Finding TDC

How does one go about finding top dead center on a Chevy V8? Follow along...

You can get close by pulling the #1 plug and placing your finger over the hole. Then crank the engine over and when the pressure builds up to its greatest, you know you are close to top dead center. This works well for determining if the motor is about to fire the #1 cylinder (vs. #6), but isn't accurate enough to find out if your balancer ring has slipped.

A much more accurate way is to use a piston stop and a degree wheel. Thread in a piston stop (can be made from an old spark plug and a bolt) into the #1 plug hole, fasten a degree wheel to the crank and make a rigid pointer to go with the degree wheel. Be sure the pointer is stiff and mounted solidly so that it won't move on you A wire coat hanger with a loop at the end bolted to the block or head works well.

Turn the motor until the piston lightly hits the stop (we don't want to damage the piston, so take it easy; if you make you own piston stop, make sure the end of the bolt is smooth with no sharp edges that could gouge the aluminum piston).

Write down the number the pointer is indicating on the degree wheel. Now rotate the engine back the other way until it lightly hits the stop again. Write this number down as well. TDC will be exactly halfway between the two numbers that you recorded. Remove the piston stop and rotate the motor to this "middle" number. Look at the timing mark and check to see if it's lined up with zero on the timing tab. If its off by a bunch, your balancer is toast. If its just off a degree or two, just make a new mark on the balancer that lines up with zero on the timing tab.

A good way to create your own mark is just to run a strip of masking tape across the wheel about 1/16" from TDC, then apply another strip 1/16" from TDC on the other side (so there's a 1/8" gap). Then just lightly spray or brush on a little white paint. When it dries, remove the tape, and you'll be left with a bright, durable timing mark at top dead center.

If the heads are off the engine, using a dial indicator is a very easy way of finding TDC (just rotate the assembly until the dial indicator, pointing at the #1 piston, reaches its highest reading), but not all of us have access to such equipment.

If you have any questions or comments please click the "Contact Craig" link and let him know.

Copyright 2009 Bruce Johnson and Craig Watson