I have a 1969 Ford Thunderbird 429 Thunderjet and replaced the distributor with a stock distributor with a Pertronix electronic ignition. In the process of troubleshooting why it wouldn't start, we turned the distributor.

While I was at work, my brother and a neighbor suspected the timing was 180 degrees out. Considering how timing is set by making sure the #1 piston is all the way up, I was wondering if there is more to it than simply making sure the #1 piston is all the way at the top. Is there something else to consider in terms of the position of the pistons when installing a distributor?

I've seen on forums where some posters suggest to question-askers that they have their timing "180 out", but it seems to me that is very unlikely to happen if you're at top dead center? Is it possible for the piston to be all the way up but you're still not at top dead center?


The words you want to remember while setting the timing is top dead center ready to fire.You want both valves closed.They make a tool that inserts in the spark plug hole that whistles as you rotate the crank to alert you that both valves are closed.Or put your thumb on it and feel the pressure.You want to be TDC on the compression stroke which is 180 degrees opposite of TDC of the exhaust stroke.Both TDC will read 0 on the crank timing ring. The 180 degrees refers to the camshaft position relative to the crankshaft.the camshaft is turning at 1/2 the crank speed.Think of it as TDC for the crank is always with the piston at the top of the cylinder but the camshaft can be 1/2 a rotation out of position(180degrees) By rotating the crank 1 full revolution you will rotate the cam 1/2 a revolution .Hope this clears it up.

  • Thanks! This is helpful +1, but I'm still having trouble picturing this in my mind... So for TDC on the exhaust stroke, is the #1 piston physically located in the same distance from the plug as it would be in TDC of the compression stroke? In other words, for TDC on the exhaust stroke, is the piston still X inches from the plug on both strokes, or is it located at the bottom of the motor at TDC on exhaust? I hope my question is clear, I'm not sure I'm using the right terms. :) – jmort253 Apr 1 '12 at 17:47
  • Yep, it doesn't matter if you're on exhaust stroke TDC or compression stroke TDC for the piston's position relative to the spark plug. In both cases it's at the top of its travel. – Timo Geusch Apr 1 '12 at 18:38
  • The thing to remember is that one cycle of a 4-stroke engine takes two revolutions of the crankshaft (and thus the pistons), but only one revolution of the camshaft. Therefore #1 piston is at TDC (closest point to the spark plug) twice in the cycle, and you need the spark to occur the second time - at the end of the compression stroke. – Nick C Apr 2 '12 at 8:56
  • It would be great if you could add a diagram to clarify – Cullub Jul 5 '16 at 20:47

There are 2 points in the engine's cycle when the piston is at TDC: The transition between compression/combustion, and the transition between exhaust/intake. The first one is the TDC you're looking for when concerning ignition timing. The #1 piston will be at the top of the cylinder and your crankshaft timing mark will be aligned to the 0 witness mark.

Pull the distributer body from the engine before rotating your engine's alignment to TDC. Then, carefully reinsert the distributor so that its spark transfer blade is pointed directly at the #1 port when bolted down. Install plug wires according to proper firing sequence. Crank, and use a timing light to advance your ignition timing as needed.

Assuming your crankshaft and camshaft(s) are properly timed to each other via the belt or chain system, the above process will serve you well for all gasoline cylinder engines.

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