Finding TDC

[68_Step]

I am getting ready to rebuild my distributor on my 1978 K30 454ci and I want to verify TDC before putting the distributor back in. I made the below list, using several threads I found on the forum. Does everything look right? If something is out of order or wrong point me in the right direction.

One thing I am still fuzzy about is how far into the cylinder i screw the stop.

1. Insert the piston stop, then turn the engine by hand until the piston contacts the stop. Mark the balancer at that point where the 0* is.

2. Then rotate the engine back the opposite way until it hits the stop again. Mark the place where it stops using the 0* as a reference

3. TDC is exactly between the marks. If that is where the factory timing mark hits, nothing needs to be done. If it dosn't make a new "permanent" mark.

4. Rotate the engine to where the the harmonic balancer is lined up on 8 degrees to the right of the 0*

5. Holding the dist above the hole, point the rotor to the #1 piston and drop the dizzy in. May need to turn the oil pump shaft.

6. If the rotor rotates away from the #1, pull and adjust, drop in again. Adjust oil pump shaft again if needed.

7. Install the dist cap, whichever tower the rotor is pointing at is now #1 - should be front right corner

8. Install the plug wires in firing order sequence: 1 8 4 3 6 5 7 2 working clockwise around the cap

9. Cut wires to fit and istall the plug wires to the plugs.

So other than hope I don't goof it up, does the list look OK? Thanks guys!

[enaberif]

TDC #1 = #3 intake valve all the way open

[rich weyand]

No, it's not OK. How do you know WHICH top dead center you are on, the one between compression and power strokes, or the one between exhaust and intake strokes? Since the crankshaft rotates twice for every rotation of the camshaft, the same crankshaft position could be correct, or could be 180 degrees out on the distributor.

The correct TDC is when the valves on #1 are both closed.

Another way to check it is to pull the plug and mechanically turn the engine over with your thumb on the spark plug hole. The correct TDC will push your thumb off the hole.

BTW, for making the timing mark, it doesn't matter which TDC you use. The timing mark goes past twice every distributor rotation.

[Pampas56]

Rich:

See previous post.

You are correct in that both valves are closed both times #1 hits top dead center. the difference is the intake valve that is open on another cylinder.

#3 is open on #1s firing stroke and #2 is open on intake stroke.

In the old days if it wouldn't fire on startup, we would pull the distributor and rotate it 180 or just rotate the plug wires and time with #6

[rich weyand]

No, the valves are both completely closed when #1 hits TDC between the compression and the power stroke. Both valves are partially open at TDC between the exhaust and intake stroke.

The exhaust usually closes 10-20 degrees after TDC, while the intake usually opens 10-20 degrees before TDC.

That's why the "push your thumb off the spark plug hole" test works. Another test for the correct (ignition) TDC is to put an air hose into the plug hole with a rag around it and hit the air. If the rag blows out, it is the ignition TDC. On the wrong one, the air will blow through the (partially open) valves.

Another test is to move the engine either way from TDC. If the valves on #1 stay still, that's ignition TDC. If the rockers move, that's the wrong TDC.

In any case, the OP didn't say anything about determining which TDC he was on before stabbing the distributor, and that gives you a 50/50 chance of being wrong. Murphy's Law says it's actually more like 80/20.

[Pampas56]

I understand.

The valves are just beginning to open and close. Too subtle a difference for my old eyes. The easier way to tell is to look for the proper wider open valve on #1s firing stroke.

An alternate method is to watch the valves on #1 as you rotate. As soon as #1s exhaust valve opens wide you should be on the exhaust stroke. one full rotation should close that valve and bring you up on the TDC for the power stroke.

An old mechanic told me years ago that the valves should be closed or close to it near TDC or you end up with bent valves.

Good discussion.

[Bigstevex4]

Or just take note of were the rotor is when pull the distributer and put it back in the same spot. Even if your off a tooth you can compensate for it. Forget about top dead center.

[68_Step]

Originally Posted by Bigstevex4

Or just take note of were the rotor is when pull the distributer and put it back in the same spot.

I don't want assume the PO had it in the right spot to begin start. I have the time right now so I figured I would learn how to find TDC to set the timing from scratch.

[68_Step]

Originally Posted by rich weyand

No, it's not OK.

I forgot to start that list making sure I am on the compression stroke. If I do that, is the checklist OK then?

BTW, this is a good discussion - I have learned a lot.

[rich weyand]

Originally Posted by 68_Step

I forgot to start that list making sure I am on the compression stroke. If I do that, is the checklist OK then?

BTW, this is a good discussion - I have learned a lot.

That'll do it.

[cmax87v3500]

put your finger in the #1 spark plug hole, have a friend rotate the engine until you forcefully feel air moving past your finger go until it stops and you should be close.