I am confused about top-dead-center

So I am adjusting my cam chain tensioner for the first time and it says to line up the timing mark to 'T' on either the 1-4 or 2-3 mark, which is TDC, right? But then I look at my cam lobes and none of them are lined up exactly in the middle of the shims. The closest one is near the edge of the bucket.
I know this is a basic question, so be gentle. I know the piston is at the top at TDC. I would think the cam lobes would be in line with the pistons. Thanks.

You are either TDC on the compression stroke or the exhaust stroke. The cams will be in a different pisition for each.

Jay

On any given cylinder, at top dead center of the compression stroke, you should have both the intake valve and the exhaust valve CLOSED, but no, the lobes for either one will not be parallel to the valve, because they have both been closed for the last 90 degrees of their rotation. The exhaust lobe will be approx. 180 deg past first close, and the intake cam should be Approx. 90deg ahead of the exhaust cam.

Wish I knew how to draw on this contraption, but if you rotate the engine over by hand you'll get the drift I'm sure.

You wouldn't want your valves open when the piston is at its highest point (TDC). This would cause the valves to hit the piston and that is not a good thing

The strange thing is the cams only rotate at 1/2 the speed of the crank, so it takes 2 crank revolutions for one cam revolution. For example, you can be at the T mark for 1-4 and the #4 cylinder cam lobes will be facing each other and then the next time you're at the 1-4 T mark, they will be pointing away from each other :S That one took some getting used to for me!!

Ok, now I get it. I was also wondering about why the 1 and 4 lobes (and the 2 and 3) were not lined up, so thanks for answering that 79.

You've succeeded in clearing a little more of the fog in my brain.

Ray J wrote:

Ok, now I get it. I was also wondering about why the 1 and 4 lobes (and the 2 and 3) were not lined up, so thanks for answering that 79.

You've succeeded in clearing a little more of the fog in my brain.

Now that would be interesting, make a four cylinder KZ cam with the 1 & 4 and 2 & 3 cam lobes lined up so 1 & 4 fire as one and 2 & 3 fire as one, that would give you one strong power pulse per revolution, kind of like a four cylinder two cylinder
Bet it would have a unique exhaust sound

Might be hard on the crank though.

I seem to recall that Yamaha (and maybe others) did that on some racing bikes.

Post edited by: steell, at: 2007/09/03 10:53

They did soemthing similar on some racing v-twins sometime in the early 2000's. It was called the "big bang theory" in one of the bike rags (Motorcyclist or Cycleworld). It made a twin run like a thumper. (But because it was a V, the big bang was really just two bangs very close together.) I don't recall what the logic was for it, but apparently it wasn't a total disaster.

Post edited by: loudhvx, at: 2007/09/03 11:20

Some of the old Sears allstate bikes they sold in the late 50s/early 60s had 2 pistons and only one rod both fired at the same time dont ask me how the connected 2 pistons to one rod. im not sure I really wanna know.