How to Find the TDC of a Chevy 350 With a HEI Distributorby Jack Hathcoat
Determining top dead center (TDC) of a Chevy 350 engine is essential if the engine was rebuilt and is ready to be started. When the engine is set to TDC, the number one cylinder is positioned to fire the number one spark plug. That means the piston is at the top of it's compression stroke, both the valves are closed, and the gasoline and air mixture is compressed. When a spark is applied by the spark plug, the cylinder fires and power is transmitted through the crankshaft. Unless the location of TDC is known, the HEI distributor cannot be installed and timed properly.
Raise the hood of the vehicle and remove the fuel pump fuse. Use a socket wrench to unscrew the number one spark plug. On the Chevy 350 engine, as you face the engine, this is the right front spark plug.
Shine a flashlight on the lower crankshaft pulley, just behind the belt. The pulley bolts to the harmonic balancer, which is directly attached to the crankshaft. A series of timing marks is scribed onto the balancer and one mark is labeled TDC. Locate this mark and highlight it with a paint pen.
Crank the engine over and hold a finger over the number one spark plug hole. Have an assistant crank the engine repeatedly until you feel compression pushing out of the hole. When this occurs, the number one cylinder is approaching TDC.
Fit a socket over the crankshaft bolt that holds the harmonic balancer to turn the crankshaft and perfectly align the paint mark on the balancer with the pointer that is bolted or welded to the engine. When the mark and the pointer are aligned, the engine is set and ready for the HEI distributor installation.
- The distributor rotor can be set to point to any spark plug wire location. On the bottom of the distributor is a gear that meshes with the camshaft that is located inside the engine. Below the gear is a tab that fits into the oil pump so it will spin and pump oil. By shining a flashlight down into the hole where the distributor fits and turning the oil pump drive slot, the drive tabs can be aligned to set the distributor rotor at any desired location. The conventional way is to align the rotor so that it points to the number one cylinder.
Things You'll Need
- 3/8-inch socket set
- Paint pen
- Although skipping all the steps and simply adjusting the harmonic balancer pulley to TDC and installing the distributor may seem like a direct approach that is quick and easy, it doesn't always work: the crankshaft travels around twice during the cylinder's full operation, and the exhaust cycle also shows up at TDC on the harmonic balancer. If the distributor is set at the exhaust cycle to deliver a spark, the engine will not start.
Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.