Sonic’s workshop: All about valve clearances

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So what are valve clearances and why do I need them adjusted?

Glad you asked.

 

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The valve clearance is the gap between the cam heel or the rocker tappet (depending on which system your bike uses) and the top of the valve. This clearance is required to allow for the components expansion as they heat up when the engine is running. Due to normal engine wear and tear, this clearance can change over time and so must be periodically adjusted to maintain proper clearance.

Each engine has a pre designed clearance that must be maintained in order ensure maximum performance. If this gap is allowed to get too small, the resulting expansion will close the gap to less than nothing, and the valve will be unable to close fully. If the valve is not completely closed compression will be lost, the valve may be damaged and the engine will run poorly. On the other hand if the clearance is too great the top end of the motor will sound noisy, performance will be reduced and premature wear of the components may occur.

There are a number of methods used by the various manufactures to adjust these clearances, and are as follows:

Locknut over screw

With the lock nut and screw type there is a threaded screw running through the rocker arm and is the part that actually comes into contact with the top of the valve. The clearance is measured using a feeler gauge between the screw and the top of the valve with the piston at top dead center on the compression stroke (to ensure maximum clearance).

Once the measurement has been taken with a feeler gauge, turning the screw in or out as required can then adjust to the correct clearance accordingly. Once the adjustment has been made (and rechecked), the screw is held in position and prevented from moving by the lock nut. Some engines that have this type of adjuster also use an access cover to get at the adjusters easily without having to remove the entire valve cover.

Shim over bucket

The second type, the shim over bucket uses a quarter sized piece of precision machined hardened steel (the shim) that sits in a bucket, located in-between the cam lobe the top of the valve. Each shim is stamped with a size that identifies how thick it is. In this method the valve clearance is measured between the cam heel and the shim itself.

Once the measurement has been taken and determined if it’s greater or less than specified in the manual, the shim must be removed and replaced with another one of the correct size. A special tool is used to depress the bucket and valve to allow the shim to be removed from the top of the bucket. Once removed, the shim is identified by its stamped mark and then a larger or smaller shim is installed in its place depending upon what measurement was originally noted.

With the new shim installed the special tool is removed and the clearance is rechecked. Once the tool is removed there is no way for the shim to come out, so the clearance remains the same until normal wear requires the clearances to be adjusted again.

The benefit of this system is there is no chance of the adjustment accidentally changing due to the slipping of the lock nut. Another benefit is because the cam lobes run directly on top of the shims there is no need for rockers and adjusters, this reduces weight and results in increased horsepower.

Shim under bucket

Last but not least is the shim under bucket type. This system is similar to the shim over bucket with one very important difference, the shim is located beneath the bucket instead of on top. The principle for measuring is the same as in the over bucket system; the feeler gauge is placed in between the top of the bucket and cam heel. Adjusting the clearance is also the same as in shim over, requiring different sized shims to change the gap between the bucket and lobe.

So, if the shim is under the bucket, how do you get it out in order to change it? Remove the camshafts! No small job. So why do it? Reliability, weight and horsepower.

The shim used in the under bucket design is much smaller than the over bucket system. These shims are about the size of an eraser on the end of a pencil. The smaller the shim the less it weighs, less weight equals more horsepower and a lighter bike (yes, it’s down to saving grams!). Also, because the shim is now under the bucket it can’t be scooped out by the cam. This is especially important with modern high revving sport bikes.

Because you must remove the camshafts to access the shims, you must also measure and record every clearance before you remove the cams. Once the camshafts have been removed there is no way to check any of the other clearances until the cams are reinstalled again. That means that you’d better keep good records and be good with your math (and you thought you’d never need basic math in the real world). Once all the shims that needed replacing have been replaced, you still won’t know if all is well and good until the cams are back in and torqued down. If you haven’t calculated correctly, you’ll be doing a repeat performance.

And finally …

Valve clearances should be checked every six months, 5000-6000 km. or every time a major tune up is done. In fact, most shops will not attempt synchronizing carburetors until the valve clearances have been checked. If the clearances are not correct it will give fluctuating readings and make it nearly impossible to set up the carbs.

Oh, there is one other type out there and that is the hydraulic lifter. I like it the best because it maintains a constant clearance without ever needing adjustment. A quiet maintenance free valve train. The down side is they can rob some horse power, but unless you’re a knee down throttle pegged sort of rider you won’t miss the small amount that is actually lost.

So there you go, that’s valve clearances in a nutshell. Keep this stuff in mind if you’re buying a new bike also. Remember, if it has shim under bucket and you’re not prepared to do this sort of thing yourself then be prepared to pay to have it done.

Hope that’s cleared your mind,

Sonic.

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