Method 1: Using the Timing Chain Cover (Most Reliable):
1. Access the Timing Cover: This method requires removing the timing chain cover. This is generally a labor-intensive job, requiring removal of various components depending on your comfort level. This might be the only reliable way to be absolutely sure.
2. Rotate the Crankshaft: With the cover off, you can visually see the timing marks on the crankshaft and the camshaft (if accessible). Rotate the crankshaft slowly until the timing marks align. Your repair manual will show the exact location of these marks. This is the most precise method.
3. Verify TDC: Once the marks align, double-check the #1 piston is at TDC by inserting a long object (like a straightened coat hanger) through the spark plug hole. The piston should be at its highest point.
Method 2: Using the Piston Method (Less Precise):
This method is less precise but avoids major disassembly.
1. Remove the Spark Plug from #1 Cylinder: Locate the #1 cylinder (usually the front passenger side on a small-block Chevy). Remove the spark plug.
2. Insert a Straight Object: Insert a long, thin, and straight object (a wooden dowel or a straightened coat hanger works well) into the spark plug hole until it hits the top of the piston.
3. Rotate the Crankshaft: Slowly rotate the crankshaft (using a wrench on the crankshaft bolt) until the piston reaches its highest point. Mark the crankshaft's position.
4. Continue Rotating: Continue rotating the crankshaft until the piston moves down, then back up again, reaching its highest point a second time. This second highest point will be your TDC. This is a critical step to ensure you don't mistake the first compression stroke with TDC. The piston should be at its very highest point.
5. Mark Your Position: Mark the crankshaft's position again. The mark you made should be your TDC.
Important Considerations:
* Repair Manual: A repair manual specific to your 1993 Chevy G20 350 is invaluable. It will have diagrams showing the timing marks' locations, the #1 cylinder location, and the proper crankshaft rotation direction.
* Crankshaft Rotation: Rotate the crankshaft in the direction the engine normally runs (clockwise when viewed from the front of the engine).
* Accuracy: Method 1 (using the timing cover) is far more precise than Method 2. If accuracy is critical (e.g., for precise timing adjustments after distributor replacement), Method 1 is strongly recommended.
* Safety: Be careful when working around the engine. Use appropriate safety equipment, such as gloves and eye protection.
After finding TDC: Once you've accurately determined TDC, you'll use this position as your reference point when reinstalling the distributor and setting the ignition timing. Again, your repair manual is crucial for this process. Misaligning the distributor can cause serious engine damage.
