How to Find Compression TDC on Cylinder 1
A safe method for bringing cylinder 1 to compression top dead centre, and how the distributor rotor identifies the No. 1 cap terminal.
Nearly every ignition-side procedure starts here. Getting it wrong by one crankshaft revolution puts cylinder 1 on exhaust TDC instead, and everything downstream is then 360 degrees out.
The procedure
Remove the cylinder 1 spark plug with the engine cool. Rotate the crankshaft in its normal direction while feeling for compression with an appropriate method, then bring the timing mark to zero or the specified installation position. Never use the starter with hands, tools, or loose clothing near rotating parts. A piston-stop tool requires a dedicated procedure and should not be improvised.
Identifying the No. 1 terminal
On a distributor engine, remove the cap and observe the rotor. The cap post over the rotor becomes the practical No. 1 location for that installation. If the vacuum advance, wiring, or housing cannot be positioned correctly, the distributor may need to be re-indexed, but re-indexing is separate from changing the firing order.
Values, sequences and intervals are vehicle-specific. Use the specification page for your exact year, engine and component alongside this procedure, and follow the manufacturer document it cites.
How to Find Compression TDC on Cylinder 1 FAQs
Is the timing mark at zero enough?+
No. The mark reads zero twice per four-stroke cycle. You need compression TDC specifically, with both valves closed.
Can I bump the starter to get there?+
Not with hands or tools near rotating parts. Rotate the crankshaft by hand in its normal direction instead.
Does the No. 1 distributor terminal have one fixed clock position?+
Not necessarily. The terminal over the rotor at cylinder 1 compression TDC is No. 1 for the installed distributor position.