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RE: Tappet talk

To: fot@autox.team.net
Subject: RE: Tappet talk
From: Bill Babcock <BillB@bnj.com>
Date: Fri, 6 Jan 2006 19:01:33 -0800
Okay, so I sound a little strident about this, but it's a leftover bias from
my motorcycle days. I thought a lot of "standard practice" like this was
based on "the way we always do it" rather than anyone actually figuring out
if something was a good idea. 

There are two problems I see with domed lifters. A dome always moves the
contact point toward the center of the lifter and makes it revolve less. Any
way you draw it, it comes out the same--a dome moves the contact point
towards the center compared to a flat lifter. 

It also makes the contact area very small, increasing wear and spalling. The
pressure on the lifter is pretty ginormous. Decreasing the contact patch
increases the pounds per square inch dramatically. I've never seen a domed
lifter with any miles on it that wasn't spalled in the center. 

Before I got the ceramics I did a lot of work to my lifters to make them
lighter. I bought mass quantities of the cast lifters that BPNW supplies,
shortened them, drilled holes in the faces, and tapered the insides. Then I
had them ground flat with a little break at the edge, and got them hardened.
The guy who did the grinding and hardening is no longer with us, and I don't
know where to get that kind of work done now. I suspect the EPA is just as
glad--his shop was scary.

I don't even like the idea of ramping a cam face--no matter what Ford says.
We're talking about a lot of pressure against the face. No cam has thrust
surfaces worthy of the term. Ramping the face creates side thrust that gets
taken up by the thrust collar, and it's a pulsing thrust that can't be
effectively cancelled out by a counter ramp on another cam face. Can't be
good. Given inertial forces and heavy springs it could be a lot of thrust. I
don't go so far as to specify no ramp on my cams, but I don't think it's
useful. But if there is a ramp on any of the Triumph cams I have, I can't
detect it. You can see a ramp on a cam by the wear pattern--its all on one
side. If your cam is shiny all the way across the face it either isn't
ramped or the ramp has worn off. Once again, our engines have cam faces that
are offset from the centerline of the lifter--gives you all the lifter
rotation you need. 

You pays your money and you takes your choice. I just can't see how it helps
a TR motor in any way.  


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