In this article I would like to talk about rocker arm geometry. Yeah I know, you gave up geometry class back in high school. I promise this is only basic math. On any motor that has long pushrods, rockers and pushrods are a problem area. Now a VW motor is worse than most, in that the pushrods are not straight in relationship to the lifter. Not only are they at an angle up and down to the lifer, they are at an angle left and right to the lifter. This just compounds the problem. Where I am going with this is that getting the right geometry at the rockers is important to keep this problem at bay. “I don’t care, my engine is stock” Well have the heads ever been flycut? Do you have barrel shims? Anything that affects the distance from the centerline of the rocker shaft to the centerline of the cam will effect how long the push rods needs to be. So please read on and at least check what you have.
So here is an easy way to check what you have to make sure you're in the ballpark. Open the valve on one cylinder half way. Do an exhaust valve so you can get a good look at the valve stem. Keep in mind you will be looking through a spring. The best way is to use a dial indicator on the valve spring retainer. If the motor is in the car this is kind of tough to do, so just use what you got and do your best to find out total lift of your cam at the valve.
Here comes the math. Divide that number in half. Whatever it is, open the valve that far. So half way open. Please use something to measure it. Guessing doesn’t work here. If your adjusting screw is at the valve, look through it and see if it is in line with the valve stem. In other words if you drew a straight line along the valve stem that line should continue through the valve adjusting screw. Here is a picture describing what I am talking about.
If it is then yahtzee! You are good to go. If not then read further. If your adjusting screw is at the push rod end (ratio rockers) then again open the valve half way and site through the push rod and the adjusting screw. “Awh man, mine is not, now what?” Well the reason it matters is we want the same angle, when the valve is all the way closed as we do when the valve is all the way open. If it's not then you run the risk of the push rod jumping out of the “cup” it rides in on the rocker end. Now keep in mind this does not cure valve float. That’s a spring problem.
Ok so it's not right, what can I do about it? Well you can first try shimming out the rockers. They make shims that you can put under your rocker arm stands that will make the distance from the cam further. This in effect shortens the push rod. Now as a general rule I will only put up to .090 under the stands. That is just my rule. If I can't get it with that then it’s all about getting shorter pushrods.
How do I go about getting the right push rods? What I do is use an adjustable push rod. You can buy one if you don’t have one, or make one with an old push rod by cutting one about and inch from one end and threading the hollow ends and installing a stud. Keep playing with the length until you have what we talked about before. Once you have it, measure the length and call someone who can make them for you. Please keep in mind on a stock deal it is very rare that you have to have custom push rods made. Also keep in mind if this motor is bigger than stock you usually have to have custom rods made. Hey, it's just part of the deal when you go with a bigger motor.
Is there better ways to do this with more math? Sure there is, but this is a way an average Joe working in a home garage can do it and keep their pushrods and valve train happy.