my75baby

Hello all, I need to figure out what TDC on my SBC 400. I changed the timing chain and have the distributor out and need to put it back in on #1 cylinder. I lined up the chain gears dot to dot (6 oclock on Cam gear and 12 oclock on Crank gear) I think this would make the engine fire on #6 but not sure? What position are the valves on #1 at TDC. I think they both should be closed (lifters sunk in motor), but is this the only time the valves are this way or could the valves be closed at some other time in the rotation? (other then ready to fire on #1?) the whole motor is pretty much open now I just want to set it right before I put it back together.

larrywalk

If you are at dot-to-dot, #6 should be at the firing position, but check it - here's the easy way: Just look at the #3 intake valve spring - it the #3 intake valve is open, the engine is at #1 firing; it it's not, it's at #6 firing!

About your other question... At TDC, the valves at #1 will be closed at firing, or very slightly open at overlap, or maybe not open at all if using hydraulic lifters and the cam has little overlap. This method of determining #1 firing is not very reliable compared to the first method.

Lastly, are you at TDC? Early harmonic dampers (prior to ~69) had the TDC line located in line with the crank keyway. Later dampers had the TDC line 10 degrees counterclockwise from the keyway. This fact makes it easy to check for the oft maligned slipped HD! In any event, the crank keyway is always where the #1 crankshaft journal is located for SBCs.

my75baby

Thanks Larry, one more question, if I line up timing gears dot to dot, can I just rotate the crank until cam gear dot is at 12 oclock and crank gear is at 12 oclock and put a straight edge across to make sure of perfect alignment, wouldn't that set up for #1 firing order so that I can just drop the distributor in at #1? doesn't that have to be at #1 firing at TDC? (I could also make sure that #3 intake valve is open and #1 valves are closed. This makes sense to me, but then again I can convince myself of anything, I am a pushover that way.

Manuel Azevedo

Yes both dot's at 12 oclock is fireing at #1 and you can put the dist. in with the rotor at number one.

larrywalk

For each turn of the crank to TDC, the timing dots will alternate dot-to-dot (6 to 12 o'clock), and both dots at 12 o'clock. (I prefer doing dot-to-dot because it's easier to see the alignment since they're closer together.)

And, if you're a bit lazy like I am, if you find that you're at #6 firing and you don't want to turn the crank another whole turn, you can just as easily stab the distributor by pointing the rotor to the #6 spark plug wire post. (Don't forget that the rotor turns an additional ~30 degrees clockwise after it first contacts the cam gear.)

Summary: At Top Dead Center, either #1 or #6 is at firing...
#1 is at firing if the #3 intake valve is open
#6 is at firing if the #2 intake valve is open

After you've got the distributor installed, you can time the engine by hooking the timing light to either #1, OR #6, whichever is more convenient!