How to adjust the valves on a single overhead cam engine
Of all the regular maintenance tasks on my vintage cars, the one I enjoy the most is valve adjustment. There’s a reason for that.
In his classic repair manual How to Keep Your Volkswagen Alive: A Manual of Step-By-Step Procedures for the Compleat Idiot, author John Muir explains why you, the car’s owner, can do a better job adjusting valves than any shop. It’s because the procedure must be done when the engine is dead cold. Thus, it is best done first thing in the morning. A repair shop is unlikely to let a car sit for that long; those folks will likely pull it into a repair bay when one opens up and work on it when it’s convenient. Learning that I could not only adjust my own valves but could do it better than a shop was one of my entry points into wrenching.
But once you do it, it’s addictive. It’s certainly very intimate. That’s a camshaft you’re touching, the thing responsible for opening and closing the valves that allow air in and exhaust gases out. In contrast, hanging a muffler and replacing brake discs barely scratches the surface of a car. For this reason, I’ve long felt that valve adjustment is one of those bellwether mechanical tasks, a predictor for whether you get sucked deeper into wrenching.
And what, exactly, are we adjusting?
A fair question. “Adjusting the valves” means adjusting the gap between the top of the valve stems and whatever is pushing them open, which could be the camshaft lobes themselves or a set of mechanical contrivances interposed between the two. You need to adjust the valves because, over time, due to mechanical wear the clearance between these two parts changes. Typically it gets bigger over time, which causes the valve train to clatter and the valves to not open quite as far as they should. But the more dangerous situation is when the gap is too small. If the clearance is too tight, there’s the possibility that the valve may not completely close, and that’s really bad. It prevents the cylinder from sealing fully during compression and combustion. If exhaust valves don’t close fully, the valve seats can start to burn very quickly, and then the head has to come off for a valve job.
Variations in cam location and adjustment mechanism
Up until the 1960s, most engines had the camshaft inside the block, and used long pushrods to reach from the cam lobes to the valve train in the head. Eventually, most engines adopted an overhead cam design that put either a single or double (one each for intake and exhaust) cam in the head right next to the valves, reducing the slop in the valve train. The general term for a piece of mechanical linkage that sits between a cam lobe and a valve stem is a “follower,” but on most of the German cars I own, there are rocker arms that pivot see-saw-like on rocker shafts. The cam lobes lift up one end of the rocker arms, which forces the other ends down onto the tops of the valve stems, opening the valves.
The mechanism used to adjust valve clearance varies, including the possibility of no adjustment being necessary at all. Most modern cars have oil-fed hydraulically-pressurized hydraulic lifters that automatically take up the gap and remove the need for any routine adjustment. On some cars with double overhead cams, both vintage and recent, the cam lobe moves a “bucket” which depresses the valve stem. Cup-shaped shims are employed on the tops of the valve stems, making valve adjustment an exercise in measuring the gaps, pulling out the existing shims, measuring them or otherwise reading their thickness, comparing those numbers to the measured gaps, calculating what the new shim thickness should be, and procuring and installing those shims. I may cover this when I get my Lotus Europa engine back together, as its twin-cam head uses shims.
However, the prevalent method used on most vintage cars is a mechanical one where the clearance is adjustable. On the vintage Volkswagens I worked on for many years, at the end of each rocker arm is a threaded post with an adjustment slot in the end and two lock nuts holding it in place. But on the vintage BMWs that occupy most of my garage space and mental bandwidth, instead of a threaded rod, at the end of each rocker arm is an eccentric—a circular adjuster with an off-center hole. You loosen the nut holding the eccentric, then rotate the eccentric until the gap between it and the top of the valve stem is correct.
Stepping through the valve adjustment process
Let’s go through the process on my 1972 BMW 2002tii in detail. This is a car with a single overhead cam, which, as you’ll see, provides an advantage in terms of valve adjustment.
First, as with any car, you need to block the wheels, engage the emergency brake, put the car in neutral so the engine can be rotated, undo the nuts holding on the valve cover, and pull it off. The more primitive the car, the easier this is. On an old carbureted car, there’s typically nothing in the way of the valve cover, but as cars progressed through the 1980s, items such as idle air control valves began to be bolted to the valve cover, requiring them to be swung out of the way first. Cleaning the top and the outside edge of the valve cover is a good idea, as it helps prevent crud from falling into the valve train when you pull the cover off.
Rotating the engine into position
Next, you need to rotate the engine into position where the first pair of valves can be adjusted. This is a subject that confuses many people, so let’s talk about it in detail for a moment.
In order to adjust the valves for a given cylinder, the valves for that cylinder must be completely closed. After all, what you’re adjusting is the gap between the top of the valve stem and whatever presses down on it, and if the valve is under tension and partially open, there is no gap to adjust. To assure that the valves for a given cylinder are closed, rotate the engine so the piston in that cylinder is at top dead center, which is, by definition, the position at which the piston is at the top of its compression stroke. In a car where the cam is in the block, you can’t see the camshaft, so you have to rely exclusively on the top dead center mark on the crankshaft for #1 cylinder. Thus, on an internal-cam engine, put a wrench on the crankshaft pulley and rotate it to line up #1 cylinder to TDC, adjust the valves for that cylinder, then rotate the engine 180 degrees in its normal direction of rotation and adjust the valves for the next cylinder in the firing order.
And what is the “normal direction of rotation?” On most cars other than Corvairs and a few Hondas, the engine rotates clockwise as viewed from the “front” of the engine (the pulleys and belts), and counter-clockwise as viewed from the “back” of the engine (the flywheel and transmission).
You also need to know the firing order, which is usually printed on the valve cover. For example, on my BMW 2002s, it’s 1-3-4-2.
So on a pushrod engine, the sequence would be:
- Rotate the engine to the TDC mark, which puts cylinder #1 at top dead center.
- Adjust the valves for cylinder #1.
- Rotate the engine 180 degrees in its direction of rotation (usually clockwise looking at the belts).
- Adjust the valves of the next cylinder in the firing order.
- Repeat until done.
The problem with this is that it relies on being able to see the TDC mark on the crankshaft pulley and to clearly delineate it from the timing marks. You also need to have a mark on the crankshaft pulley that’s 180 degrees from the TDC mark. These are all possible sources of confusion and error. If you’re adjusting valves in an old Beetle, the crankshaft pulley is right in front of your face, but in most other cars, it and the TDC marks are buried low in the engine compartment, making the process a bit challenging.
The method of the companion cylinder
If you have an overhead cam engine, the fact that you can see the camshaft gives you the ability to judge the cylinder position by looking at the cam lobes. If the cam lobes for a cylinder are pointing down, then the rocker arms for that cylinder are on the back of the cam, which is flat, and the valves for that cylinder should be closed.
The problem is that “cam lobes are pointing down” is an imprecise description. Fortunately, there is a way to make this very precise. It’s what I call the method of the companion cylinder. What I mean by this is that in a four-stroke engine, when one piston is at top dead center of its compression stroke with both valves fully closed, there is another piston in a companion cylinder that is at top dead center of its exhaust stroke with both valves fully open, and it is easy to visually tell when both valves are fully open because their intake and exhaust rocker arms are both raised up by equal amounts on their respective camshaft lobes. This is sometimes called the overlapped position. On a single-overhead cam engine, the intake and exhaust lobes on the cam for each cylinder form a “V,” and in the overlapped position, the rocker arms hang over into the “V” by equal amounts.
Why do you care? Because, by verifying that one cylinder’s cam lobes and rocker arms are in this overlapped position, you can tell very accurately that its companion cylinder is at top dead center, and thus that its valves are fully closed and ready to be adjusted.
The way you find the pairs of companion cylinders is pretty cool. You take the firing order, divide it in half, and write it down in two rows, one above the other. The columns then give you the companion cylinders. For example, the firing order in a BMW 2002 is 1-3-4-2. Dividing that in half and putting it in two rows, you get:
You now look in the columns for the companion cylinders (1 and 4, and 3 and 2). To give another example, doing this on one of my six-cylinder BMW engines whose firing order is 1-5-3-6-2-4, you get:
This says that 1 and 6, 5 and 2, and 3 and 4 are companion cylinders.
So, how exactly do you use the method of companion cylinders? The advantage is that you never need to look for the TDC mark on the crankshaft. Instead, you just look at the cam lobes, and can start at any cylinder whose lobes are overlapped, or nearly so. Going back to my four-cylinder BMW 2002 example for a moment, let’s say that you find that the cam lobes for #2 cylinder are the ones that are closed to the overlapped position. You use a socket and handle to rotate the engine to get them as close as possible to the overlapped position, then look in the little table and find that the companion cylinder to #2 is #3. You then look at #3 and find that, sure enough, its cam lobes are pointing straight down, and the rocker arms are on the flat part of the back of the cam, just where they should be for valve adjustment. You then rotate the engine in its natural direction and step through the rest of the valves in the engine’s firing order.
Note that the engine doesn’t need to be rotated into the overlapped position to millimeter accuracy. If the rocker shafts are about equally overlapped on the cam lobes, it is close enough. The back of the cam is very flat, and a small amount of rotation about this point isn’t going to change the valve clearance. But the point is that you’re able to judge the overlapped position of the companion cylinder much more accurately than you can judge that the cam lobes are pointing “down.”
The valve adjustment itself
What you need for valve adjustment is:
- The specification. You can find this in a repair manual for your car. It is usually listed as a range. For example, on a BMW 2002 it’s .006 to .008 inches for both the intake and exhaust valves.
- Knowing the range, you need a set of feeler gauges. If the range is .006 to .008 inches, that means that you should be able to slip a .006″ feeler gauge between the top of the valve stem and the adjuster, but not a .008″ gauge. Fortunately, you can buy “go/no-go” gauges that have a step in them that makes this easy. If you use a .006/.008 gauge, and the tip of it slides in without being forced, but it stops at the step, the adjustment is good.
- A wrench to loosen the locknut on the adjuster. On a BMW four-cylinder M10 or six-cylinder M30 motor, this is a 10-mm wrench.
- A tool to move the adjuster. On cars with threaded post-style adjusters, this is simply a screwdriver. On my vintage BMWs with eccentric-style adjusters, the tool is an Allen key that’s small enough to fit into the hole in the adjuster. The right angle in the Allen key is often necessary because, as the top of the valve stem wears, the eccentric must be rotated so that the hole in the eccentric moves down to where you need the right angle to slide into it.
- Depending on the car, you may need a new valve cover gasket. On old air-cooled Volkswagens, you certainly do, as the covers sit horizontal, but on my vintage BMWs, I usually get away with re-using the old gasket.
Having rotated a cylinder into overlapped position and verified that the cam lobes of its companion cylinder are pointing down, you take the correct go/no-go feeler gauge and slide it between the top of the valve step and the adjuster. It helps to push down with your thumb on the camshaft end of the rocker arm to be certain the gap is at its widest. If the tip of the feeler gauge goes in but it stops at the step, the valve is correctly. Don’t touch it. But if it’s too loose or too tight, you need to adjust it. If you think it’s a little too tight, adjust it. A little loose is better than a little tight. As I said, too tight can burn valves.
The adjustment itself is pretty trivial. It doesn’t matter if you do the intake side and then exhaust, or vice versa. But pick a sequence and stick with it. Loosen the locknut, move the adjuster, gently tighten the locknut, re-check with the feeler gauge, iterate as necessary, tighten the locknut. But there are some nuances.
In his “idiot manual,” John Muir describes two ways to do it: The “imprisoned feeler blade” method and the “memorized slot” method. In the imprisoned feeler blade method, you leave the feeler gauge in the gap while you move the adjuster. I used to do it this way until I slightly damaged the tip of a feeler gauge by snugging down on it too much. The “memorized slot” method is so-named because, on an air-cooled VW, the adjusters are little threaded rods with a screwdriver slot in the end, and two locknuts. The act of turning the locknuts often turns the adjuster, so you need to memorize the location at which the clearance was correct, and either hold it there with the screwdriver or return it there if it moves. This is less of an issue with eccentric adjusters.
I find that, on my vintage BMWs with eccentric adjusters, a third method works best: the “slightly snug” method. After doing this for a while, you can feel how much you need to loosen the locknut so that the eccentric remains slightly snug—so it can be rotated with the Allen key, but it doesn’t flop around. With it slightly snug, I adjust it so the feeler gauge goes in, partially tighten the locknut and check the clearance again with the feeler gauge, then tighten it for good and check it with the feeler gauge a third time.
If you’ve never done a valve adjustment before, it’s a good idea to turn the engine one more time, cycle through the cylinders, and make sure you’ve gotten all the valves right. Then check to be certain no tools are sitting in the valve train, replace the gasket and valve cover, tighten down the nuts, start the car, and make sure there’s no obvious tapping or rapping sound. If there is, stop! You’ve gotten something wrong. Wait for the engine to cool down and check it again.
In writing this, I re-checked John Muir’s “idiot manual,” and was surprised that he advises the valves on air-cooled Volkswagens starting at TDC and then rotating the engine counterclockwise as viewed from the pulley side of the engine. It took me a while to figure out why. On an air-cooled VW engine, the cylinders are numbered:
- Right front
- Right rear
- Left front
- Left rear
The firing order of these engines is 1-4-3-2. Thus, if you start at #1 TDC and rotate the engine counterclockwise, you’re adjusting in the order 1-2-3-4, which means that you can remove the right valve cover, do the valves for cylinders #1 and 2, replace the cover, then remove the left cover and do the valves for cylinders #3 and 4.
In general, however, it’s advised that you turn the engine in its natural direction of rotation. Although nothing should go wrong rotating them backward, timing chains and belts are typically tensioned on one side, and there’s really no reason to take the unnecessary risk that the chain or belt goes slack or gets kinked or skips a tooth.
All those years ago, in his “idiot manual,” John Muir said it beautifully: “Doing the valves, timing, and minor maintenance on your own car will not only change you relationship with your transportation but will also change your relationship with yourself.” Truer automotive words have rarely been spoken. Lay your hands on your valve train. You’ll feel like you’re king of the world.
Rob Siegel has been writing the column The Hack Mechanic™ for BMW CCA Roundel Magazine for 30 years. His new book, Ran When Parked: How I Road-Tripped a Decade-Dead BMW 2002tii a Thousand Miles Back Home, and How You Can, Too, is available here on Amazon. In addition, he is the author of Memoirs of a Hack Mechanic and The Hack Mechanic™ Guide to European Automotive Electrical Systems. Both are available from Bentley Publishers and Amazon. Or you can order personally inscribed copies through Rob’s website: www.robsiegel.com.