Hey guys. This should be simple but im second guessing myself so i thaught id ask u guys what u would do. I got a long block for my sons dakota with a 3.9 v6. Im fixin to put new motor together and i was wondering about tdc. Is tdc acheived when the intake valve just closes and the piston comes up or when the exhaust valve closes. I always thaught it was when the int valve just closes and u turn the motor a little bit more and the piston comes up to be tdc. I noticed the new motor is set when the exh valve just closes. thats 180 degree out from the way i thaught you did it. So which way is right?

When the exh valve is closing, the int valve is just opening, and the piston will be on top dead center firing cyl #4. NOT # 1.

ok so tdc on #1 is when the int valve just closes? right?

Yes, although just after the intake valve closes, you will have to bring the crank clk/wise around 1/4 revolution to get it to compression stroke for #1.

ok. i think i got it right. just to make sure i will explain exactly what i did. I turned motor clockwise until i see int valve open then close. then about 1/4 turn i the piston is at top of stroke. BAM thats it right?? Reason i started second guessing is the long block i ordered came with the timing marks lined up but they had tdc when the EX. valve just closed. Which puts my dist cap pointing to #4 cyl tower on the cap, even though the timing marks lined up the way they had it. Also i was confused by setting tdc with the int valve just closing puts the timing marks off. It puts the crank mark where it should be but puts the cam sprocket mark in the 12 oclock position.

Sounds correct. The dots vertically next to each other is firing #4 on the 3.9.

Ok. Good. Would it run with it lined up to #4? Thanks for the help!! hoping to get this thing running tomorrow. Not much time. My son has to get back to college.