Importance of Valve Adjustment

Story by Costa Mouzouris//
August 1 2018

Most four-stroke motorcycle engines need occasional valve adjustment, or at least a valve-clearance check. Adjusting the valves means setting the appropriate clearance between the base circle of the camshaft, otherwise known as the camshaft heel, and the tip of the valve or rocker arm, depending on the valve train setup. This clearance is important because it compensates for the expansion of the various engine components as the engine heats up, thus allowing optimum engine performance and preventing engine damage.

If the clearance is too tight, for example, it may completely disappear when the engine reaches operating temperature, in turn preventing the valves from closing fully. A valve that doesn’t close fully will allow combustion gasses to escape, which reduces engine performance. Worse yet, though, is that if the valve still closes completely but the clearance is too tight, the valve won’t remain seated long enough to transfer its heat into the cylinder head, which could lead to a burnt valve, particularly the hot-running exhaust valve.

Valve clearances also affect valve timing; if you’ve ever purchased an aftermarket camshaft and looked at the spec sheet, you’ll have noticed that cam timing specs are often given either at zero clearance or a specified clearance, the latter being more realistic, since it takes into account the degrees the crankshaft rotates as it takes up said clearance on the leading and trailing sides of the cam lobe. The valve’s duration (the time the valve is open) will therefore be greater if the clearance is tighter (it opens sooner and closes later), and reduced if it’s looser (opens later and closes sooner). This has a direct effect on engine performance.

Reliability is also affected by valve clearance. If it’s too great, it will cause adjoining parts to slam into each other – hence the tapping noise you hear on bikes that haven’t been properly maintained. This pounding will wear out valve train parts prematurely.

When it comes to a brand-new bike, most manufacturers require that valve clearances be checked right after break in, or at about 1,000 km. This might seem premature, but what happens in a new bike is that valve clearances are set to spec at the factory, but tighten up considerably in the first few hundred kilometres, as the valves work their way into their valve seats. This “seating” of the valves sets them a bit deeper into the combustion chamber, and consequently, the valve stems protrude farther at the other end, taking up precious valve clearance. From what I mentioned earlier about what happens if the valve clearance is too tight, you can understand why a warranty may be voided if that all-important first inspection isn’t performed– it’s not about the oil change. Modern materials have helped extend valve service intervals, but it’s important to have a look, especially if you haven’t done so in a very long time but hear no valve noise – they might be too tight.

Valve clearance is verified by inserting a feeler gauge of the correct thickness between the valve stem or rocker arm, and the cam heel. There are three types of valve adjusters: shim over bucket, shim under bucket, and locknut and screw. The last one is the easiest to adjust, since it involves loosening a locknut and turning the adjuster screw – which is in direct contact with the tip of the valve stem – to set the proper clearance; you then tighten the locknut to secure your adjustment. Access to this type of adjuster is also typically easy, since there is usually a valve-adjustment access cover incorporated into the valve cover.

Shims over buckets are a bit more involved, since you have to remove the entire valve cover for access, and adjusting the clearance requires that you insert shims of different thicknesses. With the proper tools, it’s a relatively easy system to work with; after measuring the clearance and determining which shim is required to set it to spec, you use a special tool to compress the valve enough to remove the old shim and insert the new one, and then move on to the next valve.

The most difficult valve system to work with is the shim under bucket. Measuring the clearance is identical to the shim-over-bucket system, but replacing the shims requires removing the camshafts so you can lift off the buckets and gain access to the shims. When adjusting shim-under-bucket valves, you should record all of your measurements first, get the correct shims to set the clearances, then proceed to remove the cams. While the other systems let you measure and adjust each valve individually, removing the cams makes this highly impractical.

Finally, there are also hydraulic valve adjusters, found mostly on Harleys. If you have this system, it took you longer to read this than to adjust your valves, since you don’t have to.

Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.

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