How To Rebuild Your Ford V-8 351C-351M-400-429-460.pdf - Index of

If a cylinder is down on pressure, give it a
few shots of oil, then recheck Its pressure
after waiting a few minutes. Direct the oil
to the far side of the piston. If compression
improves, the problem is in the bore. Otherwise,
it's probably the valves.
in firmly and give it a half turn. This
helps make sure it seals. Turn the engine
over the same number of strokes for each
cylinder tested-about five will do-and
observe the maximum gage pressure.
Record the results and the procedure
until you've tested all cylinders in orderjust
to keep things organized. Now
that you have the numbers, what do
they mean
You may see pressures anywhere from
80 to 250 psi, depending on the engine
and its problems. The best approach is to
compare the cylinder pressures with each
other. All cylinders of an engine don't go
bad at once, so the weak ones will show
up like a "sore thumb." Another thing to
keep in mind: cylinder pressures of a highcompression
engine-such as the Cobra
Jet and Super Cobra Jet 429 engines-may
read lower than their low-compression
cousins at cranking speeds. This is due to
cam timing and not because of their
mechanical compression ratios. The same
doesn't apply at higher RPM. Therefore,
if you have a high-performance engine,
don't be shocked if your neighbor boasts
of higher cylinder pressures from his
tamer regular-gas-burning engine.
What should the "spread" be between
cylinder pressures A good rule is the
lowest pressure should not be less than 75
percent of the highest. Otherwise, something
is not quite right in your engine. For
example, if the highest reading is 150 psi
and the lowest is 120 psi, then the engine
is all right because 150 x 0.75 = 112-the
lowest allowable pressure.
Now what do you do if all your readings
are not within the 75-percent range Something
is wrong, but what To test the piston
rings, squirt about a teaspoon of
heavy oil in the bad cylinderls from your
oil can-40 weight will do. The best way
of determining if you are putting in the
right amount of oil is to see how many
squirts it takes to fill a teaspoon. Squirt
the same amount of oil in the spark-plug
hole while directing it toward the far side
of the cylinder. It will also help if you
have the piston part way down the cylinder
to ensure getting the oil all the way
around the piston. Don't get any oil on
the valves. The oil will take a little time
to run down around the rings, so wait a
couple of minutes before rechecking pressure.
If this causes the questionable cylinder/~
to increase in pressure, the rings and
bore are at fault and a rebuild is in order.
If the pressure doesn't change an appreciable
amount. look further.
to the cylinder being tested. The tester
monitors the pressure the cylinder can
maintain in relation to the test pressure.
This is the best way to test an engine's
condition. It eliminates factors which
affect the results of a conventional compression
test, but do not reflect the sealing
qualities of a cylinder: valve timing, camshaft
wear or engine cranking speed. The
problem with a leak-down tester is it
costs about three times as much as the
conventional compression tester and requires
a compressed-air source. Consequently
it's not practical for you to purchase
such an expensive piece of equipment
for a "one-shot deal."
Leak-down testing is done by most
tune-up shops. A leak-down tester is an
integral part of Sun Electric's electronic
analyzer. Sun recommends that an engine
which leaks 20% or more of its test pressure
needs attention, whereas a good
cylinder will have 5 to 10% leakage. So, if
leak-down testing your engine finds a
Leak-Down Testing-Leak-down testing is problem cylinder, you,ll the probsimilar
to compression testing, but rather
lem will be rings, valves or a head gasket.
than the engine doing the compressing, a
It could also indicate a crack in the cylinleak-down
tester does it. Compressed air
der head or cylinder wall.
is used to apply a test pressure via a
special fittiniihrough the spark-plug hole Head Gasket or Valves-Two candidates
Leak-down testing is the most-accurate way of measuring a cylinder's sealing capability
because it eliminates extraneous factors which affect compression-gage readings.
Because leak-down testing is done without cranking the engine, cranking speed, valve
timing and cam condition do not affect the reading. A leak-down tester requires a compressed-air
source. Photo courtesy Sun Corporation.