Checking piston valve clearance

1. Jomer B. Almozara
Reporter

2. To check this clearance, the crankshaft, piston, and rod must be
installed in the block. After installing the piston and rod, temporarily
install the cylinder head, along with the head gasket and valve train. Just
remember that if hydraulic lifters are used, they will need to be pumped up
first, and lighter test springs should be used in place of the stock valve
springs so that the valves will open fully. The clearance can be checked by
using one of two methods: the clay method or the dial indicator method.
Both methods show you the minimum amount of clearance between the
piston and the valve. The clay method gives an imprint of where the valve
and piston have an interference issue, so that you will be better able to
determine how to fly cut the pistons if necessary. The dial indicator method
allows you to check the clearance at various positions of the piston.

3. Step 1
Assemble the valve train for the number 1 cylinder.
Determine the use of solid or hydraulic lifters. If solid lifters,
obtain a pair of solid lifters from a parts store. If using
hydraulic lifters, prepare using hydraulic lifters. If your heads
are preassembled, you will need to remove the stock springs
and install the lightweight springs in their place. Have the
right pair of hydraulic lifters for that cylinder nearby.

4. Step 2
Install lightweight test springs. Put a little piece of
wood, or something similar, under the valve to keep it in
the closed position while you install the lightweight
springs. Check that the pistons are not protruding above
the deck. They should be set fractionally below the deck.
Measure the clearance by sliding a feeler gauge under a
straightedge laid across the cylinder.

5. Step 3
Make balls using modeling clay, one for each valve. Stick
them to the piston in the valve reliefs that are cut into the top
of the piston, and coat the clay with clean engine oil. Make two
balls using modeling clay, one for each valve. Stick them to the
piston in the upper valve reliefs (if present) that are cut into
the top of the piston. Cover them with a little oil to keep the
valves from sticking to the clay when they make an imprint in
it.

6. Step 4
Place the cylinder head on the block. Tighten the
head bolts by hand. Make sure the pistons are positioned
somewhere in midstroke. Lay the head in place without the
head gasket, install a few bolts in the number 1 cylinder
area, and tighten them by hand. If using a wrench, do not
tighten the bolts very much—no more than 15 foot-pounds
(ft-lb). Most head gaskets must be replaced once they have
been torqued even once.

7. Step 5
Install the valve train and adjust the valves to the
specified lash, or if using hydraulic lifters, zero lash. Install
the valve train and adjust the valves to the specified lash, if
applicable. If the engine uses hydraulic lifters, set the
clearance to zero lash.

8. Step 6
Carefully rotate the crank until you see the exhaust
and intake valves open and close two times each. Carefully
rotate the crank until you see the exhaust and intake valves
open and close two times each. If you feel solid resistance,
STOP; the piston may be contacting the valve.

9. Step 7
Remove the valve train and head. Cut each piece of clay and
remove half. Measure the distance from the piston to the
top of the valve imprint with a ruler or the depth end of a
dial caliper.

10. Now remove the valve train and head. When you
remove the head, the two balls of clay now have valve
imprints pressed into them. Cut each piece of clay and
remove half. Measure the distance from the piston to the
top of the valve imprint with a ruler or the depth end of a
dial caliper. Compare this measurement with the
specification in the service information. If it is greater than
the minimum allowed, there is very little danger of your
valves contacting the piston once the engine is heated up
and running, unless the valves float at high engine
revolutions per minute (rpm). If the clearance is smaller
than specified, the pistons will need to be fly cut by a
machine shop, or a smaller camshaft must be installed.
Clean any leftover clay off the piston.

11. Compression Test

12. Your car's engine compression can tell you a lot about the overall health of
the engine. If your car is blowing blue smoke out of the tailpipe, or if your car
is losing lots of oil, you could have a bad piston ring. This will also cause low
compression in that cylinder, and a compression test will tell you. The same
goes for a bad valve. Even if you are just noticing a general lack of power, a
compression test can help you rule out some of the more serious possible
causes.
*Note: This test was performed on an ancient Porsche engine to
demonstrate the basics of how a compression test works. Please consult
your repair manual for specific instructions on your vehicle.

13. The Compression Testing Kit
Indicator Dial Gauge
Testing Tube
Spark Plug Hole Adapters
That's it! Does it seem a little easier now? Let's go do the compression
test
The kit includes a gauge, tube and adapters.

14. Before You Begin
Disable to ignition system so the car won't start.
Before you begin the compression test, the engine needs to
be warm. Get the engine up to operating temperature by
running it for a while, or you can do your compression test
after a drive.

15. Be careful. Some parts of the engine can be very hot!
You'll also need to disable your ignition system. We are going
to need to crank the starter to turn the engine over but we
don't want it to actually start. In most cars simply disconnect
the ECU. If your car has an old school coil like the one
pictured above, remove the wire from the terminal marked
15. If your car has a coil pack type distributorless ignition,
unplug the coil pack or packs. Please consult your repair
manual to find out what's specific to your vehicle.
*Engine at operating temperature.
*Ignition system disabled.

16. Inserting the Testing Adapter
Be sure you insert the correct adapter.
Those silver threaded pieces that came with your
compression testing kit are adapters. They allow you to have the
right clearance and other stuff to properly measure the engine
compression in that cylinder.
Remove one spark plug and insert the appropriate testing
adapter. A spark plug socket will insert it easily. Tighten it snugly as
you would a spark plug, but don't overtighten it, this can damage
your engine.
*Be sure you read the instruction on your compression testing kit
and use the correct adapter! Engine damage can result!

17. Screw In the Testing Tube
Screw in the testing tube.
With the correct adapter snugly in place, screw the long
black testing tube onto the silver adapter. It's a pain in the
neck to screw in, but just keep turning the whole thing
like a giant straw until its snug.
Do not tighten the tube with a tool! Hand tight is
enough.

18. Attach the Testing Gauge
The testing gauge attaches like this.
With the testing tube firmly seated on the silver adapter,
you're ready to attach the test gauge. The gauge displays
engine compression. To install it, pull back the collar at the
end of the gauge and slide it over the metal end of the tube.
Give it a tug to be sure that it's on.

19. Take Your Compression Reading
The dial indicates the compression for that cylinder.

20. You're all set up now and ready to actually
do the compression test. Double check that you
disconnected the appropriate stuff so the engine doesn't
actually start. Now turn the key and crank the engine over
for about 10 seconds. The needle on the compression
gauge will stay at the highest indicated compression
reading. This number indicates the compression for that
cylinder only. Record it so that you can compare it to the
other readings you're about to take.
Don't remove the gauge just yet!

21. Remove the Gauge and Repeat
Release the pressure and you're on to the next cylinder.

22. Don't just remove the gauge, there's a lot of
pressure in the line and you want to release it first.
Thankfully they thought of this, and there's a little
button on the side. Depress the button and you'll hear
the pressure hiss out. Now it's safe to remove the
gauge, unscrew the testing tube, and take out the
adapter.
Replace the spark plug and repeat the whole
process on the next cylinder until you have readings
for all of them. Check your repair manual to see if the
readings you got are healthy.