[Mgs] tappet question

PaulHunt73 paulhunt73 at virginmedia.com
Wed May 20 02:02:09 MDT 2015

As far as I have seen when adjusting valves any valve is at peak lift at only one point on the cam.  The tip of the lobe is rounded, so as it passes under the flat tappet it is only when the centre of the lobe is under the centre of the follower that you will have peak lift.  Very nearly peak lift a few degrees either side of that, but not much.

You mention comparing valves 1 and 8, but talk about inlet and exhaust.  It's true that inlet and exhaust lobes are not 180 degrees apart - the lobe of an inlet is 16 degrees away from the heel of an exhaust and vice-versa, but 1 and 8 are both exhaust valves.  And because those valves are on cylinders 1 and 4 which fire one complete engine revolution apart it wouldn't surprise me if their lobes and heels *are* 180 degrees apart, which is probably why someone came up with the Rule of Nine.  All the pairs in the Rule of Nine are the same valve - inlet or exhaust - and all on 'opposite' halves of the engine.

But all this is getting into the realm of how many angels can dance on the head of a pin.  The fact remains that in theory there is a significant range of cam rotation where the clearance should be at a maximum, meaning that it shouldn't be critical to get the opposite valve exactly on the peak of its lobe.  But in practice, on a cam by cam basis, that doesn't seem to be the case and one needs to check where the point of maximum gap is on any valve, and see how that compares with the point of maximum lift on its partner valve.  As I've said a couple of times on engines I've checked the two don't always occur at the same time on various valves.  It's also the case that the period of maximum gap has been surprisingly narrow, like the period of maximum lift, whereas on drawings of various cam profiles the base circle i.e. where the clearance should be maximum occupies more than 180 degrees.

  ----- Original Message ----- 
  ... does the TL of valve 8 mean that you have found the BL of valve 1? If I remember correctly, they aren't 180 opposite, input being before or after exhaust by some degrees. Is the difference of opposition so insignificant that it doesn't matter?
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