Is exact TDC absolute?

chrisf · 10-13-2006, 07:04 AM

I’m doing my first valve adjustment (been working on it for about two weeks now, as other issues have been arising!) & have read the Haynes Manual, 101 projects, as well as the pelican tech articles and having been wondering about a possible degree of acceptability in regards to the TDC for which to insert the feeler gauge.

Once the intake stroke is complete, the intake valve is closed (as well as exhaust), and compression stroke begins, the rockers should not be in contact with the cam lobe, or maybe it’s the valve stems, which ever, by the desired 0.004”, theoretically.

This should hold true though out the compression stroke, to TDC as well as the combustion stroke and to BDC, when the exhaust valve begins its travel. During the describe travel, the cam lobe’s concentric portion should be rotating under the rockers, and the recommended clearance should be seen.

Again, theoretically, couldn’t this be equated back to a certain +/- degree before and after (more so, after) TDC when one could check for necessary clearance?

JohnKo · 10-13-2006, 07:21 AM

If I understand you correctly, you're right -- to a point. In theory, the valve openings and closings are all discreet events that occur within one of the four strokes. In practive, valve timing is more complicated. For example, an intake valve that theoretically opens at the beginning of the intake stroke actually opens before the end of the exhaust stroke.

In practical terms, the valve adjustment procedure is designed for making the adjustment when you are sure the valve is closed. Since you are adjusting valve clearance when the cam lobe is on its base circle, exact alignment with the TDC mark or the other two marks is not required. Being off by a bit will not have any negative effect on the accuracy of your adjustment. I just find it handy to use the marks, as it's a no-brainer and it works.

If you're looking for a way to adjust two cylinders' worth of valves at once, I wouldn't want to go there.

Good luck! Don't overtorque the cover fasteners!

John