I ask this here because I trust you guys more than the other message boards I've seen. My buddy has an 83 GMC S-15 with a carbed 2.8, and we were checking it out today..the entire motor is in the bed of the truck and basically, EVERYTHING is removed. Hell, the camshaft can be slid in and out of the motor freely. Which worries me....I'm betting his lobes are gonna be eaten up within a few hundred miles after he assembles it.. But the question is.....the cam and the crank both have the gears and all removed from them, and both have been moved around separately....completely out of alignment. So how in the hell do we line up the crank and cam so we can get the gears and chain on? Thanks guys...

 

ok, bear in mind that I have never messed with a timing chain or anything, but I would think that it would be pretty simple. The cam is keyed with the sproket right? so is the crank, right? put the srockets on, and bring the crank to TDC, then line up the marks on the sprockets. I would think that it's that easy, but then again, never had to do it. Since everything is already taken apart, you should be able to find TDC on that motor fairly easily. Good luck and let us know what happens.

Jeff

 

It is as easy as 2_point8 posts. Just line up the dots.

Put the crank gear on and bring dot to 12 o'clock.

Put cam gear on and bring dot to 6 o'clock.

Remove cam gear, install chain, re-install cam gear with the dots lined up.

RBob.

 

And the cam and crank sprockets will only go on one way right? Are there any sort of things like...seals...any extra pieces that we need to be concerned with for the sprockets to go on? Just wondering what we're in for thats all. Which is a lot of work I'm sure. Lol....

 

Holy Crap!!! I was right?!?!?!

Yeah, I think the cam and crank are keyed so that the gear will only go on one way. Putting it on wrong would take a lot of grinding away at the little nub that keys it.

 

Now how is it held in at the key? The crank key looked like it was blocked off....the keyway was there but the end of the spout was closed so there'd be no way to get the crank gear on. Is that a removable block that you chisel out or something?

 

ok, it looks like the cam is definitly keyed, but I can't seem to see if the crank is in the book that I have. It says that in order for everything to be in alignment, the number 1 and 4 cylinders must be at TDC, then you just install the sprokets so that the marks line up. It looks like the crank sprocket is just pressed on when you install the balancera nd pully, but that really doesn't make sense, because what happens if you don't install them right away. I'm sure you just work the sprocket until it goes on or something.

 

Rbob is on the right track here, but that alignment is for a V8 #1 TDC. The V6 #1 TDC has BOTH the cam dot and the crank dot at 12 o'clock. If the cam dot is at 6, and the crank at 12, then you have #4 cylinder lined up at TDC for the compression stroke. When #1 is at TDC, #4 is also at TDC and vise versa. Either way can be used to line them up, but for the record, 12 o'clock for both on a V6 is #1 TDC. Be sure to inspect the tensioner while it's apart. I had one shatter on me one time. Not fun to fish pieces out and replace . Both cam and crank are keyed. To install the crank gear, get a rubber mallet or something to that effect and tap it into place. Once this is done, you should be able to install the new chain and then the cover. Be sure to put a new seal on the timing cover, and check the damper for ridges on the shaft that will go in to the cover. If there is a ridge, it will leak oil. You can get a sleeve kit that will go over the shaft, and have a nice smooth surface to seal. They are inexpensive and will save future headaches.

 

QTA, I don't know about that. The cam controls what cycle the valve train is at (intake/compression/power/exhaust). By putting the dot on the crank gear straight up, you get TDC of #1. The cam gear doesn't control TDC of any cylinder.

My GM Service Manual had me align the dots just as RBob said; crank gear dot facing straight up (12 o clock), cam gear dot facing straight down (6 o clock).

His crank might have a 2-piece key; I forget what year the changeover was. They can be tapped out with a brass punch and hammer; in fact, it's usually recommended that you do that before you bring the crank to a machine shop, so the shop doesn't lose the key.

If you have a balancer installer, you can use that to pull the crank gear on- just put a piece of pipe betwen the balancer and crank gear, so the balancer "pushes" the crank gear on via the pipe.

 

If you really want to get confused. . . It doesn't matter if the CAM gear mark is at 6 o'clock or 12 o'clock. The CRANK gear must be at 12 o'clock.

Why? Every time you rotate the crank 360 deg the CAM mark will be at either 6 o'clock or 12 o'clock while the CRANK mark is at 12 o'clock. The cam rotates at half the crank speed. The reason to use 6 on the cam & 12 on the crank is to ease the lining up of the gear marks.

If you need to stab a distributor at the same time (as in a cam swap), turn the engine so that the CAM mark is at 12 o'clock. That is TDC #1 for both the V6 & V8.

RBob.

 

Yep; you confused me.

The cam's rotation lines up the valves, right? So, say from 12 o-clock to 3 o-clock is intake... 3 to 6 o'clock is compression, 6-9 o-clock is power, 9-12 o'clock is exhaust. (Just taken roughly! Not sure of the actual positions to match the cycles!)

Now either way, and using my above example, the crank gear's dot is "up" for either the beginning of intake, or the beginning of power. The crank only knows "TDC" or "Not TDC", it's not concerned with int/comp/pwr/exhaust cycles.

So, how would it make any difference for cam installation if the cam dot points up or down?

Oh... wait... doing the distributor at the same time... I think I get it... if the cam sprocket's dot is at 12, that starts the power stroke? And that's why you'd put the distrib in at that time?

But I still don't see how it'd affect just installing a timing chain, since TDC is just controlled by the crank gear...?

 

The thing to remember is that there are two times in a 4-stroke that the crank is at TOP #1. It takes two rotations of the crank to fire one cylinder.

It is the CAM that decides which of the two (at TOP) rotations fires the cylinder.

As an example, place the alignment marks both at 12 o'clock. Remove the chain and rotate the crank one full rotation, replace chain. Has anything really changed? Yes, what was the exhaust stroke is now the compression stroke. But you can't really tell, until you notice that the distributor is firing 180 out.

RBob.

 

Tom, I know the cam in no way controls the TDC of any cylinder. What I was saying here is that if the dot is up top (12) then the valves are lined up for compression stroke on #1. If it's at 6, then it's compression for number 4. this is strange because my shop manual is telling me that #1 is with the cam at 12 o'clock. Maybe it's a typo, but that's what I've always done to change a timing chain, and never had problems. Although I wouldn't because either way the cylinders are lined up . It would make sense with the cam at 6, keeping it kind of standardized for chevrolet motors. Now I'M confused.

 

Can you explain to me why it takes 2 rotations to fire a cylinder once? Doesn't it compress, detonate, push back down, and during the return, expel the leftovers? Or does it require an extra un-fired stroke to expel the spent combustibles??

 

Oh, okay, then it was a typo. You wrote...

...which completely threw me off.

You answered it yourself! You typed in the 2 rotations! It takes two rotations because:

rotation 1:
piston down (intake)
piston up (compression)

rotation 2:
piston down (power)
piston up (exhaust)

Those 4 cycles are only one rotation of the camshaft.

That's why I can't see why it's "proper" to line the cam dot up at 12 o-clock. Just like you said, RBob, if you rotate the crank once, what's changed... nothing; just the crank.

So say we put the cam dot pointing at 6 o-clock. Put the gear on. Rotate the crank once. Voila! The cam gear points up at 12 o-clock, and the crank gear dot is pointing up.

I don't see how it helps to put the cam dot up at 12 o-clock, that's all. Guess it's just a difference of opinion.

I could line them up my way, crank dot pointing up, cam dot pointing up. Then I could ask you to leave the room. I then rotate the cam gear 180 degrees so the cam gear dot points downward, and install the chain. Then I rotate the crank 180 degrees, and ask you to come back into the room. All you would think is that I only put the timing chain on.



But in the case of installing a distributor at the same time, THEN it makes sense to get the compression stroke of #1... which is different then the TDC of #1.

 

Tom, It threw me off too. My manual here is telling me both at 12 gives you #1 TDC at the compression stroke, and there's more than one person telling me here that it's what you've already described. I'm not trying to argue here at all. But what you posted above is exactly right.

 

Ok, I gotcha Tom. I was thinking that each cycle was one crank rotation...as in while the piston went up-intake, and detonation, then during the bottom fo the down travel and back up, it was expelled . So the second stroke is unfired right---and the piston pushes all the gases out to make room for the next fired intake stroke?

 

Second stroke might be fired, nixon... that's the power stroke. Although I guess it could be argued with timing advance, that the power stroke occurs during the compression stroke. Beats me. If you haven't checked it out yet, I think howitworks.com had something on basic engine operation.

And I'm not arguing either, QTA! This is one of those things that I never bothered to think about before; I'm just trying to learn it 100%, without any "guessing", that's all.

[edit] Okay, found it... it was howSTUFFworks, not howitworks.. oops. http://auto.howstuffworks.com/engine.htm The link shows you the four strokes, and how the cam rotates once while the crank rotates twice. My fiance has a funny way of remembering each cycle; she surprised me with it once at a diner... I forget how the conversation came up.. I said something like "that's like if I expected you to know the 4 cycles of a car engine!" And she shocked me right then and there with the 4 cycles. I'll ask her later today; it was pretty funny.

 

Tom..thanks for the link Tom....that's great. What was confusing me was how the intake opens during a down-stroke, and the pistons returns back up before it detonates it...so that's a complete unfired revolution. So there's one firing per two crank revolutions.