How to Adjust Valve Clearance to Improve the Condition of the 4-stroke Engine?

The intake and exhaust valves of a 4-stroke engine are important components that control the flow of air mixture in and out of the exhaust. They open and close in response to the rotation of the camshaft, but there must be proper valve clearance between the cam and the valve. The engine will perform much better when the valve clearances are checked and adjusted with a thickness gauge.

Valve clearance can't be too narrow or too wide.

An example of a valve clearance measurement. An adjusting screw is attached to the end of the rocker arm that moves like a seesaw as the camshaft rotates, and the tip of the screw pushes the tip of the valve stem to open and close the valve. The valve clearance is measured by how many millimeters of a thickness gauge can be inserted into the gap between the screw and stem.

A four-stroke engine is a four-stroke engine in which the mixture flows into the combustion chamber when the intake valve is open, ignites the mixture with the intake and exhaust valves closed, causing explosive combustion, and then opens the exhaust valves to expel the exhaust gases. The valves are opened and closed by the camshaft, and there is a certain gap between the valves and the car, known as "valve clearance" or "tappet clearance".

The heat from the engine heats up the rotating camshaft and the linear reciprocating movement of the intake and exhaust valves, which are each subject to thermal expansion. In particular, the valves are exposed to the combustion of the mixture, and the exhaust valves, which are in contact with hot exhaust gases, become particularly hot. Valve clearance is required because the valve is closed to the camshaft as it expands thermally due to the rise in temperature. Conversely, proper valve clearance allows the intake and exhaust valves to operate ideally when the overall engine temperature rises.

Each engine has its own set of valve clearances, but for the reasons mentioned above, the exhaust valves are usually larger than the intake valves.

So what can happen if the valve clearance is out of the proper range?

If the clearance is too narrow, the valve may be left open all the time because when the valve expands, it hits the camshaft. This will cause compression to leak out even when the piston reaches its upper compression dead center, and engine horsepower will be reduced. If the clearance is narrow for certain cylinders in a multi-cylinder engine, idling problems can occur.

On the other hand, if the clearance is wide, the camshaft mountain will not be able to push the valve as far as it should go, so the valve will not stay open as long and the amount of lift will be reduced, and intake and exhaust efficiency will be reduced. Also, when the camshaft pushes on the valve, the sound of contact and hitting will be louder, and you may hear a strange "tick-tock-tick-tock ......" sound from around the cylinder head.

In general, valve clearance tends to decrease as the mileage increases. This is a symptom of the valve umbrella (valve face) and cylinder head valve seat wearing out as the valve opens and closes, due to the strong force of the valve's umbrella (valve face) and the cylinder head's valve seat striking each other as it opens and closes, causing the valve to retract slightly, bringing the end of the valve stem closer to the camshaft.

Although the service manual does not specify this as a regular maintenance item, it is not a no-maintenance solution, as wear and tear will inevitably occur over the course of the mileage.

There are two ways to open and close the valve.

If adjustment is necessary, loosen the lock nut and turn the screw. At this point, keep the thickness gauge in place and turn the screw and slide the gauge in and out to find a tightness that is not too much or too little resistance. Do not turn the screws together when tightening the locknut.

Adjusting screws and nuts. The two screws on the left are new and have smooth screw tips that make contact with the valve stem. As the mileage increases, the tips may become unevenly worn and the contact position with the stem becomes incompatible when the screw is turned, and the valve clearance may not be measured correctly. If the screw tip is scratched, replace it with a new one.

There are two types of mechanisms that open and close the valve, although proper clearance between the camshaft and the valve must be checked and adjusted to maintain proper clearance.

The first is the "direct drive" type where the camshaft presses directly against the end of the valve stem, and the second is the "rocker arm" type where cam movement is transmitted to the valves by a seesaw-like part. The rocker arm type is often used in OHC engines that have a single camshaft, and both direct drive and rocker arm types are available for DOHC engines.

Adjusting valve clearance requires some sort of adjustment mechanism, and the direct-drive type and the rocker arm type are very different in that regard. With the direct-drive type, a thin metal disc called a "shim" is inserted between the camshaft and the valve, and the thickness of this disc is used to change the valve clearance. There are two types of these shims, inner shims, and outer shims.

In contrast, with the rocker arm type, the valve is opened and closed by the tip of a screw called an adjustment screw at the end of the rocker arm type, and the clearance is adjusted by turning this screw. In other words, it is a direct-drive system and can be adjusted without the necessary shims.

However, some rocker arm engines use shims, which require a change of shims.

Whether it is a direct-drive or rocker arm type engine, the valve clearance should be checked using a set of thin metal plates called a thickness gauge. The thinnest of these metal plates, called reefs, can be as thin as 0.01mm, and the set contents vary depending on the measurement range.

Measurements are taken with the engine cold and the clearance values listed in the service manual are used to determine if adjustments are necessary. Clearance varies depending on the engine, for example, Honda Ape and Super Cub are 0.05mm for both intake and exhaust, while Kawasaki Zephyr χ is 0.08 to 0.17mm for intake and 0.17 to 0.26mm for exhaust. The Kawasaki Z1 has an intake and exhaust range of 0.05 to 0.10mm.

The Ape and Super Cub are zero. The reason there is a wide range between the two Kawasaki models is that the former uses rocker arms with an adjustment screw, while the latter uses shims and direct hitting, compared to the 05mm.

When adjusting shims, you need many shims with different thicknesses. It would be ideal if it could be prepared with a difference of 0.01 mm, but it is not realistic to set such detailed parts in a mass-produced commercial vehicle. Therefore, in Z and Zephyr χ, the thickness of the shim is set in increments of 0.05 mm. Therefore, a certain width is allowed for valve clearance. Zephyr χ has a clearance range of 0.09mm, but with this range one of the 0.05mm shims will be in the strike zone.

Some models (Honda CBR250R) have a shim thickness of 0.025mm, which allows for a narrower range of valve clearance, 0.16±0.03mm for intake, and 0.27±0.03mm for exhaust.

When measuring valve clearance with a thickness gauge, it is done at the top dead center of compression where the camshaft is not pushing the valve. On a multi-cylinder engine, even if one cylinder is at the top dead center, the other cylinders are pushing the valves open, so the crankshaft should be turned to the top dead center for each cylinder. At this time, if the spark plug is attached, the air is compressed in the combustion chamber and the crank will try to turn with momentum, so it is better to remove the spark plug to make it easier to reach the upper dead center.

In this state, insert a thickness gauge into the gap between the cam and the valve, but there is no need to stick to the specified value, as it is important to know the current state of affairs depending on the thickness of the leaf. If the specified value is 0.0 for 05mm..If it only goes in an 02mm gauge, that's okay, because that's the measurement to adjust it.

If you have a single-cylinder, 2 valve Ape, you only need to take 2 measurements. But for the Zephyr χ, you have to take the measurements at all 16 points.

Shims that require extensive adjustment to the adjustment screw

Outer shim type Kawasaki Z1 cylinder head. The shims sit on top of the tappet, so the shims can be attached and removed by holding the outer edge of the tappet with a special tool. Be careful not to turn the camshaft unnecessarily, as the camshaft can be scratched by the tool.

The shims are about the size of a 5 USD and are common to all the intake and exhaust valves. The larger the adjustment range, the more shims are needed. By changing the position of the shims on each intake/exhaust valve and cylinder, you can adjust the clearance to the correct range.

If the results of the measurement are out of the specified value, adjustments are made.

If it is an adjusting screw, loosen the lock nut and turn the screw. At this point, if it's an Ape, slowly tighten the screw with a 0.05mm thickness gauge between the screw and the valve, and adjust it so that the thickness gauge moves with resistance that is not too loose or too tight. It is often said to be like inserting a knife into a kettle, although it is more dependent on the senses than on the use of a precise gauge.

Once the position of the adjustment screw is determined, tighten the lock nut while leaving the thickness gauge in place so that the screw does not move. It is easy to turn the screw as you tighten the nut, so keep the screw in place.

If the feel of the thickness gauge changes after the nut is tightened, adjust it again. If you are not used to the work, the gauge will be reluctant = you are tightening the nut and the screw at the same time. However, if the nut is not tight enough, and it loosens or comes off during driving, it can cause major problems, so the tightening itself must be done firmly.

In contrast, shim adjustment is a large and time-consuming process.

I mentioned earlier that there are two types of shims: outer shims and inner shims. In the outer shims, the shims are on the outside of the cylindrical part, which looks like an upside-down cup, the "tappet" where the cam pushes on the valve, and are in direct contact with the camshaft. The inner shim, on the other hand, has the shims on the inside of the tappet and is not visible from the outside.

The inner shims are superior to the outer shims in every way in terms of performance. However, the big drawback is that you can't replace the shims until the camshaft is removed.

If the measured clearance is larger than the specified value, then replacing the shims with thicker ones will reduce the clearance. Using the example of the Zephyr χ exhaust valve with the specified value of 0.17-0.26mm, if the measured value is 0.28mm, then the clearance should be 0.23mm if the shims are 0.05mm thicker than the current clearance.

However, if the clearance is 0, or the camshaft is pushing the valve open, the gap cannot be measured and the best shims cannot be selected. In the case of the Zephyr χ, if the lower limit of the exhaust valve is 0.17mm, and the current clearance is exactly 0mm with no 0.01mm of leaf in it, then selecting a shim that is 0.20mm thinner than the current clearance should bring the clearance to 0.20mm.

If the valve is not tight to the valve seat, then you may need to make it 0.20 or 0.25 or even 0.30mm thinner. To check that, you'll have to assemble and measure the thin shims anyway, but you'll have to remove the camshaft each time.

The camshafts may seem easy to install and remove, but in the case of a 4 cylinder engine, even if one cylinder is at the top dead center of compression, the camshafts in the other cylinders are pushing on the valves and the cams are being pushed up by the strong tension of the valve springs. So when you loosen the cam holder bolts, of course, the cam holder and bolts will have a strong force applied to them, and if you repeat the same process, the female threads on the cylinder head side may be damaged.

Also, whether you use outer shims or inner shims, you have to buy the shims that are needed for the valve clearance adjustment each time you change the shims. By the way, the Zephyr χ's stock inner shims are around 7 USD for each size, and the Z outer shims are around 7 USD for each size, so if you want to replace a number of shims, it will cost you a lot of money.

Nevertheless, the intake and exhaust valves are a vital part of the 4-stroke engine, so knowing how they work and how to adjust them will surely help.

Kawasaki Zephyr χ, which is an inner shim type, the tappet makes contact with the camshaft and the shims are on the backside of the tappet (lower side). No adjustment is necessary if the intake and exhaust are within the range of 0.08 to 0.17mm and 0.17 to 0.26mm respectively.

The inner shim goes into the center section of the valve retainer, which is pinched with your fingers. When replacing the shims, the camshafts and tappets must be removed, so the work is extensive. If the measured clearance is close to zero, the number of ranks to be thinned will determine the number of times the job is done.