Just stumped on the timing chain.

I was down to the block and had it machined. Just installed the crank, rods, and cam.

Its a double roller chain i had it on there before my engine failure, for about 2000 miles. i Just forget how it goes, second guessing myself.

I know the the dots should just line up at 12, and 6. However, my crank chain sprocket has 3 ways you can put it on. Which would change where the pistons are.

On the sprocket there is Choice A, Choice B, and then the dot.

I installed it with the key at the DOT. So i guess to relieve stress. What piston should be at TDC when aligned at 12, and 6


So my questions is

 

#1 and #6

THINK about a 4-stroke engine for a minute...

Imagine the #1 cyl, at firing. Crank is at TDC. Spark plug ignites the fuel. Crank turns, piston goes down. This is called the "power" stroke because it's the motion that extracts the energy from th enow-hot gases in the cyl and turns that energy into mechanical motion. 180° of CRANK rotation later, piston is at the bottom. Exh valve starts opening. Piston starts coming back up, pushing exh gas out the open exh valve. This is called the "exhaust" stroke, I'll let you figure out why. 180° later - now 360°, or EXACTLY ONE COMPLETE CRANK ROTATION, we're back at TDC again. But the spark plug is NOT about to fire this time. Instead, the exh valve closes, and the intake valve opens. Crank continues to turn. Piston goes down, allowing the atmosphere to push air & fuel mix into the cyl through the int valve. This is the "intake" stroke. When the piston reaches the bottom 180° later, the int valve closes. The crank continuees to turn and the piston goes back up, compressing the gas. I'll let you figure out what this stroke is called. After 180° more degrees of crank rotation, the piston reaches the top A SECOND TIME, the plug fires, and we start all over again.

OK so what did you just learn? Hopefully, that the crank turns TWICE for each ONE firing of the #1 plug; that the valves moved ONCE for every TWO crank rotations; that the piston was at TDC TWICE during the engine cycle, once that was its firing instance and once that was NOT firing but rather was when the valves were swapping over.

OK now that you've got that all firmly in your mind, remember that we're working on a V8. 8 cylinders. If a full engine cycle is 2 revolutions, that means 4 cylinders have to fire during each crank rotation. That means one cyl fires every 90°. So take a look at the firing order 18436572, and observe that the 1st and 5th cyls (1 & 6) will therefore always have their pistons in the same relative motion at the same time; as will the 2nd and 6th (8 & 5), 3rd & 7th (4 & 7), and 4th and 8th (3 & 2). Each of these pairs of pistons will be at TDC at the same time. However, at any given instance of TDC, ONE will be firing, and THE OTHER will be swapping between the valves.

OK, so what does that mean for your timing set? As it happens, it is designed so that when both dots are at 12, it's #1 & #6 TDC, and #1 firing; and when the crank dot is at 12 but the cam dot is at 6, it's ALSO #1 & #6 TDC, but instead it's #6 firing. Meaning, you can build the motor "dot to dot", and it's easier to see when it's lined up right; but that's NOT #1 firing, it's #6 firing. You would have to rotate the crank one rev from there, to the 12 & 12 position, which will again be #1 TDC, and this time it will be #1 firing.

You have 3 keyways in your crank sprocket; one at 0, one that advances the cam 4° with respect to the crank, and one that retards the cam 4°. Your markings should be "0", "A", and "R". Use the dot with the "0" mark.

 

i believe its numbers 1 & 6. cam mark at 6 oclock, crank mark at 12 oclock. after the chain is installed correctly, the motor is set to fire the number 6 cylinder. if you turn the crank over 1 revolution so the marks are both at 12 oclock, you can drop the distributer in & it will be set to fire the number 1 cylinder.

 

Okay thats exactly how its done aligned witht the 0 mark and chain put on and ready to go

Thanks for the help and reasurance.

 

Throw it away and buy a new one. With an engine failure, you have no idea what kind of abuse the chain took. Inexpensive chain sets are cheap for a reason. Good sets come with billet gears but the chains will still eventually stretch.

As for installing the chain. When the crank and cam gear dots are both at the 12 o'clock position, #1 cylinder is at TDC compression stroke. It's easier to line up the dots at 6 and 12 which will have #6 cylinder at TDC compression stroke. You'll probably be rotating the crank a few times before final assembly anyway. Just make sure the dots are both at the top or know when #1 is at TDC compression before dropping in the distributor.

As mentioned above, install the crank sprocket with the 0 at the keyway. Unless you're doing all the checks to degree in the cam, don't worry about the advance or retard settings.