BACK to High Performance 101 index page Next >>

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.

With the springs setup, you can now move on to the actual geometry setting part. The idea here is to setup the rocker arms to reduce wear on the their contact points, wear on the valve guides, and wear on the valve stems. The more lift at the valve you're running, the more important this becomes. Now this is a little different depending upon your rocker style, basically there are two. One kind has the adjuster at the pushrod side (wiper pad or roller tip), the other has the adjuster contacting the valve stem. For the adjuster at the valve stem, you want the adjuster to be in line with the valve stem at half lift. For the style with the adjuster at the pushrod, you want the pad or roller to contact the valve stem in the center of its contact patch. Another way to view this is that if you drew a straight line where the valve side of the arm was, it should be perpendicular to the line of the valve stem. This ensures that any side loading of the valve stem is kept to a minimum through its arc of travel. Starting to see where the 'geometry' part comes from? Finding half lift is easy; install some temporary pushrods and the rocker assemblies as though you were buttoning it up. Now crank the engine over by hand and use your dial indicator to find max lift. Divide this by two and you have your half lift figure. While you're at it, compare this max lift number to the numbers on your chart and verify you clearances are still acceptable. With your recorded full lift numbers take an average of them all and find half of the average value. If you do a little math you can also now find what your rockers 'true' ratio is by comparing numbers to your cam card.

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.
Time to actually set the geometry. If you're using 1.4 or higher ratio rockers, you may need to start with a .060" shim under the rocker bases or you'll experience coil binding that you shouldn't be seeing. With the rocker assembly installed on the head, the dial indicator in place and zeroed and no lift (turn the engine over if needed) on the valve you're working on, turn the engine over by hand until you see your average half lift figure occur. Look closely at the contact point to see how it's sitting. If it's sitting high on the valve stem, i.e. the adjuster is tilted up at the valve or the wiper pad/roller is touching below its halfway point then you need to add shims. If it's sitting low, i.e. adjuster tilted down at valve or wiper pad/roller touching above its halfway point, then you need to subtract shims. Finding the right thickness shim for your need is hit and miss, stacking shims is allowed. Trial and error is usually what it takes to find what combination of shims will work for you. If you find you have no shims but still need to subtract shims, then double check everything first. If everything checks out, then you might need to have your rocker base machined down a little.

Well the hard part is done; time to figure out pushrod length. For this you'll need an adjustable pushrod, you can either buy one or make one. To make one, cut about 1" out of the center of a spare pushrod, tap it to 6x1.0 and use a 6x1.0 stud with two jamb nuts to secure it's position. Your adjuster should still be locked in place with the oiling holes lined up. If you are using stock adjusters with no oiling holes, then just center it in the rocker arm. Turn the engine over so that the valve is at zero lift, then install the pushrod from under the engine and adjust the stud to expand the pushrod into position. You'll need to remove a pushrod tube for this. This is just like setting valve clearance, you'll be at TDC, but instead of turning the adjuster, you adjust the pushrod length itself. This ensures the pushrod is the right length for the geometry set where it is, and that oil will pass through the oiling holes without restriction.

Take that adjustable pushrod (keep it set to that length with the jamb nuts) to your local VW shop and tell them you need a set that length. The cost to cut and assemble them should be around $30, and the cost of the pushrods will vary greatly depending upon what you choose. When you get back from the shop, install them like you would normally and set your valve clearance. Note the position of the adjuster and be sure the oiling holes still line up; if the new pushrods are the right length the all should be well. Button the engine back up making sure to replace that one pushrod tube. Fire it up and listen for signs of any obvious problems.
Then drive it like you stole it.

1-This is how the shaft on the dial indicator should line up with your valve stem when taking all measurements. To better illustrate this the dial is located on the valve stem itself, when taking your measurements it will be on the rim of the retainer.
2-Another benefit of using spring shims is that it prevents chaffing damage to the spring seat. The worn off copper coating on this spring shim is proof of this.
3-When checking for retainer/keeper to valve guide clearance you simply press the valve stem down with the retainer and keeper halves installed and take your measurement.
4-Checking for inner spring coil bind. The inner spring is will often bind before the outer spring so pay close attention here and record everything
5-Checking for outer spring coil bind. Since the outer spring is heavier, it will take a little extra effort to force it to bind up for this measurement.
6-If you drew a line through the adjuster screw here you'd see that it is way out of line with the valve stem. This particular photo illustrates the need to add rocker base shims.
7-A little harder to see, this photo illustrates the opposite, the need to remove shims or move the rocker assembly closer to the head.
8-This is what you're going for here. Do all your trial and errors mockups with different shims on one valve. Once you think you have it, check the other valves. Some may vary due to machine tolerance differences; you'll have to find a happy medium here. In most cases the differences won't be enough to worry about
9-Good rocker base shims should fit snugly on the rocker stud. I generally have to use a deep socket and tap them down on the shoulder of the stud.
10-If you shop stocks and adjustable pushrod you may just want to buy one, they're really cheap. Mine didn't, so rather than wait for a special order I made my own. Don't try to turn the engine over with this installed, dual springs are strong enough to easily bend the little 6mm stud.

BACK to High Performance 101 index page Next >>

About us | Contact | Events | History | Home | Images | Interactive | Links | Tech