Does anyone know where Top dead center is on Piston #1 in a 1985 305 z28 Engine? This engine doesn't have a Harmonic Balancer, or a Distributor so it's hard to tell. When the timing chain DOTS are aligned is that TDC for Piston 1? I have to do a Valve Lash soon so I need to know. Thanks.

 

With the number one piston at the top of the bore and the number 6 rockers are rocking the number one piston should be at TDC on the compresion stroke.


I think, you should get a book that has this type of info

 

if i recall correctly you want the dots straight up and both valves will be closed for TDC on the firing stroke of #1. dot to dot would also be TDC, since the cranks spins around twice for each cycle of the engine. if you have to run the valves having #1 at TDC isn't going to be of much help for the other 7 holes. just look for both valves to be closed at each cylinder you want to adjust. if you have the intake off it isn't hard to tell when the cam lobe is up or down on the lifters.

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JM,

Today's helpful hint:

The keyways cut in the crankshaft snout are in line with the #1 throw on the crank. When the keyway for the harmonic balancer is at the 45° position, the #1 cylinder is at TDC.

Determining if it is on the compression or exhaust stroke is your business.

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Later,
Vader
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Down with the Sickness
Adobe Acrobat Reader

 

But if you're doing a Valve Lash, It doesn't matter if it's on the Compression or exhaust stroke right? As long as it's Top Dead Center right?

 

Imagine this, If the Camshaft Sprocket Dot is at 6 o'clock and the Crankshaft sprocket is at 12 o'clock,is that TDC for piston 1? Then, if the Camshaft Sprocket is at 12 o'clock and the Crankshaft sproket is a 6, it that TDC for piston 6? Also, does the firing order make a difference?

JUST PLEASE TELL ME HOW TO DO A VALVE LASH OTHER THAN HAYNES!

 

JM,

The timing markers aren't always keyed to the engine for a given position. As long as the markers are in the correct relative position for the crank and camshaft, they'll work for installing the timing set and may not be a good clue to crank position.

Some sprockets have "plus" and "minus" markers for advancing or retarding the valve timing, and the markers don't have to relate to crank position directly.

Is the engine in the car or out?

Is the intake manifold installed?

Are both rocker covers removed?

Do you have a degree wheel? Layout dye? Permanent marker?

If you presume a position based on the crankshaft snout keyway, you can set the valves.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

since it sounds like your engine can't or won't start try this. rotate the crank till a sae of valves are closed on a cylinder. tighten the rocker nut while you spin the push rod. when the push rod stops spinning ad another 1/2 turn. you have to watch the lifter doesn't start spinning doing it this way. that should allow you to fire the engine. after it runs set the lash by backing off a rocker nut until it clicks then tighten until it stops plus 1/2 to 3/4 turn more on the rocker nut.

 

#1 is at TDC twice during each engine cycle. So it is possible to have the #1 piston at TDC, but still not be at the correct point for valve lash adjustment.

With most timing sets, when the dot on the crank gear is "up" and the dot on the cam gear is "down" (dots together) the #1 piston is at TDC but it is at the other instance of TDC besides firing. Specifically, it is at the point where the #1 cyl exhaust stroke is finishing, and the intake stroke is beginning. So if you rotate the engine slowly through that point, you will see the exhaust valve still being held open somewhat, and the intake valve already beginning to open.

A quick lesson in how a 4-stroke engine works might be in order. If you watch its actions as the crankshaft rotates, starting at #1 firing, you will see that the piston begins to be pushed down by the hot expanding gases in the cylinder. After the crankhas rotated 180°, which is ½ revolution, the #1 piston will be at the bottom. Somewhere near this point, usually somewhat before it gets there, the exhaust valve will begin to open. The crank continues to rotate; the exhaust valve reaches maximum opening when the piston is about halfway back up; then the valve begins to close. 180° of crank rotation after the start of the exhaust stroke (and one full rotation after firing), the piston reaches TDC again. The exhaust valve may or may not be fully closed at this point. The intake valve has probably already started to open.

As the crank continues to rotate, the piston descends in the bore again, this time drawing fresh air-fuel mixture in through the open intake valve. When the piston is somewhere near about halfway down the intake valve reaches its fully open position, and begins to close; the crank continues to rotate; 180° after this last TDC occurrence, the piston reaches the bottom. Somewhere near this point the intake valve closes. The crank continues to rotate; the mixture in the cyl is compressed; as the crank approaches TDC again, the spark plug receives its spark, and the mixture is ignited, a few degrees before the piston reaches TDC.

Keep in mind that the crank rotated twice during this cycle. The cam however rotated once, since it performed each of its valve events exactly one time during these 2 crank revolutions. In a V8 there are 8 cylinders (duh); their firing points are evenly spaced throughout the 2 crank revolutions that it takes to make one complete engine cycle; so during each crank rotation, 4 cylinders fire. The firing order in a GM V8 is 18436572. So #1 firing occurs at the instant that the #6 valves are "rocking", and vice-versa, since #1 and #6 are 4 cylinders apart in the firing order.

So: since from what you have been saying you can see the crank & cam gears (your timing cover must be off), what you should do to get htis done the easiest way is align the dots so that they are "together". That will be #6 firing. At that point, with a stock hydraulic cam (or any other hydraulic cam that will run in a TPI motor for that matter), you can adjust both #6 valves. Pick one of the #6 rockers, loosen it way up, and then tighten it while jiggling its push rod up and down. As you tighten the nut, the pushrod will have less and less play in between the rocker and the lifter; presently the push rod will become tight in there, which is the spot you would also look for by spinning it between your fingers; when you reach that point, tighten the nut exactly ½ turn more, and do the other #6 valve.

When you are happy with the #6 valves, turn the engine exactly 90° in the clockwise direction viewed from the front (in other words, the same direction you would turn the bolt in the end of the crank to tighten it). Remember that 4 cylinders fire during each crank rotation, evenly spaced; that means one fires every 90°, which is ¼ revolution. So at that point, the next cylinder in the firing order is firing and ready to be adjusted. Go back and look at the firing order, you will see that it will be the #5 cylinder. Do that one exactly as you did #6. Then turn the engine another 90° and do the next one, which will be the #7. Proceed all the way through the firing order, all 8 cylinders, and you're done.

Rotate the engine almost until both dots are "up", which should be 90° plus almost one more full crank revolution from where the engine now is. Leave the engine positioned to where the crank is about 10° before it reaches #1 firing. DO NOT move the crank from this position before you put the distributor in, in its correct alignment. If you can't get that done (most likely the distributor won't seem to go in the last ¼" or so) then I'll describe the technique for lining it up.

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[This message has been edited by RB83L69 (edited October 14, 2001).]