2011 - Crane Cams

VALVE TRAIN
Adjusting the Valve Train
Mechanical Lifters
All pushrod engines using mechanical (solid) lifters, or
mechanical roller lifters, must have an adjustable valve train
so that precise adjustment for “Valve Lash” can be made to
match the camshaft’s requirements. Valve lash is the running
clearance that exists between the tip of the valve stem
and the valves mating surface of the rocker arm. (It is
expressed in the Crane Catalog as “Valve Lash” and on the
camshaft specification card as “Valve Setting”. Both terms
mean the same thing.) The amount of valve lash can vary
between camshaft profile designs, being as small as .010” on
some and as great as .035” on others. It is important to use
the recommended valve lash when you first test the performance
of the engine. You must also be concerned with thermal
expansion of the engine components. This is especially
true if using aluminum alloy cylinder heads, or block. For
this reason, Crane requires that the valve lash be set with
the engine “Hot” on all pushrod engines using mechanical
lifters. This will insure that the minimum required clearance
(valve lash) is maintained throughout the engine’s operating
temperature range.
Compensating for a Cold Engine when Adjusting Valve
Lash
When installing a new cam, the engine will be cold but the
lash specifications are for a hot engine. What are you to do?
There is a correction factor that can be used to get close. We
mentioned that the alloy of the engine parts can be affected
by thermal expansion in different ways, therefore the
amount of correction factor to the lash setting depends on
whether the cylinder heads and block are made out of cast
iron or aluminum. You can take the “hot” setting given to
you in the catalog or cam specification card and alter it by
the following amount to get a “cold” lash setting.
• With iron block and iron heads, add .002”
• With iron block and aluminum heads, subtract .006”
• With both aluminum block and heads, subtract .012”
Remember this correction adjustment is approximate and is
only meant to get you close for the initial start up of the
engine. After the engine is warmed up to its proper operating
temperature range, you must go back and reset all the
valves to the proper “hot” valve lash settings.
Setting Valve Lash on Mechanical Cams
All the valves must be set individually and only when the
lifter is properly located on the base circle of the lobe. At
this position the valve is closed and there is no lift taking
place. How will you know when the valve you are adjusting
is in the proper position with the lifter on the base circle of
the cam? This can be accomplished by watching the movement
of the valves.
1. When the engine is hot (at operating temperature)
remove the valve covers and pick the cylinder that you
are going to adjust.
2. Hand turn the engine in its normal direction of rotation
while watching the exhaust valve on that particular cylinder.
When the exhaust valve begins to open, stop and
adjust that cylinder’s intake valve. (Why? Because when
the exhaust is just beginning to open, the intake lifter will
be on the base circle of the lobe, so the intake is the one
we can now adjust.)
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3. Use a feeler gauge, set to the correct valve lash, and place
it between the tip of the valve stem and rocker arm,
unless otherwise specified. Adjust until you arrive at the
proper setting and lock the adjuster in place.
4. After the intake valve has been adjusted, continue to
rotate the engine, watching that same intake valve. The
intake valve will go to full lift and then begin to close.
When the intake is almost closed, stop and adjust the
exhaust valve on that particular cylinder. (Again, when we
see the intake valve almost closed, we are sure that the
exhaust lifter is on the base circle of the lobe.) Use the
feeler gauge and follow the procedure described before
in step 3.
5. Both valves on this cylinder are now adjusted, so move to
your next cylinder and follow the same procedure again.
In the future you may find shortcuts to this method, but it
still remains the best way to do the job correctly.
Using Valve Lash to Help Tune the Engine
The engine only responds to the actual movement of the
valves. Since the valve cannot move until all the running
clearance (valve lash) has been taken up, the amount of
valve lash you use affects the engine’s performance. For
example, if you decrease the amount of (hot) valve lash, the
valve will open slightly sooner, lift higher, and close later.
This makes the camshaft look bigger to the engine, because
of a slight increase of actual running duration and lift. If you
increase the amount of (hot) lash the opposite occurs. The
valve will open later, lift less, and close sooner. This shows
the engine a smaller cam with slightly less actual running
duration and lift. You can use this method on a trial basis to
see what the engine responds to and keep the setting that
works the best. Just remember, the more lash you run, the
noisier the valve train will be. If the clearance is excessive it
can be harsh on the other valve train components.
Therefore, for prolonged running of the engine we do not
recommend increasing the amount of hot lash by more
than +.004” from the recommended setting. Nor do we recommend
decreasing the hot lash by more than -.008”.
Warning:
“Tight Lash” camshafts cannot deviate from the recommended
hot lash setting by more than +.002” increase, or
-.004” decrease. “Tight Lash” cams are those which have recommended
valve settings of only .010”, .012”, or .014” on the
specification card. These lobe designs have very short clearance
ramps and cannot tolerate any increase in the recommended
valve lash. The extra clearance can cause severe
damage to valve train components.
With “Tight Lash” cams, we recommend using only the prescribed
amount of hot valve lash, and that close inspection
of the engine be maintained.
Please realize that changing valve lash settings from the recommended
design specifications will change the harmonic
characteristics of the valve train, possibly causing valve
spring deteriation and breakage.