Top Dead Center – T.D.C.

Top Dead Center, otherwise known as TDC, is the culmination of the engine electrical system. If you don’t have TDC you don’t have a car that runs, you have a lawn ornament!

You can change the spark plugs, replace the distributor or install a new coil, but none of this will matter if you don’t have T.D.C.

T.D.C. is the starting point of the rotation that the the cylinders go through in their firing order. The camshaft, located below the pistons, which are below the spark plugs, within the cylinder the spark plugs are screwed in to, with it’s varied lobes sets the pace for the order.

When you set the timing it is according to the first piston being at the T.D.C. of the first cylinder. Once you have that set you can set the distributor cap to the approximate location where the cam will be. Then, the vehicle will start and you can go on with the timing.

There are methods for finding T.D.C. One in particular I didn’t appreciate is the idea of putting my finger on the spark plug hole while someone turns over the engine until I feel the pressure of the air being expelled by the piston.

I’m not a tall person. To reach the spark plug hole on the first piston I have to climb into the engine bay of this Chevy 3500. Not the place I want to be while someone is cranking the 454 engine over!!

Manually turning the engine with a breaker bar……well, yeah right.

I came up with a method that is totally safe, far more accurate than the finger thing and kinda fun at the same time!

What I did is locate a hollow item with the same threads as the spark plug. This turned out to be really easy! Connectors and fittings for my air compressor have the same exact thread as a spark plug! And they are hollow too!

Balloon and compressor attachment,

So, with my threaded air compressor fitting I attached a party balloon to the end without threads. I blew into the other end of the air compressor fitting to inflate the balloon a little to make sure it would stay on the fitting. If it pops off when you blow into the fitting put a rubber band around the lip of the balloon or a tie wrap or whatever to make it stay.

Now screw the threaded end into the spark plug hole, but NOT SO FAR THAT YOU CAN’T REACH IT. I actually stuffed the balloon into my spark plug socket and then I held the socket with my fingers and screwed it in a few turns. When I removed the socket the balloon was still attached to the air compressor fitting.

I was able to stuff the balloon into the spark plug socket to make screwing it in easier.

Now, when you turn the engine over, locating T.D.C., the balloon will inflate when the number one piston rises in the cylinder! When the piston reaches the T.D.C. it starts back down the cylinder thus air is sucked back out of the balloon.

Here’s the balloon in place of the spark plug.

When I cranked it the second time I knew how big the balloon would get when it was at its fullest IE: T.D.C. When the balloon filled up again I could clearly see it from the drivers seat and I removed the air compressor fitting with the balloon attached (I was able to twist the balloon and it twisted the fitting in the spark plug hole and it came right out. I could see the top of the piston with my flashlight!

And there it is T.D.C.!

Now I knew I had the piston at T.D.C. and continued with the timing process!

When I had T.D.C. I was able to unscrew the balloon by had by twisting the balloon several times until it started to back out. I could have put the socket back on it to remove it, but twisting the balloon was just as easy.

Now you have another method for finding T.D.C.!

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