Ken Landaiche wrote:
>
> Tony,
>
> Before we sign off on this thread, what is the layout of the compression
> tester? Up for some ASCII art?
LEAKDOWN TESTER PLANS
Here it is as it appeared in the September, 1990, RIGHT LINE.
All rights for reproduction remain with Steve Soar.
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Build your own Leakdown Tester!
List of ingredients:
*A spark plug with the insides removed (arguably the hardest part of
the whole deal)
*Air tool Whip ( an 18" length of air line with 1/4"NPT male
fittings on both ends)
*Block of aluminum about 1" X 1" X 2 1/2"
*#60 drill (or smaller)
*1/4 NPT Tap
*0-100PSI pressure gauge
*Epoxy
*Male quick-disconnect air coupling with female 1/4" NPT threads
Assembly Instructions
Break the insides out of an old spark plug. Beat on the ceramic with
a hammer and a drift until the pieces can be removed. You want all
the insides out, just the threaded metal shell left. Cut off the
ground electrode with a hacksaw. File on the non-threaded end of the
male quick-disconnect until it fits inside the spark plug where the
ceramic used to go. Clean it and the spark plug with a clean-
evaporating solvent like brake cleaner to get off the finger grease
and epoxy the quick-disconect into the spark plug shell. This gives
you a way to attach the air whip to the spark plug to get the
pressure into your cylinders (By the way, you did use a spark plug
that fits your car, didn't you?)
Prepare to drill your aluminum block so that it looks like the
figure. A drill press is handy but you could do it by hand.
_________________________
|____ ________________|
\ /
\ /
/ \
____/ \________________
|_________________________|
Cross section view
Explanation: Use the drill appropriate for 1/4" NPT tap (I forget if
its 3/8" or whatever). Drill into one end only far enough to
adequately tap threads. Drill into the other end far enough to come
near, but not break into, the hole you drilled from the other end.
Note that these two chambers are *not* connected (yet). NOW, take
your #60 drill and your pin vise (Note:: a double-slotted X-acto
knife handle will do for a pin vise in a pinch), drill the small #60
hole to connect the two chambers.
Now use your big drill bit (for 1/4 NPT) and drill a hole in the
side of the block. This is where the gauge will screw in.
____________ ______
|____ ___| |______|
\ /
\ /
/ \
____/ \________________
|_________________________|
Cross section view
Now, using the 1/4"NPT tap, tap all the big holes.
Time to assemble
Use a good sealer on all threaded connections as any leak will
introduce an error to the tester. I used RTV silicone gasket maker,
seemed to work OK. Screw the gauge into the hole at the top of the
diagram. Screw one end of the whip into the hole into the long
chamber (at the right of the diagram). Screw the epoxied spark plug
assembly into the other end of the whip. The short chamber at the
left of the diagram is where your shop air will enter. Screw in a
quick-disconnect here that is compatible with your particular shop
setup.
Congratulations, you're the proud owner of a leakdown tester.
Theory of Operation
The tester works by measuring the pressure drop across a flow
restriction (the #60 hole in the aluminum block). The lower the
pressure reading on the gauge, the worse shape that cylinder is in
(WRT leakage). To believe that this is so, consider the limiting
cases.
Case 1--The spark plug fitting is sitting in free air. there is no
resistance to the flow of air and the gauge reads 0 psi. You might
actually see this reading with a holed piston or a split valve head.
Case 2--The spark plug fitting is closed off with an extremely
tight-fitting cap. Since there is no flow through the orifice, the
is no pressure drop and the gauge will read whatever the inlet
pressure is, *exactly*. You will never see this reading in real life
but you may get close with a newly assembled racing engine built to
close tolerance.
Caveat: Professional leakdown testers read out in "%" leakage,
converted from psi. There must be a calibration factor used based on
the amount of pressure drop for a given orifice size but I have been
unable to determine what the math for this might be. Any Fluids
types listening out there?
Fortunately, even though the tester doesn't read out in "%", it is
still extremely useful.
Application Hints-
Warm up the engine to be tested, then stop it and remove the spark
plugs. Bring piston under test to top dead center on its *firing*
stroke (or else either intake or exhaust valves will be open, making
any measurement useless). Be sure the piston is *really* at top dead
center, or else when you connect the air to the cylinder the engine
will turn over and you will have to set top dead center again.
Screw the tester plug into the cylinder. Connect shop air to the
input port of the tester. Be sure the inlet pressure doesn't vary or
you will get unrepeatable results. For my setup, I just ensure that
the compressor has just shut off from filling the tank and the
regulator is wide open. An improvement for some additional $$ would
be to add an inlet regulator and gauge to the tester, then you
wouldn't have to worry about this.
Now you've got air going into the cylinder and your engine is
hissing away. Write down the gauge reading. On my recently rebuilt
Datsun motor, with 90 psi in, and #60 hole, cylinders read ~77 PSI.
On the Jetta I bought with a burnt exhaust valve valve, the bad
cylinder read 65 PSI. Your mileage may vary.
Now run around and figure out where the hissing is loudest. If
loudest at the carb throat, the intake valve is the culprit. If you
hear it at the exhaust pipe, the exhaust valve is bad. If its
loudest at the oil filler cap, then worn rings are likely. You can
also check the head gasket by looking for bubbles in the radiator,
though you may have to remove the thermostat for this to work.
Seeing as this tool is not absolutely calibrated, its use is mainly
as a relative measure. All cylinders should be within several psi
of one another. Furthermore, if you find a low one, the listening
test should give you a good idea of where the problem is. This is
why leakdown is a better diagnostic aid than a simple compression
test.
It also a good idea to record leakdown figures for your race motor
as soon as it is broken in. For one thing, this will tell you
whether the rings and valves have properly seated (or, whether you
will have to take the %#$!!XX%& thing back apart). Also, if you feel
your new motor going soft in mid-season, you can do another leakdown
test and compare with your fresh rebuild numbers. This is perhaps
more accurate than seat-of-the-pants feel and fading memories.
Someone else on the net suggested taking apart a compression tester
to get the whip and the spark plug inlet. KD tools makes what
they call a "valve holder" an adapter from 1/4" NPT to spark plug
designed to put pressure in the cylinder to hold the valves up while
you remove the springs to replace stem seals without removing the
head.
Send inquiries to: Steve Soar, 3709 NE 157 CT, Vancouver, WA 98682
ssoar@tekig5.pen.tek.com
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Phil Ethier, THE RIGHT LINE, Saint Paul, MN
"I like things that go fast" - Huey Lewis
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