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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
My 'fixed' heads did not fail. With all the discussion about this topic I would be almost embarrassed if they did. The 'fixed' heads after 5500 miles had two intake valves out of 'spec' when I checked them this summer. Exhaust valves were very tight!
History: Feb 2014 @ 14400 miles, wiggle test showed several exhaust valves considerably out of spec. GM under warranty replaced the heads, valves, springs etc. I put 5600 miles (many track miles) on the car/engine since then and decided to check valve guide clearances. First via (not accepted) wiggle test to get an idea of what was going on. I found several intake valves out of spec. After these findings, talking to dealer, we sent the heads to a machine shop. Two intake valves were out of spec using a valve guide bore gauge. The exhaust valve were extremely tight with the largest clearance being 0.0017". I am having the heads re-worked.
My 'fixed' heads did not fail. With all the discussion about this topic I would be almost embarrassed if they did. The 'fixed' heads after 5500 miles had two intake valves out of 'spec' when I checked them this summer. Exhaust valves were very tight!
History: Feb 2014 @ 14400 miles, wiggle test showed several exhaust valves considerably out of spec. GM under warranty replaced the heads, valves, springs etc. I put 5600 miles (many track miles) on the car/engine since then and decided to check valve guide clearances. First via (not accepted) wiggle test to get an idea of what was going on. I found several intake valves out of spec. After these findings, talking to dealer, we sent the heads to a machine shop. Two intake valves were out of spec using a valve guide bore gauge. The exhaust valve were extremely tight with the largest clearance being 0.0017". I am having the heads re-worked.
Wait - so the heads you are referring to were replacement heads from GM, not "fixed" heads from an aftermarket vendor, correct?
If so, your results are exactly as expected - GM is still shipping Ti intakes with surface finish issues - that's techically called an engineering flaw.
Wait - so the heads you are referring to were replacement heads from GM, not "fixed" heads from an aftermarket vendor, correct?
If so, your results are exactly as expected - GM is still shipping Ti intakes with surface finish issues - that's techically called an engineering flaw.
So what's a good aftermarket set of heads to get? I was looking at bare Trick Flow castings and then have them built to my specs.
Give Tony Mamo a call. He is currently in the final stages of developing and testing his own CNC program for these heads. He may have more than one program for different applications.
On paper, I think they are a terrific design. I love the rocker mounting system they have that uses a base plate.
Goal is really just to have a reliable car that I don't have to worry about coughing up a valve or two leaving me with a very expensive repair. I'd like to pick up some additional power but not at the expense of reliability. Budget would be to hopefully spend no more than 5k all said and done.
Give Tony Mamo a call. He is currently in the final stages of developing and testing his own CNC program for these heads. He may have more than one program for different applications.
On paper, I think they are a terrific design. I love the rocker mounting system they have that uses a base plate.
Thanks!!! I like the design of the Trick Flow heads too, probably number one right now on the list of aftermarket heads I'm currently considering.
YT's suck. LOL - I'm kiddin.... Just had to say that to get Dan spun up. I really have no personal experience with them. My engines don't normally get along well with anything that's not built to take a lot abuse.
I installed the Crower system on my personal car's engine. I think they're engine jewelry. The quality is excellent. The Geo is spot on. The witness marks in the pics below, are after full engine rotation with a solid lifter. 650/670 lift.
If I have to do my heads again, I am going to consider the Crower's. Thanks for your input.
Chadyellowz, a member that rebuilt heads for living, claimed he ran across several rebuilt heads, from reputable shops with bronze guides, that had valve guides out of spec.
Originally Posted by arkus
This is important
Is there a study on the percentage of "fixed heads that failed"
I wouldn't expect there are a lot that have failed. There just hasn't been enough time or miles on them for them to exhibit that failure mode. Remember machine shops really didn't start doing wholesale head repairs until a couple of years ago. Even stock engines that saw a lot of heavy track usage and had 20K to 30K miles on them went 3 or 4 years before they failed. Ones that didn't see a lot of heavy usage went 7 or 8+ years before a failure.
On top of that we have an issue of what failure means. Just because they are out of spec doesn't mean they failed. We have heads that have failed and we have heads that were out of spec when checked. Failures are when a valve head breaks off the valve stem and drops into the cylinder. As the guide wears and becomes grossly out of spec it becomes a likely cause of a valve drop, however, guides that are slightly out of spec are far less likely to be the cause of a valve drop than other mechanical issues. My engine builder thinks the issue could be weakened valve springs that lose their ability to control the valve at high rpms over time and eventually a valve bounces off a piston at an rpm level below the rev limiter setting. Some others think the real problem is a machining issue that causes the valve seats not to be concentric with the valve guide which leads to stress on the valve stem causing it to eventually break off. If that is the case just replacing the guides may be the best solution.
Wait - so the heads you are referring to were replacement heads from GM, not "fixed" heads from an aftermarket vendor, correct?
If so, your results are exactly as expected - GM is still shipping Ti intakes with surface finish issues - that's techically called an engineering flaw.
Not an engineering flaw. It is a production flaw. However, all of that stuff could just be a red herring. If you check the actual engine failures reported on the forum since 2010 dropped exhaust valves are the ones that are most prevalent. Dropped intake valves are approximately 10% of the total. If you look at the reports of guides out of spec the ratio between worn intake and exhaust guides is much closer to 50%.
We we are actually looking for what causes valve heads to break off and drop into the cylinder and comparing those two sets of data indicates there is a problem with the assumption that worn guides are the main cause of dropped valves. Otherwise, the number of intake and exhaust valve drops would correlate closely with the out of spec guide data.
If I have to do my heads again, I am going to consider the Crower's. Thanks for your input.
You're welcome. I have always employed a roller tipped rocker, whenever I could. The need for a roller tip increases as total lift increases. For these particular cylinder heads, you don’t have that many options. T&D, Comp, Crower and Jesel are about it. The T&D system is aluminum, and I think Comp’s system is just a re-branded T&D. I have not ever been sold on aluminum rockers, and stick to steel. You can get the T&D in steel, but the price adder for this upgrade almost doubles the cost. Jesel makes a couple different systems. The steel system is pretty expensive, almost double the Crower. Very nice, but just too much money for a street rig….
Keep in mind that if you do go with this set, and wish to use OE heads, they will require machine work as the rocker pedestals get milled down. Depending on total valve lift, the amount of milling will vary. After milling, the fastener holes will need to be drilled out further and re-tapped. The fastener locations over the intake runners will most likely punch through, into the intake runner, so you will need to use a thread sealant or Loctite on those fasteners. The rockers also take up more space under the covers, so you will need to either use a valve cover spacer, or taller valve covers. I only mention these things to you, and anyone else who may stumble onto this, to be aware that there are other costs and considerations that need to be accounted for when using a rail system. Those additional costs can easily exceed $1000. The Crower system also has a roller tip option, that’s about $100. The upgrade gives you needle bearings, verse a pin at the tip roller.
Not an engineering flaw. It is a production flaw. However, all of that stuff could just be a red herring. If you check the actual engine failures reported on the forum since 2010 dropped exhaust valves are the ones that are most prevalent. Dropped intake valves are approximately 10% of the total. If you look at the reports of guides out of spec the ratio between worn intake and exhaust guides is much closer to 50%.
We we are actually looking for what causes valve heads to break off and drop into the cylinder and comparing those two sets of data indicates there is a problem with the assumption that worn guides are the main cause of dropped valves. Otherwise, the number of intake and exhaust valve drops would correlate closely with the out of spec guide data.
Bill
No it wouldn't necessarily correlate at all
The error with your assumption is that the intake and exhaust valves have the same strength and subject to the same stresses.
They don't and they aren't.
Your opinion doesn't account for how a intake valve may or may not be more resilient to a loose guide condition than an exhaust valve.
You're welcome. I have always employed a roller tipped rocker, whenever I could. The need for a roller tip increases as total lift increases. For these particular cylinder heads, you don’t have that many options. T&D, Comp, Crower and Jesel are about it. The T&D system is aluminum, and I think Comp’s system is just a re-branded T&D. I have not ever been sold on aluminum rockers, and stick to steel. You can get the T&D in steel, but the price adder for this upgrade almost doubles the cost. Jesel makes a couple different systems. The steel system is pretty expensive, almost double the Crower. Very nice, but just too much money for a street rig….
Agreed...beautiful system, but cost prohibitive/unnecessary for most.
On top of that we have an issue of what failure means. Just because they are out of spec doesn't mean they failed. We have heads that have failed and we have heads that were out of spec when checked. Failures are when a valve head breaks off the valve stem and drops into the cylinder. As the guide wears and becomes grossly out of spec it becomes a likely cause of a valve drop, however, guides that are slightly out of spec are far less likely to be the cause of a valve drop than other mechanical issues. My engine builder thinks the issue could be weakened valve springs that lose their ability to control the valve at high rpms over time and eventually a valve bounces off a piston at an rpm level below the rev limiter setting. Some others think the real problem is a machining issue that causes the valve seats not to be concentric with the valve guide which leads to stress on the valve stem causing it to eventually break off. If that is the case just replacing the guides may be the best solution.
Bill
When a part is deemed to be out-of-spec, it has failed in my book. There are different levels of failure: form reaching its wear limit to catastrophic failure.
When a part is deemed to be out-of-spec, it has failed in my book. There are different levels of failure: form reaching its wear limit to catastrophic failure.
Especially when that failure (out of spec) occurs well before what the parts useful/expected life span should be.
When a part is deemed to be out-of-spec, it has failed in my book. There are different levels of failure: form reaching its wear limit to catastrophic failure.
08-20-2015, 11:32 AM
That's how I'm reading it.
