[Mgs] tappet question

PaulHunt73 paulhunt73 at virginmedia.com
Tue May 19 03:21:40 MDT 2015

As said elsewhere TDC and BDC are not relevant, if you are referring to the piston.  If you look at the valve timing the inlet lobe is at its peak 70-odd degrees before BDC, and the exhaust valve is at it's peak about the same distance after BDC.  So the back of the lobes are under the followers 180 degrees from there, which is more than 70 degrees before TDC and after TDC respectively.

But it shouldn't make a difference, look at the cam profile, and the base circle should extend for significantly more than half way round the cam, in theory giving more than 180 of engine rotation where the clearance should be the same.

Note in theory, which is why I said it's one of the reasons why I find the position of greatest gap to adjust them, which on mine (and others I have done) can be several degrees either side of the strict RON point.  

Also, whilst the dynamic forces on the piston etc. are slightly different on compression and expansion etc, statically TDC is directly above BDC, how could it be anything else?


  ----- Original Message ----- 

  Regarding Rule of Nine, I understand having 8 all the way up to set 1. But there is a significant rotation between "real close to the top" in front of, and after TDC. This can be seen with rotation the engine and seeing very little to no movement. Also, if I am not mistaken 8 TDC is not perfectly in line with BDC of 1; there is a bias. This means that when 8 is TDC, 1 is not absolutely BDC ready for measurement. So the question is, how do you ensure, with all of that slop, that you truly have BDC?
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