When it comes to hardcore race products – especially valvetrain related – Crower Cams and Equipment can rightly be called OG. For more than 50 years, the team at Crower has been designing and manufacturing go-fast parts for the competition market. In that time you might say they’ve become experts on the matter. As such, they have developed and refined the existing practices of determining valvetrain geometry and simplified it on their kits to the point of caveman-simple.
Rock Solid Rockers
Crower offers shaft rocker systems made from aluminum, stainless steel, as well as billet steel, covering a range of performance applications, from mild to wild applications. For this article, we’ll be focusing on their stainless steel offerings.
“Our rockers are made from a 17-4 Stainless Steel material with a 52100 [ball bearing steel] shaft, heat treated to our specs,’ explains Don Flanagan, Crower’s Production Manager. “We have a variety of tip options available, and they come complete with the stands, the adjusters, the hardware and shims.”
Crower has also recently released a set of rockers made from billet steel for the most extreme applications. “Those start from a blank billet of steel, are fully machined in-house, and then heat treated to our specifications,” explains Flanagan.
“You aren’t just opening that valve against spring pressure, you’re trying to open it against exhaust pressure, too. A lot of times, especially with blown cars, the cylinder pressure is so high, it can play havoc on the rocker arms especially on the exhaust side. Usually, if they are going to break, they’ll break the exhaust rockers. So those applications are when you’re really want the stainless or even the billet rockers.”
Geometry Isn’t Just For High School
There are a lot of factors to consider when setting the proper valvetrain geometry. Understanding what it is you’re trying to accomplish is the key to properly setting your rocker arms. “In a rocker arm’s geometry, we’re talking about two circles. In the center of the larger circle is the shaft of the rocker [which is the rocker arm’s pivot point] and then the outside circumference of the large circle is the center of the valve,” Flanagan explains.
Those two distances will vary by manufacturer and application, but ideally, you want the roller tip of the rocker arm to line up with the center of the valve. In this application, we’re using a big-block Chevy. “For a big-block Chevy, the measurement from the center of the fulcrum to the center of the valve is 1.650 inches,” says Flanagan.
That 1.650-inch radius, translated into a 3.300-inch circle, scribes the arc that the rocker arm will follow when in motion. “Now, ideally, at half of the max lift of the camshaft, you want the point where the valve and rocker meet to be exactly perpendicular to each other,” Flanagan explains.
“Most of the pressure comes in on the opening ramp of the cam lobe. It creates more stress on the rocker arm at mid lift, than at full open, which is why we want the strongest configuration of rocker arm to valve at mid-lift.” Getting that 90-degree orientation, or “square” as Flanagan calls it, at exactly half of the camshaft’s maximum lift is what setting valvetrain geometry is all about.
Nothing To It But To Do It
While setting the proper valvetrain geometry may seem like a very complicated task, Crower has made it exceedingly simple to do with their rocker arm kits. “What we’ve done is we’ve made a gauge for setting the rocker stand up,” says Flanagan.
“In order to do that, we’ve taken half of the diameter of the roller tip of the rocker, and half of the diameter of the shaft. We add those two together and that gives us a number. Then we take the cam’s lift number – the kit’s base dimensions use .750-inch lift. So you take half of that, which is .375 and subtract that from the number we got earlier, and in this case that happens to be .212-inch.”
Because Crower knows the dimensions of all the parts involved, and makes a specific assumption on the only variable (the camshaft max lift) they can boil all of the math down to a simple gauge that has the precise offset required to achieve the perfect 90-degree angle between the rocker arm and valve at exactly the middle point of the cam’s lift.
“On a big-block Chevy system we have a one-piece intake stand, where they are all joined together, which also acts like a girdle for the head. The first thing to do is to lay the stand on the head, with no shims, and put a couple bolts in there to hold it in place,” directs Flanagan. “Then I drop the shaft analog in the stand, place the gauge on the tip of the valve and measure the distance between the ‘shaft’ and the end of the gauge.”
Assuming you are using a .750-inch max lift camshaft, the math is easy at that point. Just measure the gap when you set the gauge on the valve, and shim the stand to close the gap. However, what if you are using a different max-lift on your camshaft?
“If you have less lift, you need to halve the difference between your camshaft’s max lift and .750, and use feeler gauges to shim the stand gauge off of the valve by that amount,” Flanagan says.
“If you have a cam with a max lift larger than .750 inch, then you do the same measurement, but apply the feeler gauges to shim off of the shaft end. More lift, and the bracket has to go down. Less lift, the bracket has to come up. That will give you the proper geometry, putting everything in the strongest possible position.”
Centering The Roller Tip
Even with the measuring tools simplified as much as these are, Crower still gets questions about how to set up the rocker arms. “We get a lot of calls from customers saying, ‘I can’t get the tip right in the middle of the valve.’ They are trying all sorts of height changes to get the rocker tip right in the middle of the valve stem. The thing is, having the tip of the valve centered isn’t as important as getting the height of the stand correct. The important part is to get the geometry correct,” says Flanagan.
After you get you your stand height right, you can mark your valve and check your wear pattern. If you think that it can stand to move one way or another, you can always notch the stand mounting holes slightly and slide the stand over to get the roller in the center of the valve. Since you aren’t changing the height of anything, your geometry is still correct.”
With their engineering experience and racer’s mindset, Crower has taken the complex subject of valvetrain geometry and simplified it into a genuine bolt-on part that shouldn’t take much effort on the part of the end-user to get right.