Im trying to find TDC on my new engine while its on the engine stand so that I dont have to do it when it is in the car. I have the hose that came with my leakdown tester screwed into the #1 plug hole with a balloon on the end. The balloon will inflate with air as I turn the engine and I watch the two valves and I stop turning when both are in the closed postion. My problem is that the rest of the valves on the drivers head can end up in 2 different postions sometimes. Im not sure if you get what Im saying but when I think I have TDC, I look at the rest of the valves and remember their postion. I then keep turning the engine until the ballon fills again. I watch the #1 valves and stop when their closed again and the rest of the valves are now in a different postion. Is this right or am I looking to much into this? The timing marker lines up perfect at TDC when the #1 valves are closed so I know Im doing something right. I just want to make sure it is 100% correct before I drop the dizzy in. Can the engines valves' be at different positions when im at TDC on #1?

 

You're on the #1 compression stroke when neither valve has moved during the last ~half turn of the crank to TDC.

When at the mark, both #1 valves can appear to be closed, but one has actually been moving and the other will when you continue turning. That's compression for #6.

 

so both valves will be closed even before I get to to TDC?

 

Both valves will be closed as you approach TDC, and will remain closed as you pass it.

 

Think about the 4-stroke cycle engine. The intake valve opens and the piston moves down drawing in air/fuel. The intake valve closes and the piston moves up, compressing the mixture. The spark plug ignites the compressed mixture and the piston moves down. The exhaust valve opens and the piston moves up, expelling the spent gases.

So, yes, both valves have to be closed as the piston moves up in order for the air/fuel mixture to be compressed. And remain closed past TDC in order for the pressure created by combustion to drive the piston down. And, after the exhaust stroke, the exhaust valve will be closing as the piston approaches TDC, and the intake valve beginning to open so air/fuel can be drawn in as the piston passes TDC and begins moving back down.

Also, don't forget that "TDC" and "#1 firing position" are two different things. The former describes the position of the crank/piston, the other the position of the cam/valves. Each piston passes through TDC twice through one 4-stroke cycle.

 

ok I got it now. Ill spin the engine over a few times and visually try to run that sequeance in my head. Im sure after a few roatations, I will get it and know when im at TDC. Thank you.