How to Adjust Your Chevy Valve Lash

How to
Adjust Your Chevy Valve Lash
by Lars Grimsrud
SVE Automotive
Restoration
Musclecar, Collector & Exotic Auto Repair &
Restoration
Broomfield, CO
Rev. A 2-7-01
This tech paper will discuss the adjustment of
Chevrolet hydraulic lifters (“valve lash”).
The
procedure outlined here differs slightly from the Service Manual, and is
based on my years of experience doing this work in the quickest, least
painful, most economical way while keeping the level of quality high. It
is recognized that other people will have different methods of doing
things, and may disagree with specific methods and procedures that I
use.
Overview, Theory and my Thoughts on Lash Settings
Hydraulic
lifters are wonderful little innovations which reduce valve train wear and
virtually eliminate required valve train maintenance.
Without the
use of hydraulic lifters (mechanical lifters), the valve train must be
adjusted with a certain amount of “slop” in it
(“lash”). This lash is necessary, since the various
components in the valve train tend to “grow” and expand as
they heat up from normal engine operation. As the components
“grow,” they take up a large portion of the lash, but some
lash must still be retained as a safety margin. If there were no lash,
there would be a risk of the valves not closing fully, resulting in poor
engine performance and burnt valves. This lash, however, results in a bit
of valve train noise as parts “clank” together, and this
clanking induces wear of the valvetrain components. This wear, in turn,
requires that the lash be re-adjusted at regular intervals. If only there
were a way to eliminate the lash…. hmmmmm….
Enter the
hydraulic lifter. Believe it or not, but the internal components of a
hydraulic lifter are the most precise, close-tolerance parts on a vehicle.
The basic operation and principle of the hydraulic lifter is as
follows:
When the hydraulic lifter is at the “low”
point in its bore (the valve is closed), the body of the lifter is
exposed to pressurized oil in the lifter oil galley. The lifter body has
a little hole in it, and this hole allows oil to enter and/or exit the
lifter body. The pressurized oil in the galley thus enters the body of
the lifter, and pushes lightly on a plunger in the roof of the lifter
body. This plunger is about a half inch in diameter, giving it a total
area of approximately 0.12 square inches. If you’re running 60
pounds of oil pressure, that means that the oil is pushing upwards on the
plunger with a force of about 11 pounds max. This 11 pound force is not
enough to open the valve, but it will remove all slack out of the valve
train.
As soon as the lifter starts moving upwards in its bore (the
cam is opening the valve), the oil hole in the lifter body moves out of
alignment with the oil galley. The lifter body is sealed off, and oil
can’t get in or out of the body. The lifter, thus, goes into
“hydraulic lock,” and suddenly acts like a solid lifter. The
oil under the plunger is not compressible, so the lifter now opens the
valve.
As the lifter comes down the bore after completing its valve
opening chore, it is once again exposed to the oil pressure in the lifter
galley, and the pressurized oil once again assures that all lash is taken
out of the valvetrain before repeating the opening cycle. As the
valvetrain wears, the oil pressure simply constantly pushes the plunger
upwards to remove any slack caused by the wear. The plunger can be pushed
upwards in the lifter bore within the design limitations of the lifter,
and will eventually be stopped by a snap ring retainer in the top of the
lifter body. Once the plunger reaches the retainer, it can no longer
provide effective valve train adjustment, and the valvetrain will start
making noise.
The distance the plunger is compressed into the
lifter body when the lifter is at the low point in its bore is referred to
as “lifter preload.” This is the “valve lash” or
“valve adjustment” on a hydraulic lifter. The further the
plunger is depressed, the more wear the lifter can “absorb”
before reaching the snap ring retainer. However, the more the plunger is
depressed, the more prone the engine becomes to “lifter float”
or “valve float.”
As we noted earlier, the oil in the
lifter is not compressible. If, somehow, the lifter body were filled with
just a few drops of oil too many, and the lifter were moving so fast in
its bore that the oil did not have a chance to bleed out and re-stabilize
the valvetrain lash at the bottom of the lifter travel, the lifter would
keep the valve open when the valve should be closed. Further, if
aggravated, this condition could cause the lifter to open the valve beyond
its design limitations, out of time with the intended valve cycle. This
is what is known as “lifter float” or “valve
float.” It can have disastrous consequences if the valve were to
hit the piston. We, therefore, adjust hydraulic lifters with some
pre-load, but not too much. So what’s the right
spec…?
Obviously, we can eliminate valve float completely by
simply adjusting our lifter pre-load such that the plunger is right at the
top of the lifter body; right up against the snap ring retainer. The
problem with this approach is that there is the possibility of the same
hydraulic lock conditions exerting so much force on the snap ring that the
snap ring is forced out. With nothing retaining the plunger, we would
have the same disastrous ending to our engine… Also, with no
plunger travel available, the non-maintenance feature of our hydraulic
valvetrain is defeated, and we must now constantly adjust the valves as if
they were mechanical.
The factory setting on a Chevy lifter
pre-load is � to 1 turn lifter preload with the lifter on the low side of
the cam (valve closed). This eliminates valvetrain maintenance for at
least 100,000 miles, and is a good compromise setting. However, it can
allow the valves to float at rpms as low as 5700. This, effectively,
becomes a factory-installed rev limiter: if they can make the valves float
lightly around 6000 rpm, GM can reduce warranty claims from customers
over-revving their engines. Hey… these boys and girls designing
this stuff in Detroit aren’t dummies, are they?
So for a
performance application, we split the difference. A � turn lifter
pre-load will raise the rpm limit of the engine, yet it will still provide
quite a bit of plunger travel so the lifter can do its valvetrain wear
adjustment thing.. It will also keep the plunger away from the snap ring
retainer, and it will keep our operation safe. Safe, reliable, improved
performance and good durability/life: what more could you ask
for?
Procedure
This procedure typically takes me about
30 minutes from start to finish on a Chevy without air conditioning, but
I’ve done it a few times. Allow yourself an hour or two for a
leisurely pace of wrenching and beer drinking.
General
tips:
Keep your work area clean and organized. It’ll make the job
seem much easier. I like to lay a clean towel out on the ground by the
car or on an adjacent workbench. As each bolt, screw, nut and component
is removed, I lay the parts out carefully on the towel. Whenever
possible, I put screws back into the holes that they came out of after the
component is removed. Wipe up spills and sweep the area as you progress
to keep things clean and pleasant. You will be leaning across the fenders
on pre-C4 cars, so use a fender apron.
Step-by-Step:
�
Park the car on a level surface. Set the parking brake and block the
tires. On manual cars, put the trans in neutral. Pull the coil wire that
goes from the distributor cap to the ignition coil (on HEI cars,
disconnect the connector out of the distributor) and ground it.
�
Turn the engine over until you can see the timing mark on the harmonic
balancer. Using a piece of chalk or other visible marker, place three
more timing marks on the balancer: one mark every 90 degrees around the
balancer (one exactly opposite the factory mark, and two in between these
marks: just get it pretty darned eye-ball close, it doesn’t have to
be exact.)
� Remove the valve covers. You may have to remove some
accessory brackets in order to do this.
� Rotate the engine over
(either by “bumping” the starter or by inserting a socket and
breaker bar onto the harmonic balancer bolt) until the factory timing mark
lines up with “0.” Observe the pushrod for the exhaust valve
on the #1 cylinder: if the pushrod moves as you come up on Top Dead
Center, you’re on the exhaust stroke, and you need to rotate the
crank one more time. If neither pushrod moves as you come up on the
timing mark, you’re on the compression stroke and ready to go.
�
Loosen the adjustment nuts on both the rocker arms for cylinder #1 using a
deep socket and a �” drive ratchet. One at a time, adjust them as
follows:
� Place the pushrod between you thumb and forefinger of your
left hand (or right hand if you’re left handed…). Rotate, or
“twirl,” the pushrod back and forth between your fingers and
notice how lightly and easily it spins. As you do this, slowly tighten the
rocker arm nut. The instant you feel the “twirl” friction
change between your fingers, you are at “0” lash. STOP. Now,
notice the position of your ratchet handle. Tighten the nut exactly �
turn from your current position. Do the same to the other rocker arm for
#1 (when doing this, make sure that the friction you feel as you swirl the
pushrod is not caused by your ratchet and socket pushing or binding on the
rocker arm – keep things straight and aligned, and watch for false
indications caused by your tools). That’s it for #1.
�
Now, here’s the trick:
What’s the firing order for a GM
V8?
1-8-4-3-6-5-7-2
How often does a cylinder fire in a V8?
Every
90 degrees
That means we can now rotate the crankshaft 90 degrees at a
time, and go right to the next cylinder in the firing order for the valve
adjustment, with confidence that both of the valves for that cylinder will
be closed and ready to adjust.. So rotate to your next chalk line, and
adjust #8 as described above. Rotate to the next line and adjust #4.
After you’ve rotated the crankshaft twice over (using the starter
and “bumping” is the easiest way), you’ve finished your
valve adjustment! No oily mess, no worrying about if you missed a valve.
Just a nice, simple, structured procedure!
� Pop your valve covers back
on with a fresh set of gaskets, re-install any accessory brackets
you’ve removed, and start it up with confidence. You now have a
correctly adjusted valvetrain that will operate quietly and with
outstanding performance and reliability.:
Questions, Comments &
Technical Assistance
If you have questions or comments regarding this
article, or if you notice any errors that need to be corrected (which is
quite possible since I’m writing this from memory…), please
feel free to drop me an e-mail. Also, if you need any technical
assistance or advice regarding this process, or other maintenance issues,
feel free to contact me:
lars.grimsrud@lmco.com