Finding True Top Dead Center

 

Finding True Top Dead Center

 

 

Does your Chevelle's engine exhibit symptoms of the timing being off but you've set it accurately and had the distributor checked out?  It's possible the timing marks on your balancer and/or timing tab aren't accurate.   When the zero mark of the balancer is even with the zero mark of the timing tab, the #1 piston should be exactly at TDC, or Top Dead Center.  Several things could've happened on a 30 plus year old engine.  The outer ring of the harmonic balancer may have slipped, moving the zero degree line.  The wrong balancer with the mark in a different location could be there.  The only way to verify the timing marks are accurate is to find TDC of the #1 piston (when it's at the very top of its travel).  The only way to accurately do that is with a piston stop.  Even if you can visually locate the piston at the top of its travel, it can be off by a few degrees because it moves very little for several degrees of crankshaft travel at and near TDC.

With a piston stop in place, GENTLY rotate the crank BY HAND until the piston contacts the stop.  Make a temporary mark on the balancer even with the zero mark on the timing tab.   Then GENTLY rotate the crank by hand in the opposite directly until the piston contacts the stop again.  Make another temporary mark on the balancer even with the zero mark on the tab.  Exactly halfway between the temporary marks on the balancer is where the true zero degree mark should be.  Make a permanent mark there.

This is easy to do if a head is removed but can still be accomplished on a complete engine.  You can buy a piston stop that goes goes in the spark plug hole or make one.  To make on, take an old plug that fits your engine and knock the porclain and center electrode out of it.  Force-thread a bolt through the inside of the base and round off the bottom end of the bolt.  When using the plug hole type of piston stop, remove all spark plugs so the crank can be more easily turned by hand with a socket on the balancer bolt and make sure the piston in the cylinder you install the stop in is well down in the cylinder when you install the stop.  CAUTION: BE VERY CAREFUL.  Take the keys out of the ignition and put them in your pocket.  Even a slight bump of the starter will likely result in a broken piston.

Accurate timing marks equal accurate timing.