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John's suggestion about the rod through the hole above the #6 piston is very
useful, but a bit of caution is in order, especially for anyone installing a new
or modified camshaft.
Using a dial indicator through the hole to directly read piston position will
produce a good "piston" TDC, but it will NOT produce an accurate "crankshaft" TDC.
Directly reading piston TDC is grossly inadequate for exacting work like degreeing
a cam, in which case you must use a modified "double measurement" or
"before-and-after TDC" method of reading piston position relative to the
crankshaft position. Here's one variation of this technique for marking a more
exact TDC on the crankshaft:
1) Rotate the crankshaft CLOCKWISE until the piston is some particular distance
below TDC (no particular distance, say about 0.25 inch).
2) Set up a dial indicator with its rod contacting the piston top (this can be a
rod down through the hole in #6). Make sure the rod is contacting a CLEAN area on
the piston and zero the indicator.
3) Rotate the crankshaft a tiny bit further clockwise until you produce a reading
on the dial indicator (no particular reading), and make a note of the reading.
4) Make a temporary mark on the crankshaft pulley next to the pointer.
5) Carefully rotate the crankshaft COUNTER-CLOCKWISE until the piston contacts the
dial indicator rod and produces exactly the same reading that you had noted
before.
6) Make another temporary mark on the crankshaft pulley next to the pointer.
7) With a flexible tape, mark a point exactly halfway between the two temporary
marks on the pulley. This is the "more-or-less exact" TDC.
There are at least half a dozen variations on the above technique, documented in
various overhaul and blueprinting manuals, but they are all done for the same
reason: As the piston position approaches TDC, an increasingly greater angular
change in crankshaft rotation is required to produce a given unit of vertical
movement in the piston. By measuring the piston position at two equal points at a
reasonable distance before and after TDC, you can avoid this "area of uncertaincy"
and obtain far greater accuracy.
Another common approach is to use "piston stops" that are simply adjustable bolts
that stop the upward movement of the piston. The crank is rotated in one direction
until the piston hits the stop and a mark is made on the pulley, then the crank is
rotated in the opposite direction until the piston hits the stop and another mark
is made. Then the distance between the marks is halved to locate TDC on the
crankshaft.
Note that this kind of accuracy is NOT critical for setting ignition timing, since
the timing specs given in the manual are just a beginning point---optimum timing
being affected by fuel composition (very different now than when the manual was
written), air temperature, humidity, engine temperature, engine condition,
altitude, etc. The only way to obtain optimum ignition timing is by experimenting.
One other thing to keep in mind: DO NOT trust the factory TDC marks on the pulley.
Besides the fact that there are minor variations between engines, there is also a
good chance that it might not be the original pulley. This is a common occurrence
on M37s because of the propensity to change generator and alternator systems over
time. For example, my M37 has a military-retrofit 100-amp alternator, which
required a four-belt crankshaft pulley to replace the original pulley. If you are
installing a new camshaft it is always a good idea to obtain an accurate
crankshaft TDC for degreeing the cam.
Have fun,
Alan
G740@aol.com wrote:
> Just a bit of trivia for all you Dodge owners. Located on the rear of the
> cylinder head directly above the #6 piston is a small hex head pipe plug.
> Cylinders 1 & 6 are twin cylinders, meaning they travel identically. By
> removing the plug, and placing a long throw dial indicator thru the hole, you
> can measure the exact point of TDC. This hole was put in just for this
> purpose, so you can always get the oil pump, distributor and camshaft timed
> correctly.
> Inquiring minds want to know!!
> Regards to all,
> John Bizal
This archive was generated by hypermail 2b29 : Mon May 01 2000 - 05:30:05 PDT