I never understood why tightening the upper timing chain did not have an effect on the position of the cams. Seems the cams would move a bit at least…or is this built into the mechanism somehow…
It does… which is why they need to rechecked… and finely calibrated by use of the circumferential verniers.
Changing chain tension does change timing, but by a small amount, unless the chain started out really, really loose.
Chain wear (stretch) affects timing. Retensioning just takes up the slack.
You are pulling the camshaft down at one end, so it’ll try to flex along its length and not be quite straight. I’ve found that with the v12, it is necessary to do each cam bearing cap up to exactly the same torque every time to consistently measure the same value when doing valve timing.
Tightening the cam chain can have no effect on cam timing. What does have an effect is the wear on the chain - if the chain has become slack due to wear then clearly the tension side will be longer and thus delay the timing. In certain cases (and cars) this can become extreme and lead to failure.
With the XK engine I don’t think that’s true. When you tighten the chain you move an eccentric pulley, which pulls more chain from one side than the other, inevitably changing the timing of one cam relative to another.
However, as mentioned above, that amount is tiny and is essentially irrelevant.
It definitely does have an effect which is why the manual says it should be tight before you check that the timing is correct. Moreover, the tension cam will make the chain tight by turning in either direction. I’ve found that there will be several degrees difference between those to directions so pick a side and use it always. I think the manual says counter clockwise.