For all you guys comparing the LS7 to the stovebolt and 69 Z/28....

Have you considered that these engines had NO valve lift and NO spring load? The 30/30 cam was like .450 net lift after lash, and 200 lb open load on the springs. Less geometry issues when you have a fixed pivot rocker and you aren't moving the valve tip much due to low lift.

Older engines were far more forgiving than modern high lift deals. Just my opinion.

fine, I understand that. Now explain all the miles I stacked up with a solid with almost the same lift and just as much spring, and nowhere near these issues. Both SBF and SBCs. Not to mention solid rollers with WAY more spring. I guess my knowledge of valvetrain geometry is superior to that of the GM engineers. Considering that much of what I know came from either GM's own performance books for the SBC, stolen for the most part from Miller, or from Miller himself. Now they forgot how?
Sounds too much like todays physicist unable to build a 1945 A bomb because they never learned that technology.

I'm not buying it.

[Kevin Johnson]

The comparison should be made to earlier or concurrent versions of the LS engines NOT exhibiting the same types of failures at a statistically significant level.

Then argue why specific elements of LS7 architecture violate accepted sound engineering principles as exemplified within the same family. I do not think you will find the basis for such an argument.

The next typical step would be to attack quality control, i.e. the engineering was sound but the components were off-spec. Be sure to run parallel studies, again, on earlier and concurrent LS engines NOT exhibiting the same types of failures at a statistically significant level.

Keep in mind that, yes, some owners thrashed their engines; others modified them. However, some owners clearly pampered them and failures did not occur during or as a result of abuse. Compare these patterns/behaviors to concurrent versions of the car but with different editions of the LS engine.

[Mark200]

And until someone actually does a Rockwell test you're throwing darts blindfolded. all this discussion and nobody's done a rockwell test to any of these parts yet? its at the most a $100 test. at least you can rule out if fancy oil is going to cure the problem.

I'll look into it. I don't think it can be done locally but I have discussed NDT on another issue with a lab in St. Louis previously; I'm sure they can handle it via mail order. I don't think they will turn out too soft, but it will be an interesting data point nonetheless. In fact, since I suspect the spalling is cracking, they may be too hard (brittle).

What number do you think would be reasonable? C scale, what? 40? 45? Higher?

I travel a lot so it will take some time for me to get it set up, get them shipped, then get the results. Probably a month or more. But I'll update the thread... I do like data points

[Mark200]

Maybe that nubbly alligator hide surface finish on the rocker tip (no matter how hard it is) has something to do with it?
I'm certain that same portion on a stovebolt rocker is at least a smooth machined radius (no matter how hard it is).

Not all the valves are showing it; offhand I don't recall any of the intake lash caps showing any spalling. The earlier photo was a worst case; the following is a best case (also 44K miles); many are somewhere in the middle (speaking of the exhaust side, no lash cap):


. . .

[Mark200]

[...]If the thousands of experts on the internet advising how to set up no-roller rockers are any indication,you would have to conclude that no-one knows how to do it and avoid scrub and side thrust.I actually found one person on this site who understood about the valve stem,valve tip and centre of rocker spindle forming a right angle at the instant of zero-scrub.Not bad,after 10 years or so of searching the internet.

Has anyone checked the geometry to see if the point of zero-scrub happens at a suitable valve-lift,or if it even happens at all?Somewhere below 1/2-lift would be suitable,but it depends on the rpm you plan to use.

I intend to check this next time I get a chance to fiddle with the heads (I'd planned to check it with an aftermarket rocker anyway, would be prudent to check w/the OEM rocker as well). I presume you mean the CL of the rocker pivot -to- tip/contact being perpendicular to the valve stem vertical CL @ 50% lift?

Spec redline on the LS7 is 7000 RPM, fuel cutoff 7100.

[X-file]

If you read the link,you'll see that it's better to have it square at less than 1/2 lift.Loftfan explains it fairly well.

[Kevin Johnson]

If you read the link,you'll see that it's better to have it square at less than 1/2 lift.Loftfan explains it fairly well.

On the Corvette forum an engineer took the time to make a graph paper model of the valve to rocker arm relationship.* I imported that into a drawing program and obtained a very approximate H/L value of .32 .

I hope this is good enough for a general idea. Again, I think it is a mistake to assume that GM engineers do not really understand valve train theory. An analysis between and within groups of the LS family should be performed.

* He was exploring the effects of manufacturing errors on the height of the rocker arm fulcrum with regards to scrub.