How to Determine Top Dead Center

How to
Determine Top Dead Center
(With the Engine in the Car)
by Lars
Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic
Auto Repair & Restoration
Broomfield, CO
Rev. New 4-26-01
For some reason, Chevrolet engines have a bad
tendency for the harmonic balancer to “slip.” The failure
mode is very subtle and difficult to detect, and can cause all kinds of
problems when trying to tune the engine. The outer “ring” of
the harmonic balancer tends to rotate, or “slip,” on the
rubber cushion that connects it to the inner part of the balancer. When
this happens, the timing can no longer be correctly set on the car, and
the car will have the symptoms of running horribly, with no power, after
the timing has been “correctly” set. In fact, the timing is
as far off as the balancer ring has managed to twist itself, and there is
no easy or obvious way to detect this. This tech paper will discuss how
to determine where the actual Top Dead Center of the engine is to see if
the timing mark on the balancer is in the correct location.
As
always, I’m going to include the disclaimer that many of these are
my own comments and opinions based on my personal tuning experience. It
is recognized that other people will have different methods of doing
things, and may disagree with specific methods and procedures that I use.
This procedure, used as outlined in this paper, does work, and will
provide reasonable accuracy. I have made every attempt to present
factual, technically accurate data wherever possible. If you find factual
errors in this information, please let me know so I can correct
it.
Tools and Equipment Required
As a minimum, you will need
the following tools & equipment:
1. Long 14mm Bolt
2. Spark
Plug Socket with Ratchet & Extensions
3. Socket that fits the Harmonic
Balancer Center Bolt with Extensions and Breaker Bar
4. Sharp Tipped
Felt Marker
5. Flexible Scale or accurate Tape
Measure
Procedure
� Park the car on a level surface,
set the brake, block the wheels, and put it in “neutral” or
“park.”
� Disconnect the battery ground terminal.
If you don’t want to loose all your late-model radio station
pre-sets, disconnect your coil primary lead wire (for standard ignitions)
or the distributor “BAT” wire (for HEI systems). Just do
something to make darned sure the ignition system won’t
fire….
� Remove the #1 spark plug. To make the job easier,
I recommend that ALL of the spark plugs be removed, since you’ll be
turning the engine over by hand during this process. Whether or not you
pull all of the plugs depends on how easy (or difficult) your plugs are to
get to. Pull as many as you can.
� Using a socket and a breaker bar
(or good �” drive ratchet) on the Harmonic Balancer center bolt,
turn the engine over by hand (clockwise as seen from the front) until the
timing mark on the balancer starts coming up towards the edge of the
timing plate. Stop.
� If your engine does not have a Harmonic
Balancer Center Bolt, you can remove the pulleys on the Harmonic Balancer
and then insert two of the pulley attach bolts back into the balancer in
adjoining holes. You can then wedge a screwdriver between the two bolts
and use the leverage to turn the crank over as described above.
�
Smooth and de-burr the end of the 14mm bolt by slightly rounding the
edges. Wire wheel it to smooth out the grinding and to keep the threads
in good shape.
� Screw the bolt into the #1 spark plug hole. Screw
it in by hand until it touches the top of the #1 piston.
� With your
socket and breaker bar, turn the crank to put the piston softly but firmly
up against the bolt. You’re not trying to drive the bolt through
the top of the piston, so use a little finesse and “feel” when
doing this to avoid gouging the piston with the bolt. The piston should
come into firm contact with the bolt when turning the crank clockwise (as
seen from the front).
NOTE: If your timing mark was positioned
as noted above, you inserted the bolt until it hit the piston, and turning
the crank clockwise as described moves the piston AWAY from the bolt, then
your timing mark is WAY, WAY off, and you must re-position your crank so
that the piston is on its way up towards top center. You can do this by
removing the bolt from the spark plug hole, shining a flashlight into the
plug hole, and observing the piston. Use of a mirror is handy when doing
this. When the piston is visually near the spark plug hole, insert the
bolt as described above and proceed.
� Using your sharp felt
marker, place a mark on the harmonic balancer at the timing plate
“0” line.
� Now, using your socket and breaker bar,
turn the crankshaft counter clockwise until the piston once again comes
into contact with the bolt. Again, have a little “feel” for
this so you don’t bash the piston into the bolt with excessive
force.
� Once the piston is up against the bolt with the same soft
but firm feel as before, use your felt marker to place a line on the
harmonic balancer at the timing plate “0” mark.
�
Measure the distance between the two resulting lines on your harmonic
balancer and split the distance exactly in half. This is exact Top Dead
center. If it is not right on top of the engraved line on your balancer,
you have a problem that needs to be corrected.
If the factory
engraved line and your new felt-marker TDC line do not line up, you either
have a slipped balancer outer ring, or an offset key installed in the
crank. Most offset keys are only 2 degrees, so if you’re off by
more than 2 degrees, you most likely need a new balancer. It is also
possible that the timing plate on your timing chain cover is not the
correct setup for your engine combination. Whatever the reason, you need
to correct the issues before trying to set your timing for proper
performance.
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