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Shown below is a used rocker arm for illustration.
As you can see, especially with the picture on the right the stock castings are not the cleanest. All of these casting imperfections are a potential problem and are stress risers.
Think about it, if you take a piece of wire and put a tiny nick in the insulation and start bending it back and forth, where is the wire going to break? Where the nick is right. Or if you have something sticking out of something smooth and it rubs against something its going to get broken off, usually taking material below with it. This is what these stress risers are doing to the parts, especially on cast aluminum. To fix this we are going to do the same thing as Beam Polishing on connecting rods. This also helps lighten the part which by the time you are finished peforming this mod you will have lightened up the valvetrain by the weight of 1 full hydraulic rocker arm which is about 80 grams. We will also reshape part of the rocker arm and do some minor drilling to increase oil flow to the cam lobes.
Beam polishing is removing casting flash and burs off the surface of the part which can cause potential failure under high stress. Since we are actually removing metal to make the part stronger it is very critical that you know how and where to remove it otherwise you will weaken the part.
To do this you will probably need a dremel tool, a carbide bur like in the pictures below or sanding drum, 300 grit and 600 grit sand paper. You want to take the carbide bur or sanding drum and remove all of the burs and stuff sticking up from the surface, only take off the minimum amount of metal to bring it level with the surface.
When taking metal off you have to be careful which direction the tool is turning in relation to the part. You want all of the scratches to be going the same direction as the long axis of the rocker arm. This would mean that if you are using a carbide bur you want to try to keep it like I have shown in the picture above. If you are using a sanding drum you will want to try to keep the drum at a 90 deg angle to the long axis of the rocker arm, otherwise the scratches will make it more prone to break (like that notch in the wire).
Now that you have gotten all of the casting slag and burrs off it is time to slightly round most of the sharp corners. Also any abnormal indentations in the rocker arm need to be very carefully blended. Sharp corners are also stress risers. Think of when you strip a bolt or nut, the points are the first part to go. You don't need to round them much, maybe about 1/16" depending on where you're rounding it.
Now we will get a little more involved. We are going to remove that great big round spot from where they poured the metal into the mold. This spot will probably be the largest effect on the strengthing of the rocker arm. It is big and bulky adding to weight of the rocker arm and it is on the lash adjustor side of the rocker arm meaning it affects recipricating more. Just think think at around 4,000 rpm each of these rocker arms will be moving up and down at about 33 times a second.
Next we are going to cut the oil groove. This is going to be right above the cam follower. The best tool to use for this is probably the carbide burr shown above as it will give the parabolic smooth shape you want.
On the stock rocker arm oil puddles in the pocket where this groove is, the result is as the rocker arm is moving it slings oil up towards the valve cover. Oil all over the inside of the valve cover doesn't really help out lubrication much and the oil slung all over the place causes windage. This groove allows the oil to flow down the rocker arm and onto the cam lobe which should help decrease cam and cam follower wear not to mention reduce windage and rocker arm weight, not to mention the oil will be getting towards the bottom of the cylinder head sooner allowing it to get back into the oil pan sooner. Note that we are NOT removing material here to reduce weight, lighter weight just comes with the modification, the main purpose of the oil groove is to increase oil flow to the cam lobes.
Finally take your 300 and 600 grit sandpaper and polish the aluminum casting, you will want to move the sand paper along the long axis from the cam follower end to the lash adjustor end. You don't want the rocker arm to be perfectly polished. Engine parts need surface area to disapate heat to the air and oil, polishing the rocker arms perfectly smooth will reduce this surface area causing them to build up heat. Basically all you want to do with the sand paper is even out the overal surface of the rocker arm to distribute stresses more evenly.
The last modification probably only needs to be done if you are converting from hydraulic to mechanical rocker arms. Inside the block there is a restrictor to meter the amount of oil going up to the rocker arm assembly. On engines with hydraulic rocker arms this restrictor is sized to allow more oil to the rocker arm assembly because the hydraulic rocker arms require much more oil than the mechanical. When we convert to mechanical rocker arms there are fewer and smaller oil passages, this will result in less oil flow and higher oil pressure. To bring the oil pressure back down a little and get more oil on the cam lobes we can enlarge the oil port on the bottom of the rocker arm pointed towards the cam follower.
You will need a drill with a drill bit slightly bigger than the oil port, when I mean slightly I mean by the smallest drill bit you can find that will still enlarge it. An extra .002" or .003" is about all you need. Note be extremely careful and take your time while drilling, most likely you will break atleast one drill bit, also take some tape and put a small ring around the drill bit so that you don't drill through and hit the machined surface on the other side.
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