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TDC #1 means #4 is at overlap. (long) valve adjustment theor


Guest Anonymous

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Guest Anonymous

Nick, when you said:

"The way all BMW techs were trained was to put cyl #1 on tdc and adjust both intake and ex on cyl# 4,

Next cyl #3 at tdc and adjust #2..etc till all 4 cylinders are done.."

There's a problem with stating it that way.

Technically, both #1 and #4 are at TDC at the same time, so you could describe the sequence as above, but it is generally misleading.

That is because in common language, TDC means TDC of the compression stroke.

When cyl #1 is at TDC, cyl #4 is in overlap with both valves open. Surely, you did not have the same definitions in your statement above?

BMW states the sequence of adjustment starts with cyl #1 at TDC (while cyl #4 has open valve overlap).

That means you check the clearance for both intake and exhaust on a given cylinder while it is at TDC.

I'd bet the only reason BMW mentions the the other cylinder (#4) being in overlap is to help provide additional visual confirmation that the correct valve timing stage is set for the cylinder being adjusted (#1 @ TDC).

There is a wide enough interval on the cam timing around each cylinder's TDC where both the intake & exhaust rocker arm pads are over their respective lobe's base circles at the same time. There's really no need to rotate the cam so that each individual lobe is 180 deg opposite the rocker pad. Better that you use that extra time toward repeating the sequence to check your work an extra time or two.

Standing at the right hand side of the engine bay, the engine is canted toward you which allows for an easy viewing angle to confirm the cam position.

Start with #1 at TDC, and you can see the timing mark on the cam sprocket flange at 12 o'clock. While cyl #1 is at TDC, both rocker pads are on the base circles, and you should just be able to see an equal amount of both lobes visible from around the bottom side of the cam (get your view angle perpendicular to the head and look straight at the cam.)

The base circle takes up nearly one-half the cam lobe. Think about the stock cam, if it is 264 deg duration, that means each valve is closed for 456 degrees of crank (with the clearances set for checking timing).

The base circle duration is not quite the entire time the valve is closed, as the ramp angles have to start earlier to take up the valve lash. The effective base circle on the stock cam is roughly 340 crank degrees.

The stock intake and exhaust lobes have similar event durations, but they're offset from each other by 492 crank degrees. Assuming 340 crank degrees of base circle for both intake & exhaust lobes, it follows that the stock cam would present 112 crank degrees where both rocker pads were on the base circle at the same time. That means there is a large enough window to set both valve's clearance on one cylinder at the same time, no doubt that's why the factory service manual shows it that way. Just remember that 112 crank degrees is only 56 degrees at the cam, and you still need to "eyeball" it pretty carefully.

One last word about the task ..

ALWAYS have spare eccentric bolts/nuts ready when you perform this work. Nothing is worse than breaking one on a Sunday afternoon, and losing use of the car until you can get parts. They are cheap enough to keep extras in your toolbox

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