How do you check your geometry?
I check for the usual items. The roller tip is well positioned on top of the valve, the pushrod cups don't interfere with the bottom of the rockers and pushrod holes in the intake have enough clearance. I try to avoid anything that involves the use of Math. Keep in mind I'm just a home builder much like the majority here so I leave the technical stuff to those who know what they are doing.
In the past 12 years of racing I've bent a total of 2 pushrods and broken zero rockers. Did lots of other damage but rocker geometry wasn't the issue. Over rev and valve float cause trouble.
So basically you guess -and I don't mean that insultingly. Geometry is all about math. Rocker geometry is so simple, yet so completely misunderstood and ignored throughout the industry and one the most important things there is when assembling a performance engine. I put it right there with bearing clearances and degreeing your cam. The internet is a great place and has really opened my eyes to how much terrible mis-information there is regarding pushrod length and rocker geometry, even by so called "experts" and "professionals". For decades, Comp Cams put in print, in their catalog, probably the absolute WORST technical information and diagram when it came to measuring for pushrod length but hey, it's Comp Cams, so it has to be right, right? A lie repeated enough times becomes the truth. People invest thousands of $$ in a performance engine, buy a custom cam, custom rockers, custom pushrods, and have NO idea whether or not their rocker geometry is even close. The engine's performance DEPENDS on that geometry being
correct in order to get the most accurate information from the cam to the valve, yet it gets passed off as almost trivial. The first and BIGGEST misconception and most repeated lie about rocker geometry is this; proper geometry is when the roller tip sweep pattern is in the center of the valve. That is absolutely wrong and you're almost
guaranteed to have improper geometry if you use that as your guideline. Another misconception is that you use pushrod length to establish proper geometry. Pushrod length is a result of proper geometry. Shaft rockers can be set up with the heads on the bench. You don;t even need a pushrod. All you need to know is your net lift @ the valve. Proper geometry is this; when the centerline of the rocker is at 90* to the valve at mid lift. The center line of the rocker being an imaginary line that passes through the center of the rocker pivot, and through the center of the roller tip. When this imaginary line is 90*to the valve stem at mid lift, you have proper "mid lift" geometry. There are those who prefer a slight variation to this but for the most part, this is the industry standard. What this accomplishes is to give the minimum amount of roller sweep over the valve tip. Contrary to popular opinion, it is not being off centered that causes guide side loading and excessive wear, but it is excessive roller sweep on the valve tip that causes premature guide wear. Of course, in a perfect world we would like to have minimum s weep and be perfectly centered on the valve tip but that rarely occurs. Being centered is secondary to proper geometry and having the minimum amount of sweep. I use the center third of the valve tip as my "safe" working range for the sweep pattern. If it's within that range, it's good to go. If outside that range, then we have to do something about it and start looking at options.
My 390, basically all stock as far as deck height, cyl head milling, head gasket thickness and valve length needed .220" spacers to get the geometry right with Sharp roller tip rockers. That's not even close!!!! And yes, when I got the rockers where they should be, I had to clearance them for the pushrod cups. Oh well. That's what you have to do sometimes. I talked to Randy Jr at PRI about this and he seemed receptive to the suggestion of raising the stands. Of course, he's probably got bigger fish to fry so I'm not sure where that landed, but he said he had heard the same thing from several "FE" people.
Here's a diagram to help visualize what I'm talking about afa "mid lift" geometry;
Here's a short video I did for stud mount rockers but the principles apply to shafts as well.
https://www.youtube.com/watch?v=o5is9BsH5OU&feature=youtu.beBecause I work in cylinder head and induction development, rocker geometry is a real pet peeve of mine. There is power to be had or lost with it. There is longevity and reliability to be had or lost with it, and it's about as simple as it gets to do it right. I've had many, many, professional engine builders call me to thank me after watching my video, telling me how much sense it made and how easy it was, and that they had been doing it "wrong" their whole life.
I will edit to add: this all above only applys to roller tip rockers. The factory shoe type rocker has an entirely different geometry and NONE of this works with that type of rocker.
