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C6 Corvette ZR1 & Z06General info about GM’s Corvette Supercar, LS9 Corvette Technical Info, Performance Upgrades, Suspension Setup for Street or Track
At 16k miles I pulled my heads and had the guides replaced by a reputable LS shop in southern NH as a preventative measure. He suggested, and I agreed to send the intakes back to Del West to have the shafts polished because of the rough coating that contributes to valve guide wear according to some.
He installed CHE guides, and I set the heads up with BTR 660 springs to match a mild cam that was recommended by Pat G. Some of the guides were found a little over the replace spec, but nothing was way out of whack.
After the install I continued to complain about valve train noise, to address this I put in shorter, then longer push rods, nothing changed it much, and I decided to just live with it.
It's now 2+ years later, and the noise was still bothering me - was it increasing, unknown?.
I wanted to do some track days, but before I did that I decided to do the wiggle test, which I did on two cylinders.
Results were slightly over spec, around .004 when measured and adjusted at the valve stem tip (.008 - .010 at the tip), and also with a last word indicator just above the guide.
I took some video of the results and sent them to the machinist.
He said, doesn't look right, send them to me and I'll go thru them and cover all the costs - couldn't complain about that, so pulled the heads and sent them back.
He said a couple of the intakes were a little over spec, but the exhaust were all bad, and here's the kicker, he said the exhaust valve stems were worn and scored, which was new to him, and no explanation.
I bought 8 new GM valves and with new guides the heads are back together, and the car is going back together now.
So, what could have happened?
Are worn exhaust stems common or in-heard of?
Just want to put the whole head thing out of my mind, love the car, but if I can't get the little voice out of my head I think I'll be sending it down the road...
thanks.
Can't figure out how to link the YouTube Wiggle test videos.
Let me know how, or you can just look for them on YT under my user name, mikezohsix2011
Well, unless I misread your comments, it sounds like you replaced all the guides the first time, but only polished/addressed the intake valves and put the original exhaust valves (untouched) back into the heads.......a major mistake IMHO.
The arbitrary installation of shorter, then longer, pushrods after (I'm assuming) measuring and installing the correct length pushrods to begin with, is another major mistake...again, IMHO.
I think those two events need to be clarified before you could have a discussion on speculating what happened.
Unfortunately, you don't have the guide measurements to go off, or address how the car was driven during this period, which then puts all speculation even further out in left field.
Still a failure in my eyes just the same...without any collateral damage.
See my original comments to him regarding re-using the original, untouched, exhaust valves in his original valve guide replacement.
If the exhaust valve stems were not addressed then, then any subsequent wear following that initial repair is suspect , to me.
Please see my latest post on WCCH guide measurements after 18-20K miles. to get an idea what information you really need to know before you start trying to understand what is happening.
I put in GM valves this round because that's what was there to start with, and though some have issues with the sodium filled stem, many people have no service issues, and going to a solid stem has it's drawbacks, and associated failures too.
I didn't replace, or do anything with the exhaust valve stems the first time, because the machinist had no issue with them.
There was no visible wear, and they mic'd up fine.
I don't know that there's a rule of thumb that says you have to replace the valves every time you replace the guides.
I just got off the phone with the shop, and I pressed him a little harder about what might have gone wrong, and he did say that the exhausts may not have played well with the new guides, but no reason why.
For the amount of wear, on the exhausts he said they measured up at just over .004, out of spec, at the replace level, but not horrible by any means.
On the push rod length.
On the advice of Pat G, the cam should take the std push rod, as the base circle is the same.
When I went back to him with the noise complaints, he said, measure, and see what you come up with.
So I measured with adjustable push rod and did the turn on the bolt measurement.
It came up a little shorter than the 7.800 stock, but still within the range, I put the shorter rod in, don't remember the length, maybe .025 shorter, it was the first available.
Put them in, effectively no change.
Some months later, I went back to the machinist, who said longer generally quiets them down, assuming no other issues, so, what the hell, in went the fist oversize rod.
Again, no big change, and still within the allowable pre-load, don't recall the numbers, but it's a pretty wide range.
I am assembling the motor now with the stock PR length.
I put new GM lifters in while the heads were off, so back to where I started.
The car is a street driven Z06, no track time, no drag races, no dyno pulls.
Yes, I've spun it to redline a few times during the tuning process, but generally using the power in the lower RPM ranges as you would on the street.
I'm not a kid (60 y/o), I get my kicks from the corners, not smoking the tires...
Thanks for clarifying
For the record, I also use the OEM sodium filled exhaust valves.
My concern would be the condition of the exhaust valve stem, needing to be at least polished and cleaned up at minimum, before being re-used. I replaced mine both times the guides were done, as the stem and guide are exposed to the hot exhaust gasses and the contaminants that go along with it. If the exhaust guides were worn to begin, allowing the valve to wobble and not seat properly, that in itself weakens the welds on the two piece valves and would be reason enough to replace during the initial repairs.
Thanks for clarifying
For the record, I also use the OEM sodium filled exhaust valves.
My concern would be the condition of the exhaust valve stem, needing to be at least polished and cleaned up at minimum, before being re-used. I replaced mine both times the guides were done, as the stem and guide are exposed to the hot exhaust gasses and the contaminants that go along with it. If the exhaust guides were worn to begin, allowing the valve to wobble and not seat properly, that in itself weakens the welds on the two piece valves and would be reason enough to replace during the initial repairs.
reading your thread that you referenced now.
Hindsight, should have done something with the exhaust valves, or replace them last time - oh well.
This time they're new, so maybe, just maybe, I'll get better results this time.
I bought my C5Z when the C6Z came out, and my C6Z when the C7Z came out.
Hope that I can keep this car without worrying too much until the C8 comes out, and hopefully be able to afford a C7Z.
Old Ex valves are being sent back to me now, will post back when I have them in-hand to see how bad they are...
When I pulled the head, I never took the valves out of the head and looked at them - should be interesting.
This is what my exhaust valves looked like coming out of my stock heads at 40k miles. I am willing to bet yours will look the same. The stems move a lot of heat up into the guides and once they wear into their clearances they start to coke up and fail.
Last edited by Apocolipse; 08-02-2018 at 12:16 PM.
Thermal conductivity was another reason I was sold on the Moldstar90 valve guides; their ability to transfer heat from the valve is superior to the OE powdered metal or even bronze valve guides most shops use. Also, since it was no additional cost at AHP, I went with new Ferrea F2042P exhaust valves as AHP says, while the OE GM exhaust valves are not bad, for the same price the Ferrea's are just better.
As for the pushrod lengths, I want to say you have upwards of .120 of plunger travel on a stock LS7 style lifter and so there is plenty of room to go down .025" to 7.775" length PR's. Below is a great article from just last March talking about lifter preload on LS engines. It says Katech recommends .070" preload while BTR recommends .100 for LS7 lifters. As long as you have your heads off again, I would suggest you consider a set of Johnson 2110 SLR lifters and set them at .035" of preload. Very quiet and same lifters used on COPO Camaro motors.
Also, check out these comments from Richard at WCCH from a thread a few years ago.
We have definitely seen excessive wear in some LS7 exhaust guides. Mostly coming from engines with over 30k on the clock. We have seen some heads show inconsistent wear requiring only one or two guides needing replacement. It appears that the sodium exhaut valves are transmitting an excessive amount of heat from the stem to the guide. In some cases the oil is BBQ'd in the guide and the lack of lubrication and cooling from the oil causes the excessive wear (mostly in the lower half of the exhaut guide). We've measured some guides as having over .004" of wear in the lower 1" of exhaust guide length. Porsche 911's suffer the same type of related guide wear on the exhaust side.
The next item to note regarding the sodium exhaust valves relates to the wall thickness of the sodium exhaust valve stem. It's only .040" thick and makes for a fragile exhaust valve. GM uses a good quality steel material but the design spec. renders the valves brittle at the neck. Hence the reason we choose the replace the factory units with stainless or inconel. The exhaust valves we use are manufactured with a .001" oversized stem size which allows us to hone the exhaust guides and remove most or all of the taper in the bore. In cases with excessive wear we replace the guides with bronze.
I can't comment on weather the excessive guide wear is causing the valves to rattle around in the guide on on the seat causing the head failures. It may be due to excessive heat buildup from the restricted heat flow path to the guide. As I mentioned before, there's only 1mm of wall thickness at the valve neck. Not a good situation in engines producing high specific horespower. The stainless exhaust valves don't show the rapid wear that the sodium valves have.
Given the same conditions, the bronze guides will wear as well. The size of the small heat path must be increased. With a solid stem valve heat is allowed to travel further up the length of the stem. It provides a larger flow path for heat dissipation.
The primary heat path is through the valve seat. The secondary heat path is through the valve guide. Lean stoic. conditions at cruising speeds generate lots of heat on the exhaust valves with unleaded fuels. The heat bakes the small amounts of oil that makes it to the lower portion of the exhaust guide. Once the lubrication is gone there is metal to metal contact and on a molecular level electron sharing occurs. The steel molecues of the exhaust valve stem have a greater resistance to wear. The powdered metal guides have mixture of wear resisting alloys along with lubritious properties pressed into the matrix. You don't have to beat on the engine to experience valve guide wear.
The primarly reason we use bronze valve guides is because GM will not sell the guides separately. Rather they will sell you an entire replacement head. The aftermarket parts supply only furnishes bronze alloy valve guides. I personally like the powdered metal valve guides the factory uses but the supplier does not sell piece meal to the public. Additionally we have no long term wear issues with the bronze guides and solid stem stainless steel valves.
In the last decade GM has done a masterful job improving both metalurgy and rocker arm geometry to reduce overall valve guide wear. Rocker arm desings are also superbly executed. The issues seem to only be related to the sodium valves.
bronze manganese valve guides with stainless steel valves inlet and outlet.
and 0.25 milled with i drive the heads about 5000km no issues.
With Turnion kit and stock Rocker arms
with reinforced valve springs
Do I have to be scared now?
Titanium valves are risky, as my engineers have said because they can break anytime with little erosion. In addition, Titan valves are magnetic and can attract metal particles at any time.
They would have to be thermally inspected to work properly, he said.
There are also problems with bronze manganese which does not work properly with titanium valves.
He said that the titanium valves are suitable for the racetrack because the light and high speeds but can drive for the everyday dear steel valves
Please correct me if I'm wrong.
Greetings from Germany
Last edited by hy_bmw_freak; 08-03-2018 at 08:20 AM.
The bad combo is stock ti and bronze guides.
Katech coats their valves to keep bronze guides happy.
AHP uses PM or moldstar guides with polished ti valves.
Since you are running steel valves and not ti, just keep an eye on them.
On another note, if you have the compcams trunnion kit take that out right away as they always fail. Instead order CHE (or the like) bushing kit.
Last edited by Apocolipse; 08-03-2018 at 08:52 AM.
The bad combo is stock ti and bronze guides.
Katech coats their valves to keep bronze guides happy.
AHP uses PM or moldstar guides with polished ti valves.
Since you are running steel valves and not ti, just keep an eye on them.
On another note, if you have the compcams trunnion kit take that out right away as they always fail. Instead order CHE (or the like) bushing kit.
Thank you man !! :-)
But i have Compcams turnnion kit hmm you think thats broke anytime because of the needle valves ?? so far no problems i had
Last edited by hy_bmw_freak; 08-03-2018 at 09:25 AM.
Its pretty commonly recommended to replace the exhaust valves. I did on my first head R&R.
DH
I did also with new OEM exhaust valves (CHE bronze guides from WCCH). I had no interest in running used valves from guides that were out of spec. I went with the Kateck TiMo intake valves, made for them by Victory Racing in the Carolinas - some experience with doing Ti valves in high HI engines.
I bought my C5Z when the C6Z came out, and my C6Z when the C7Z came out.
Hope that I can keep this car without worrying too much until the C8 comes out, and hopefully be able to afford a C7Z.
/QUOTE]
The way you say you enjoy driving the car I would trade it for a one of the models with a PD blower. They have beautiful smooth torque. If your not taking advantage of the high / fast reving LS7 its not worth this hassle.
08-02-2018, 10:18 AM
Just did my heads AGAIN, 6k miles later
