I was thinking, he was somehow catching one cylinder at the compression cycle when both valves on one head have to be closed at the same time. Then it seems then you could adjust both valves on the one head at the same time.

That's exactly what Chris is doing. But it's far easier said than done !

The 4-cycle engine takes 720° (2 full rotations of the crankshaft). Since the 2 cylinders fire 360° out of phase with each other, one cylinder's worth of valves can be adjusted every 360°. But ONLY one very specific cylinder !! What's happening to you is that the obscure OT mark flies by in the tiny window, and that sets you up to adjust the wrong 2 valves !

But like I said, the OT mark is only an indicator, not the actual item. It's the same as seeing checks in the checkbook and assuming there's still money in the bank. The checks and the account are associated, but not directly linked. In the same way the OT mark is associated with valve movement, but not directly linked. You need to be using a true indicator, and that should be your rocker arms.

I have been using the rear wheel bump method to rotate my flywheel; not my electric starter. With the spark plugs out it works pretty good. With your explanation, I agree, you need to rotate the engine four times to adjust four valves. I was just wondering how Chris could do it in the video with only two revolutions?

I'm going to go back and see if I can decipher your adjustment procedure from your detailed explanations of how a 2 cylinder runs. It can be a bit overwhelming... But what you said about watching the valves go up and down makes sense. I'll give it a try...

STOP! Let's come up with a fool-proof way to adjust the valves without needing the OT mark, AND which only requires 2 crankshaft positions. Sound good ? Then here it is...

Your New Personal Valve Adjustment Method
1. Ignition OFF. Spark plugs out. Both valve covers removed. Gearbox in high gear. We are going to move the crankshaft by "bumping" the rear wheel in ONE direction, forward or backward. We are going to watch all 4 rocker arms while we bump the wheel.

2. Bump the rear wheel until you see BOTH rocker arms on either cylinder moving at the same time. That is, if you bump the rear wheel forward and then backward (say 10°) thus making the crankshaft linger within a small portion of rotation, and both rocker arms on one cylinder are moving at the same instant, then you have found Top Dead Center for both cylinders.

3. Now you can safely adjust both valves on the cylinder that DID NOT have the moving rocker arms.

4. To adjust the valves on the opposite cylinder, keep bumping the rear wheel in the original direction and fairly soon the 2 rocker arms you just adjusted will both start move at the same time. It is now safe to adjust both valves on the cylinder that DID NOT have the moving rocker arms, which will be the opposite cylinder.

It's that simple. You'd need to work harder than that to fall off a bar stool !

Why does this system work ? It's a funny quirk of the 4-stroke engine, but once every 720° of crankshaft rotation, both valves are open for a very brief instant. (One opening and one closing.) And this happens exactly at TDC (what the BMW flywheel mark distinguishes as "OT").

As you can see from the simplistic diagram, the purple cam followers (both intake and exhaust) on Cylinder A have ridden up on their respective cam lobes. Therefore, the rocker arms for Cyl A will be in motion holding both their valves partially open.

On the opposite end of the spectrum is Cylinder B, and both it's cam followers are riding the base circle of the camshaft. Therefore, both Cyl B rocker arms will be relaxed and both their valves are closed. It's only when the rocker arms are in this relaxed state that the valve clearances can be properly adjusted.

Then, with a simple 360° rotation of the crankshaft, the cam lobes point the opposite direction, and the valves can safely be adjusted on the opposite cylinder.

► So by watching the rocker arms you can tell what the crankshaft position is with great precision, and thereby know when to adjust the valves.

► Watch the rocker arms because they always tell the truth. Know that the flywheel marks are lying to you 50% of the time.