<VV> Lifters and other topics

[Sethracer at aol.com]

In a message dated 2/26/2011 12:33:51 P.M. Pacific Standard Time,  
vairguy at echoes.net writes:

I  also don't see how the tappet being flat would really make such a major  
difference, and if it did it seems all it would do is wear the lifter 
and/or  pushrod which are easy to adjust and fairly easy to replace. So please  
enlighten me!  

I'm not trying to argue the  point, I'd just like to be educated in the 
mechanics  involved.


The "Tappet" is just another name for the "lifter" and they can be  solid 
or Hydraulic. The cup end holds the pushrod and the movable, internal  
plunger, is kept in place by a clip or spring. Internally, the lift acts as  a 
passageway to feed oil from the gallery, located in the case on the Corvair,  
up the pushrod to lubricate the valve spring and the rocker. The hydraulic  
lifter also uses this oil - fed under pressure by the engines oil pump -  to 
take up the clearance, by pushing up the plunger, in the valve train to  
help provide quiet operation. A solid lifter engine, in order to allow for the  
growth of metal under temperature change, has to have extra clearance 
pre-set in  the valve train to make sure the valve can close - absolutely - when 
hot. If no  clearance was pre-set, the growth of the dimensions of the parts 
would hold the  valve open causing mis-fires. Most engines have gotten away 
from solid lifters  because the noise can be distracting (Let me tell you 
about a Duntov .030" inch  clearance cam I had in a Corvette! You could hear 
me coming for blocks)  Almost all Corvairs have hydraulic lifters - a few 
racers have installed solids.  Flat tappet cams are called that because they 
present a flat face on the bottom  of the lifter to ride against the lobe of 
the cam. Roller lifters present a  cylindrical mounting interface. In both 
cases, a line of material contacts the  cam. That line, in theory, changes as 
a roller cam follower spins at the bottom  of the lifter, and in a flat 
tappet system, the cam is ground with a slight  taper which is supposed to 
"spin" the lifter around a bit during operation. Most  importantly, a flat 
tappet lifter presents a heavier (as in lbs per square inch)  wiping contact load 
against the cam, than a roller lifter. It is generally  regarded as the 
highest load applied to any surface inside the motor.  The wear on this surface 
is mitigated by the oil on this surface and the  "high-pressure" additives 
of the oil. The ZDDP - essentially a zinc-phosphate  compound, provide a 
cushion in that load. Supposedly that compound is  a contaminator of the 
catalytic converter on modern cars so it is being removed  - or greatly reduced on 
new-car oils. Thus the controversy for older cars  that, supposedly, need 
that assistance on their flat tappets. Almost all new  cars have roller 
tappets for several reasons. Wear on the tappet to lobe quickly  results in low 
or no valve opening, not good for performance. And the cam, far  from being 
easy to replace, is just about the last thing that falls out of  the motor 
when you disassemble it.  
 
Seth Emerson