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kingofny1997 06-18-2012 09:37 PM

Cam timing
What's up hot Rodders, a little on the confused side, since I hear this and then that....

I just installed a new cam and now the new gear sprocket with chain, now I have it 6 o'clock cam and 12 crank (dot to dot)... Lifters just installed, now next step I have read in certain places that 6-12 is TDC (but really #6 cylinder firing) 180 degrees out and that 12-12 is really TDC (#1 firing) do I leave it 6-12 oclock or 12-12 o'clock before I put the timing cover on and distributor rotor would be on what number

JohnTN 06-19-2012 07:47 AM

You got it right. At 6-12 the rotor should be pointing at #6, at 12-12 the rotor should be pointing at #1. You can put it back together either way.

If you put it back with cam/crank in 6-12 position and rotor at #6, when you rotate 180 degrees it will be at 12-12 and rotor at #1. And vice-a-versa.

If it is a new cam, you ought to degree it to confirm the valve timing.


kingofny1997 06-19-2012 09:20 AM

Ok understood, now when I found TDC at 6-12 the #1 cylinder had the compressed air coming out the spark plug whole, when I turn it 180 degrees, and have it at 12-12 will I feel the same compressed air come out the #6 cylinder (since it happen with #1 at 6-12)

JohnTN 06-19-2012 01:11 PM


As you rotate the engine the next cylinder to fire will be on its compression stroke. So, as you approach the 6-12 position the #1 piston is coming up in the cylinder, both intake and exhaust valves are closed, and the fuel/air mixture is being compressed so you will feel air being forced out of the spark plug hole.

The firing order is 1-8-4-3-6-5-7-2 (on the GM engines I am familiar with)

On one revolution of the cam sprocket, each of these pistons reach TDC, in turn, every 45 degrees of rotation. So at 45 degrees you would feel air being pushed out of the #8 spark plug hole, etc. -- at 180 degrees, the #6 sparkplug hole.

And of course that is why it is important to have the spark plug wires going to the correct spark plug in the right order.


kingofny1997 06-19-2012 01:53 PM

Thanks got it, really understand got it, I am going to put the timing cover and gaskets then to the water pump and pulleys

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