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RE: Tappet talk

Subject: RE: Tappet talk
From: Bill Babcock <>
Date: Fri, 6 Jan 2006 12:01:04 -0800
As usual with traditional parts like these, there's a lot of contention
about what makes a good lifter and why they have specific characteristics. I
don't consider these right or wrong, but any serious research was done a
very long time ago. There are as many flavors of cam followers as can be
imagined and each is supposed to provide some kind of advantage. For pushrod
engines most of the serious research ended when roller cams came along.

I've heard lots of explication about domed vs. flat and why cams might have
some kind of face taper. But the references I know of that include some kind
of research element say the two factors that influence lifter rotation are
the offset of the cam lobe from center and the precision of the barrel of
the lifter. Any loose fit or departure from perfect roundness of the bore or
the lifter can prevent rotation. I can't visualize how doming would help
rotation unless it was moving the contact patch off center in a cam/lifter
pair where the cam lobe was centered over the lifter. 

In fact the cam lobe contacts the lifter fairly close to the edge of the
lifter in most engines. And nobody buys their cams and lifters in pairs. How
all this stuff is supposed to match up and produce a consistent result is
beyond me. 

Perhaps there is some notion that a domed lifter under the high pressure of
valve lift becomes a flat surface, while a flat surface becomes dished. This
would make a slight dome advantageous for optimal surface area of contact.
But with the cam lobe offset so far from center this advantage seems dubious
and random. And that much flex of a hardened surface would certainly cause
spalling. I suspect it's like a lot of other notions in parts design. More
history, myth, and sales spiel than engineering. 

As far as I can tell my ceramic lifters are absolutely flat. When I place
them face to face there is no rocking at all.     

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