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The base circle is the area of the cam lobe that is at the exact opposite area of the camshaft as the highest point of lift.
Therefore when the valve is opened all the way, the lifter is sitting at the top of the cam lift. When the camshaft is rotated 180 degrees, it will be at the base circle.
You want to have the cam positioned with the lifter at it's least area of lift or the base circle for your adjustment.
If you have replaced the lifters, be careful, because they want the lash before the 1.5 turn to be at zero before the lifter plunger has begun to move. If the plunger is at the bottom of the lifter already, they your valve will not close after you make the adjustment.
I hope this explains your question.
Any other questions, please feel free to ask
Have a Great Day
So as not to confuse you and to make is as simple as possible.
Take off the distributor cap, and adjust the Intake and Exhaust valves of the
cylinder that the rotor is pointing toward.
So bring up number one cylinder while the distributor cap is off and see that the rotor is pointing to the number one spark plug wire tower on the cap.. This will place the number one cylinder intake and exhaust valves on the base circle. Adjust those 2 then move to the number 4 cylinder.
Watch the rotor move as you slowly rotate the engine to where the rotor points to the tower where the number 4 wire is connected. This places number 4 intake and exhaust lobes of # XXXXX cyl on the base circle. Adjust them and then move on through the firing order to number 2 then 5 then 3 then 6.
I hope this helps
That is correct.
As you are beginning to tighten to take the lash to zero, be very careful and have that adjustment just take out the play with a light touch. Then do the 1.5 turn.
You got it.