They Don’t Call It The “Power Section” For Nothing
Fourth Edition, Vol. 1
This month I’m going to attempt to educate you on the finer points and importance of the venerable Power Section of the internal combustion engine. It doesn’t matter if it’s a 36 hp. VW engine, a fire breathing 3,500 hp. supercharged Pratt & Whitney R-4360, or a 350 hp. Compression Ignition (Diesel) engine. Any engine with poppet style valves are in this category. Even your Briggs & Stratton lawn mower engine is subject to this topic.
First, let me start off be saying that this topic will generate a lot of discussion on whether my opinion on this subject is really that critical and if it’s really that important. So, you can read along and at the end of this article, decide for yourself.
O.K., in all of the above described engines, they all have many commonalities, but the one thing that we’re going to talk about here is the valve train. This includes the intake valve(s), exhaust valve(s), their respective seats, the valve guides and their relation to each other. When I was working for Jerry at the VW shop, he gave me some very extensive training on this subject. I then later on did some research of my own on this subject and, after some eye witness accounts, came to realize that no one really pays that much attention to this very critical aspect of engine design, repair and overhaul.
This concept is very simple, yet I am continually amazed at how neglected this process seems to be. The concept itself is fairly straight forward. It requires some specialized equipment. Either archaic or sophisticated depending on one’s attention to detail. The concept I’m referring to is valve seat concentricity.
While attending the Aeronautics course at Victor Valley College in the early ‘80’s, one of our tasks was to “dress” the valve seats on one of the engines. Our Instructor brought out this odd looking drill motor looking thing with a fairly straight drive “chuck” on the end. It had an adapter on the drive end for these different grinding “stones”, which came in different angles for the different valve seat angles as well as different diameters and stone densities. Coarse for fast cutting and fine for finishing. Now, I had heard of these machines and had even seen pictures of these elusive tools in ancient automotive and aeronautical books dating back to what seemed like Roman times, but had never seen one in person so to speak. Our Instructor proceeded to educate us on the finer points of operating this heavy, bulky, and growling behemoth onto some sacrificial valve seats on our teaching specimen. This “tool”, in no way what-so-ever, did it “finesse” the surface of the valve seat even with the finest stone wheel attached. It managed to hog out the width of the seat beyond the published dimensions in such swift fashion, that not even the quickest trigger man of our group could keep from over grinding this surface. After destroying several different cylinder heads, we finally managed to present something somewhat resembling the desired result. But, it took us all freakin’ day to get there. The surface looked good with the same width all around and at the correct angle too. That afternoon when I got to work, I told Jerry of this archaic process of dressing valve seats, and he continued to educate me on the history of this now extinct dinosaur of modern equipment of the day.
I borrowed Jerry’s set of Neway brand of seat cutters, and the next day headed off to school. I had learned to cut valve seats on this equipment and was fairly competent with it. After a brief presentation and tutorial to the class, I proceeded to compare this hand “cutting” on our Continental O-200 test specimen to the aforementioned gouging and obliteration experienced the day before with the power equipment. The first thing I did was to apply a liberal amount of blue “Dykem” to the valve seat so I could see how much material I was removing and where. I chose the appropriate expandable pilot, and selected the 45° cutter. After about four or five revolutions of my cutter, I removed it only to be shocked to see that the Dykem was only removed on one side of the valve seat. This was new to me. So, I thought that maybe the pilot was crooked in the guide. So, I turned it 180° and tried again only to get the same result. How can this be? The old style motor and stones couldn’t be that far off, could they? So, I tried it again, and you guessed it, the same thing. So, I checked both pilots to see if they were bent. Nope! What could it be? With the help of some of my fellow classmates and our Instructor, the light finally came on. Apparently, the weight of the “valvasaurus” at the upper end, was enough to bend the pilot just enough to keep the stone from making full contact on the seat surface. This, and the fact that the stone itself had “cupped” a bit kept it from making a full, flat contact with the seating surface. This phenomenon is what I call valve inconcentricity, and unfortunately, I see it all of the time.
It doesn’t seem to matter whether it’s a freshly overhauled cylinder head, or a new unit from any of the manufacturers. I have seen non-concentric valve seats on just about every cylinder head I take apart to check or recondition. Why you ask? I don’t know. When I receive cylinder heads back from the shop, (and I mean any shop, no any one in particular), I always have to re-cut the seats. Yeah, they’ve been “stoned” and the seat width appears to be the same all around the contact surface, but when I use my carbide bladed Neway cutter, it never makes complete contact all the way around the seat surface. And, I have to remove quite a bit of material to make full contact. Even new cylinder heads are not exempt from this phenomenon.
So what’s the big deal? Well, I’m getting to that. First, it is imperative that the valve faces make complete uninterrupted contact on the valve seat sealing surface. If there is a portion on the seat that the valve is not making contact with, the hot combustion gases will escape through this small opening, and over time will either burn a larger opening in the seat, or burn the edge of the valve that is not contacting the seat. This is especially true of the exhaust valve. The valve conducts the heat away through the contact surfaces of the seat, and through the valve stem, and through the valve guide, then through the cylinder head. This also brings up the other important aspect of the contact area, the seat width. If the seat width is to narrow, then the valve face will eventually “cup” caused by the heat transfer contact area being too narrow, and the valve will subsequently run hot. Conversely, if the seat width is too wide, then the seat pressure will be too low to break up any carbon deposits that can collect on the seat or pass through the port and will lead to the above scenario also. Seat pressures are very critical to proper operation and longetivity. Too wide of seat is not good, and too narrow of seat is not good either. If you don’t have any specifications to go off of, then a good rule of thumb is to machine the valve seat width to a third of the valve face width. The contact area should be at the upper third of the valve but not to the edge. You do not want the contact area too close to the edge of the valve or it will burn that thin outer edge of the valve. Especially the exhaust valve.
Now, some guys say that this concentricity that I speak of is bullshit and that the valve will eventually “seat” itself after running a bit. WHAT? Seat itself in? How does it do this? If the valve doesn’t seat completely and evenly all around the seat, then there exist some stress on the stem of the valve if only one side is making contact. “Well, the heat will help the valve find its way to full contact”. WHAT? You mean that the stress of only contacting one side of the seat will be neutralized by the heat and the valve stem will “bend” and help the valve conform to the non-concentric seat? How does this do this when the valve train is designed and machined to rotate the valve so it doesn’t continually contact the seat in the same place every time? Most engines have some sort of offset rocker arm or adjustment screw to rotate the valve when it gets pushed in. Others use a Roto-coil as well as the offset contact area of the actuating component to lessen the friction caused by the valve springs and to enhance the rotation of the valve. This “seating “ itself in concept is bullshit to me as it can’t be good when we are accepting predictable induced stress into our valve stems, especially the exhaust valve. Not to mention the uneven wear the guide will be experiencing. Have you ever heard of a “dropped valve”? I have, and I have seen it also. And, I have seen what the catastrophic results are from a valve head running loose in a combustion chamber and trust me, you don’t want to be in the vehicle when it happens. Especially in the air.
So, what do I do that’s so important? I’ll tell you. This is how I go about this task. Let’s use a Lycoming O-360/O-540 parallel valve cylinder head. First, I get out the Lycoming Direct Drive Overhaul Manual. I then refer to figure 6-26 and find my particular engine. I then look across and find the dimensions I need to shoot for in both the seat width and the overall diameter. In this case, the seat width for the Intake valve is .076 to .117. I shoot for .090 to .100. I don’t like the intake seats too wide. Next, I find the exhaust seat width. Here, it’s .058 to .077. I like .070, so I shoot for that. I get out my pre made gauges that I made years ago of these dimensions to check each seat width as I go. I apply some blue or red Dykem on each seat and while that stuff is drying, I pick out a good fitting solid pilot out of my Neway kit. I like the solid pilots as I can also use them to check the inside diameter of the valve guide. Next, I choose the 31° cutter, ( I like the 1 degree difference as recommended in figure 6-30), and give it about three or four turns. I remove the cutter and see how bad the contact surface is. I’ll continue with this cutter until I’m just over my desired target width. I then will use the 15° cutter to narrow the top edge. This is very important, as you don’t want to be too wide here or the valve contact area will be too close to the edge. I then will use the 75° cutter to narrow the bottom edge of the seat just enough to obtain the correct seat width. I’ll continue this process until I have the desired seat width, and the correct overall width. I’ll do this same procedure on the exhaust valve seat but will use a 46° for the same reason as stated above. I remove as little as material as possible. After I think I have what I want, I’ll take a valve and insert into the respective position and rotate it a few times, remove it and check for the contact area on the valve face. If it’s within the desired margins and the contact area is not too far out on the valve face, I’ll go on to lapping this valve with this seat. After lapping in each valve, I check for the desired contact area again and also for the width and to make sure I have complete contact all the way around the seat. I then will mark each valve with a Sharpie with that particular cylinder position number. Then move on to the next one and so on. If this procedure is performed properly, you should be able to install each valve in its mated seat, plug the spark plug holes, and fill the cylinder with clean solvent, and with only the weight of the valves, have no leakage of solvent into the ports from any of the valves that were done this way. If you cannot achieve this effect, then the procedure needs to be performed again.
Unfortunately more often than not, this lack of proper practices, and the result of improper sealing exists, and is being viewed as acceptable work that can and will eventually lead to, among other issues, burned valves, burned valve seats, worn valve guides, “tulip” shaped valves, necked valve stems, dropped valve heads leading to subsequent pre-mature engine failure and/or subsequent pre-mature top end overhaul. Not to mention the potential power loss of not containing all of the gasses within the combustion chamber. Plus, your compression test numbers will be lowered by any “blow by” and per TCM SB03-3, no leakage past the exhaust valve is permissible.
So, what do you think? Will the heat of engine operation help the valve head to eventually make full contact with the non-concentric valve seat and relieve valve stem stress related to this uneven application of valve spring induced pressure? Or, would you rather eliminate the risk of “dropping” a valve and do it my way? You choose.