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Old 01-21-2011, 12:28 PM
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Reworking heads questions

Is there a definitive way to tell if a head needs a valve job? I have never worked a head before but am learning and have been reading extensively about it for a few days.
I picked up a used 98 Vortec long block with 114k said miles and have been going over one of the heads, cleaned it up and lapped the valves and everything looks okay. I don't know about the angles but so far the seats and faces are within spec width wise and the valve stem run out is too. The thing looks good but I was hoping for some guidance still.

I observe valve stem play from side to side in the guide but when measuring across the head as per the FSM it is within specs. Is this a problem?

The valve stems according to what I read should be .343 and what I'm measuring is .340, granted i'm not using top line tools for this.

After lapping (tried for learning sake on this one) everything cleaned up nice, after reassembling and putting rubbing alcohol in the chambers to check leaks there is very minimal leakage toward the intake side of some exhaust valves. I let it sit overnight and it hasn't been enough leakage to cause a drop to leak out but it's wet. No leakage on intakes.

I was reading that up to 30% leakage on exhaust is ok and the compression stroke will aid that. Will I be okay to let the valves seat a bit more when the motor runs or am I past the line of being okay on this one?

Long post sorry, just a lot of questions. Thanks

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Old 01-21-2011, 12:48 PM
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So what you're saying is that you think you need a valve job and possibly valves? I would definitely take those heads to someone who knows what they're doing and can help you out on that one. The factory valve job is fairly good but it can be GREATLY improved.
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Old 01-21-2011, 01:07 PM
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I talked to my machine shop and will be taking them out there for a look regardless of what I do. I'm really more interested in learning the process of the valve train and the do's and dont's I guess. Testing my common sense has got me in trouble before but rather then just take the heads to get done I'd rather know the what for

I believe once run the valves will seat and not leak since they barely do anyway and 30-40% leak is acceptable on the exhaust side. I know it is not ideal but can someone confirm?

Seller said the motor had low oil pressure when warmed up but didn't burn oil and the shape the heads are in would confirm to me.

These will end up on an older block with around 9.5:1
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Old 01-21-2011, 02:02 PM
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Quote:
Originally Posted by Coolball
I talked to my machine shop and will be taking them out there for a look regardless of what I do. I'm really more interested in learning the process of the valve train and the do's and dont's I guess. Testing my common sense has got me in trouble before but rather then just take the heads to get done I'd rather know the what for

I believe once run the valves will seat and not leak since they barely do anyway and 30-40% leak is acceptable on the exhaust side. I know it is not ideal but can someone confirm?

Seller said the motor had low oil pressure when warmed up but didn't burn oil and the shape the heads are in would confirm to me.

These will end up on an older block with around 9.5:1

What? who told you that a 30-40% leak is acceptable, NO IT ISN'T.

Valve work is not something that can be done on a modern engine, that means for the last 50 years, by the average person with a can of lapping compound. That idea is a left over from the days of flat head engines and the great depression of the 1930's. Modern lapping is a final step done after the machined surfaces of the valve seats on the head and the valve have been restored using machine tools. These tools, either precision shaped stones or carbide/diamond cutters, are used to restore the seats. Then lapping compound is used to insure there is a gas tight fit down to a few millionths of an inch. The seats are extremely important not only from a sealing stand point, but the angles of the seats greatly influence flow into and out of the cylinder, and equally important, especially for the exhaust valve, is that the seat is a major heat flow path necessary to keep the valve inside its operating temperature range, it must make full and complete contact on the head or it melts away very quickly.

Before the seats of the head's are refurbished, the guides have to be restored. There are two big reasons for this; 1) The seats are indexed from the guides. If the guide can't hold the mandrel steady for the cutting tool the seat will not be concentric, will have waviness in the cut surface, and will not be in alignment with the guide. 2) In operation, the valve will wobble relative to the seat, it will not make a seal to hold the compression and combustion pressures allowing them to leak away their power and your expensive gasoline, you might as well take your wallet and flush it down the toilet. In addition the stem to guide is another cooling path for the valve's operating heat load. When the clearance between the guide and stem isn't correct, it greatly reduces valve head and seat life.

The tools to do a proper valve job are very costly to purchase and they consume expensive seat cutters for both the head and valve like an 8 year old going thru a candy store. So while the cost of a proper valve job may seem daunting, the fact is the average automotive machine shop has a lot of expense they need to recover just in equipment and consumable tooling let alone the salary of the guy doing the work. From an operator's stand point, if a valve job isn't done adequately and properly you're just throwing your money away in terms of fuel whose energy wasn't used and in terms of failed valves which will run too hot and will burn themselves and possibly their head seats as well. If th latter event happens then you'll really discover what expensive is when this stuff has to be fixed or replaced.

As a friend of mine likes to say about this kind of the stuff; "the easy or cheap way out, leads back in".

Bogie
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Old 01-21-2011, 03:19 PM
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Thanks for the reply Bogie I have read many of your replies in the forums.

Not 100% but I think it was an article in Carcraft and what stuck with me was 30% okay on exhaust but the intake is where the attention should be given. I could have misinterpreted the concept of the article itself. I'm almost dizzy from reading so much. It did go on to mention after testing a supercharged engine where an exhaust valve nicked a piston and bent, after the valve was replaced performance showed no difference on the dyno afterward. It kind of led me to believe I was being too cautious.

That has also made me wonder more about what passes for okay on a head. I know new guides, valves and fresh seats and having it decked would be great but just because a head has some miles on it, the most common thing I see is that it needs redone. There has got to be a happy medium?

Wouldn't the valves eventually seat properly (or better) with the motor in operation? Is 0% leakage on the valves the industry standard? I would think that all the carbon built up after some miles would greatly affect the numbers and many engines run just fine like that.

I've come to understand the theory of heat transfer from the valve to the seat in only the contacted portions. I read in that same article earlier that a wider band is okay on the exhaust because of how much hotter it runs.

Are my valves concentric~I don't know. The lap shows an even band on both valve and seat. Exhaust face is approx .095 and intake is approx .050 ish? They are both definitely wider then new but are they in spec~from what I read yes.

It's apparent the rockers were loading the valves sideways as the run out is greater parallel to the head but my book has no mention of side to side, it stated to check across the head. Correct?
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Old 01-21-2011, 05:47 PM
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searched the history and found the link to the article http://www.carcraft.com/techarticles...er/index1.html

Quote from the article: "The consensus among performance engine tuners for leakage past the exhaust valve is that it would have to be massive--over 30 percent or more--before it should be of concern."

Is there a better way to test heads off the motor for leaks then filling the chambers and seeing if liquid goes past the valves?
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Old 01-21-2011, 05:58 PM
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Valve wear is measured as total indicator runout. Excessive ANY direction. Modern valve jobs are not done with lapping compound. Not only can the seat be totally out of shape. The valve does not seal in the lapped seat at running temp. Valve head is expanded by heat and valve seat is cooled by circulating coolant.

Leakage from a cylinder over 10 % is cause for concern..

Vortec heads do not tend to wear valve guides excessively. But they have better than a 80 percent chance of being cracked. Thru the center exhaust seats and the center head bolt hole. These can usually only be seen by a magnaflux system .Unless they are really shot..
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Old 01-21-2011, 07:01 PM
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Quote:
Originally Posted by Coolball
searched the history and found the link to the article http://www.carcraft.com/techarticles...er/index1.html

Quote from the article: "The consensus among performance engine tuners for leakage past the exhaust valve is that it would have to be massive--over 30 percent or more--before it should be of concern."

Is there a better way to test heads off the motor for leaks then filling the chambers and seeing if liquid goes past the valves?
This is talking about leak-down testing. I'm not really sure where to start in saying something about that, it's a complicated subject so I'll try to distill it to its vital essence.

It's something of an arbitrary leak test where you will get different results from different manufacturer's test equipment. A test that shows 30% leakage on one tester will only show 10 percent on somebody else's tester. What's important is that you use the same tester for your own work and don't make comparisons to anyone else's test except where they have values from the same tester and test conditions of each cylinder to all others on the same engine.

The way these work is that there is an apparatus with 2 pressure gauges. Between them is an orifice. Between what we can call the base pressure gauge and an air source like a compressor is a regulator ahead of the regulator is a shutoff valve as part of the apparatus or on the compressor itself. After the orifice is the leak down percentage gauge which is connected to the spark plug hole and should be showing an equal or lesser pressure than the regulated base pressure gauge. the difference in readings is the leak-down percentage.

The engine is brought to the firing position for each cylinder to be tested and the crankshaft locked in position so it cannot rotate from the pressure that will be applied to the test cylinder. The compressor is started and allowed to run the storage tank to its shut off pressure. The shut off valve is opened allowing air through the apparatus and into the cylinder. The regulator is adjusted to 100 psi just to make the math easy to do in your head, otherwise it could be any pressure you like. Then the pressure on the gauge between the orifice and the spark-plug hole connection is observed. The low side pressure divided by the 100 pound standard times 100 gives the percent of leak down. Obviously the orifice size will have a large influence upon the percent you're observing as it's controlling the fill rate going into the cylinder. So a .050 inch orifice set up will show a lower pressure in the second gauge which is a higher leak down rate, than would a .5 inch orifice as the larger orifice can replenish the leaking air at a greater rate than it's loss from the cylinder unless the sum of all the leaking points such as around the rings or valves was equal to or greater than the area of the orifice. Obviously a burnt valve or a blown piston would tax the compressors ability to hold pressure in the cylinder.

The important part of what can be learned from this test beyond the leak down percent on your tester, is you can hear where air is escaping from to identify where corrective actions need to be taken to repair the engine.

Other lessons to be learned are several such as pressurized oil being sprayed off the rod bearings onto the cylinder walls is a major contributor toward sealing the rings. This thought, also, needs extension to the combustion pressures of the operating engine which are what actually makes the compression ring seal. So one would expect to see higher leak-down percents from a non operating engine than you would see if you could perform this kind of test on an operating engine.

A 30% loss from the valves would certainly get my attention if I saw that on my .040 orifice aviation certified tester. The first thing one would do is what the article states which is to wrap the valve stem with a hammer. The valves, like the piston rings seal tightly from the operating dynamics of the valve springs and the engine's cylinder pressures. So it is likely that static spring pressure of the non-operating engine would no more close the valve tightly as laying a 5 pound hammer on a nail will drive that nail into a board.

I would worry far more about an exhaust valve leaking than an intake, though neither is good, but the exhaust valve is working in thoroughly vicious environment. Small hot gas leaks past it quickly become eroded ditches in the seats usually followed by cracks in the valve head and sometimes complete separation of the head from the stem. The exhaust valve is operating at a cherry red temperature, it just doesn't take much to wipe it out. A mismatch of seats, a piece of carbon or metal from a piston or ring failure, or the valve orbiting or sticking from wear of the stem and guide is often all it takes to set up the failure chain.

Let me refer back to something you said in a prior blog regarding your observed stem wear. Back in the day when the push rod was guided on the Chevy, the wear pattern usually was seen in the guide as a line across the width of the head. In other words a line from the intake to exhaust side. This was because the contact point of the rocker made a sliding radius across the end of the valve stem that pulled and pushed the valve in that direction. The Vortec uses self guided rockers and introduces the kind of wear seen in the Ford Small Blocks which is more orbital in general and can be more extensive in the front to rear orientation of the head rather than across the width of the head. Wear in either direction is bad because in operation it lets the valve face and therefore the seat have a rocking contact, again the exhaust is running red hot and it can be deformed by that motion. In the extreme on an engine running at WOT it could cause the head to separate from the stem. The Vortec head has a taller guide than earlier heads to try and control the wear from this motion for a longer period of time, i.e. postpone problems till after the factory warranty expires. Of course shortening the Vortec guide is all the rage for guys putting in high lift cams. This where I recommend they step up to screw in studs with guide plates and hardened push rods to take the job of push rod and rocker alignment off the valve stem and its guide as these already have plenty to do.

Bogie
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Old 01-21-2011, 08:56 PM
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Are you after performance or just want to get it running? There's a HUGE difference.
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Old 01-22-2011, 10:05 AM
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They say the only tribute to teaching is learning and there is very excellent information being given here, thank you.

Onto what Bob said about run out in any direction how is this properly measured? Is it combined so say you have .002 toward the exhaust and another .002 toward the intake, do you combine each way for .004 total?

I did smack the valves with a small sledge last night and retried the fluid leak test over the night and 6 valves had seated to 0% leakage. Following what Bogie said about static spring pressure, since there's no leaks and it's not an operational set up at this point the increased cylinder pressure would also aid. The stock springs will be changed with performance springs to match the new cam which would also aid. Would this put me in the clear? Sounds like I'm trying to avoid a valve job doesn't it

The two main things would be excessive run out creating a rocking relationship between valve and seat, lessening heat dissipation and a non concentric seat basically forging the exhaust valve. Correct?

Ap72, I'm going for street friendly performance for my 3rd gen Camaro. Goal is around 350hp with the 300hp cam + vortec heads and headers.

One other question regarding lapping just as an idea I've only lapped each valve maybe a total of 10 minutes. What constitutes excessive in this procedure?
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