Rocker Geometry
By Ryan Ballou

Remember when you were in high school math class and someone blurted out, "When will we ever use geometry?" Well, there isn't a lot of math to deal with when it comes to rocker geometry, but all of the concepts that you learned in school are about to be put to use.One might think it would be difficult, but in fact it's pretty easy, if you know what you're doing.

The first thing you'll need to do is tool up a little for this job. Along with the basic array of tools you should already have anyway, you'll need a dial indicator with a standard magnetic base setup. The dial needs to have a resolution of .001" and a range of 0"-1". This entire setup can be had from about \$50 and up depending upon where you find it. You don't need ultra precision here, so don't think you have to go out and buy a Starrett or Mitutoyo brand. If you do your own engine work, you'll get plenty of use out of it for other projects as well, so don't go ultra cheap either.

The next thing you need you have a choice on. If you happen to already have a drill press then you're in luck if the head is off the engine. If not then what works best for me is a tool sold by many VW parts distributors that is used for replacing valve springs with the engine installed in the car. You can be creative here; you just need something to manually actuate the valves one at a time while you monitor the lift with the dial indicator. The catch is that you need some leverage, anything more than a stock valve spring is really strong.

The fact that you're doing geometry generally means this is a new setup for you, even if it's just the rockers that are new. If you've never checked for coil bind or retainer/guide interference before, then now is the time to do it. The first thing you want to do is remove all the valve springs from the head/s along with the rocker assemblies. Now mount the indicator however you see fit. What works best for me is to use a long (1") 8x1.25 nut like that supplied with bolt-on valve covers and use a jamb nut to fasten it to a rocker stud. Then if I unscrew the rod from the magnetic base for the dial indicator, it is threaded to 8x1.25 and will screw directly into this nut. Lock it down with another jamb nut and you're set. If yours is not threaded the same then don't worry. A versatile setup that utilizes the magnetic base can be made from an old oil pump cover bolted somewhere on the head and let the magnetic base 'stick' to it. Just make sure it's close enough to the valves so that the dial reaches them and can be aligned with the valve stems.

With the dial indicator setup and the heads stripped you're ready to check retainer to guide clearance. Slip a retainer onto the valve stem, put the keepers in place then pull the retainer up into the keepers. The dial's contact point should sit on the retainer rim and be perfectly in line with the valve stem for this to be accurate. Zero the dial with the valve in the closed position, then slowly open the valve by hand until either the keeper or retainer contacts the valve guide, make a note of what lift this occurred at and what valve it was. Now repeat the process for each valve on each head and record all your numbers and where you got them. Now would be a good time to make a chart on a piece of paper. Make columns for each valve, and intersecting rows for each measurement.

Time to check for coil bind, the condition where all the coils in the spring are touching each other, spring fully compressed. This is basically the same as what you just did, but now the coil will likely bind before the retainer has a clearance problem. If you're running dual springs, do the inner springs first then the outer springs, don't try both at the same time. Here's where that valve spring removal tool comes in handy, it provides the leverage you'll need to compress the springs. If the head is off the car you can install it on an angle plate on your drill press and use it for the leverage. Set the angle plate to 9.5 degrees to be sure the valve doesn't see side loading. Chuck a small ¼" steel rod into the mandrel and use it on the valve stem to open the valve. The dial should be setup in the same manner as before, in line with the valve stem and sitting on the retainer lip. Again, zero the dial with the valve closed and slowly open the valve until it stops on it's own. Make a note of the lift and what valve that lift corresponds to. Repeat for each valve.

A quick reference to your cam card will help you to determine if you have enough room until coil bind for the valve lift you're expecting to see. That is, if you're expecting to lift .550" and you coil bind at .540" you have a problem, likewise if you coil bind at .560" because it's still too close. A good safe number is .100" from coil bind on all valves; some people will allow it to be closer on intakes. If you find you don't have enough room then you either need to have your heads cut for the springs to sit deeper, use different springs with thinner wire, use different retainer, or any combination of those three. Retainer to guide clearance is dealt with the same way. If you're expecting more lift that you have clearance for, you'll need to get your guide cut down, in extreme cases longer valves are used for the added clearance. For retainer to guide clearance, I like to see more clearance than I did with coil bind. So if I bind at .660", I'd like to see retainer interference higher than that. In extreme situations when you float the valves, it's better that the spring bottoms out and bounces first, preventing a hard hit from the retainer.

Your coil bind numbers can be used to setup your spring shims so that they all bind at the same point and help to equalize spring pressure across all the valves. This part is easy, find the valve that hits coil bind at the lowest number install your thinnest shim here, it should be a .015" and shim the rest to match. This way all the springs are sitting on shims to protect the spring seats. So if the low valve binds at .635", install a .015" shim for .650". Then say another spring binds at .675", use a .025" (or the closest you can find) spring shim on the .675" valve to bring it down to .650". Repeat this for all the valves so they all bind at the same (or close to it) lift number. If you're running dual springs, just match the outers, the inners should end up close enough to not worry about it.

Next you should preset the adjuster screw position. If your adjuster screw has an oiling hole (adjuster at pushrod side or swivel style adjusters on stock rockers) you need to make sure this hole lines up with the oiling hole in the rocker arm itself. Screw it into the correct position and lock it down with the jamb nut, then leave it alone.

The last thing before going on to the geometry, you need to be sure your rockers are shimmed properly on their shafts, assuming you are running solid shafts. Think of the rocker assembly as three distinct sections separated by the two rocker studs. Each of these sections should be shimmed to about .004" side clearance. This is really easy to check with a set of feeler gauges. You can move the rockers side-to-side depending on where you put your shims. For wiper and roller style rockers, try to get the pad/roller to be centered on the valve stem. Aim for center of valve stem with ball type and elephant foot type adjusters on stock style rockers as well. If you are still using stock adjusters, then aim for slightly to the left of center on the valve stem; this will help the valve to spin slightly during operation keeping the valve seat clean.