How to Find the TDC on a Small Block Chevyby Don Bowman
There are two ways of finding top dead center on the compression stroke on a Chevy small block. The first and easiest way is perfect to set the firing order, the ignition timing and the valves. The second method is used primarily by race car enthusiasts to find "absolute" top dead center. All engines are slightly different due to manufacturing tolerances. This method is effective for dialing in a camshaft where power is a main concern.
Top Dead Center
Remove the bolts in the driver's side valve cover using a socket. Remove the valve cover. Remove the distributor cap if applicable, using a screwdriver.
Rotate the engine clockwise using a socket and ratchet on the crankshaft bolt. Rotate the crankshaft and watch the first two rocker arms on the driver's side. Continue to rotate the crankshaft clockwise until the first rocker arm (exhaust valve) goes down to open the exhaust valve. Continue to turn until it comes back up and the second rocker arm (intake valve) goes down opening the intake valve. Continue until the rocker comes back up and you see no further movement.
Watch the harmonic balancer as it turns and look for a scale in degrees. Look for 0 degrees before top dead center (BTDC) on the balancer. Look at the timing chain cover on the front of the block and you will see a timing plate, which is also graduated in degrees. Continue to turn the crankshaft slowly until the 0 line on the balancer is aligned with the 0 pointer on the timing chain cover. The distributor rotor should also be pointing to the number 1 cylinder, which is the driver's front left side. This is TDC or top dead center for all practical purposes.
Finding TDC with a Degree Wheel
Remove the number 1 spark plug. Install the piston position plunger in the spark plug hole. It has a rod that touches the top of the piston and a vertical stop scale parallel to it. There is a small sliding ring on the scale. Move the ring to exactly where the top of the plunger is located.
Cut the bottom of a coat hanger so you have a straight piece of wire. Curl one end over and bolt it in the cylinder head to hold it in place. This may require removing the power steering pump. The wire could also be mounted to the water pump bolt. Point the bottom of the wire or the pointer directly at the 0 degree mark on the timing plate on the timing chain cover. Tighten the bolt to hold it securely in this position.
Remove the accessory belts by using a socket to loosen the tensioners and remove the belts. Remove the accessory belt pulley on the crankshaft balancer, using a socket to remove the bolts. Install the degree wheel on the balancer.
Mark the position of the coat hanger pointer relative to the 0 degree mark on the degree wheel by marking where the pointer is on the wheel. Rotate the crankshaft counterclockwise very slowly, using a socket and ratchet while watching the piston position plunger. Stop as soon as you see the plunger begin to drop down. Mark the position of the pointer on the degree wheel.
Rotate the crankshaft clockwise and again watch closely for the plunger to just begin to drop. Stop rotating the crankshaft. Mark the position of the pointer on the degree wheel.
Count the degrees between the two outside marks. Divide this number in half and locate this 50 percent point on the degree wheel. For example, if the wheel rotated counterclockwise 23 degrees and clockwise for 21 degrees you have a 44 degree arc Split this figure and you have 22 degrees. Count 22 degrees from either outboard mark and mark this new spot on the degree wheel.
Rotate the crankshaft so the new 22-degree mark is under the pointer. Remove the degree wheel and make a white line on the harmonic balancer directly under the pointer. This is your new 0 degree timing mark.
Things You'll Need
- Set of sockets
- Spark plug socket
- Common screwdriver
- Phillips screwdriver
- Additional tools needed for absolute top dead center:
- Degree wheel
- Piston position plunger
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).