Valve
Clearances ... What's
the deal?
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Ah,
the smell of oil, the guts of the motor, the skinned knuckles .. |
So what are valve clearances
and why do I need them adjusted?
Glad you asked.
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 AND
SCREW TYPE
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
TYPE
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 TYPE
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Shim
under bucket require camshaft removal to adjust! |
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.
|
'Orrible
picture, but hopefully you can see the complexity of this 20 valve
head. |
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
...
|
Lurvely! |
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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