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The History of
MID-LIFT®

MID-LIFT®
How it Began
The other companies designed their
rocker arms in a "closed valve" state, with regard to only fitting the head,
not
the engine, thus disregarding the pushrod's influence with the valve. What Jim
Miller did that was novel, was to design the rocker arm's tangent points when BOTH the valve and the
cam were in the MID-LIFT position simultaneously. This
influenced all of the critical dimensions and equally divided the rocker arm's
circular motion (arc) on BOTH sides, which Jim emphasized was critical to not
only minimizing
back-and-forth sweep atop the valve, but providing the maximum information of
the cam dynamics to the rocker arm with the ultimate least amount of wasted motion.
This additional motion, found in all previous rocker manufacturers, required the
crank to turn additional degrees of rotation to effect the same amount of cam
lift "at the rocker arm." Miller's MID-LIFT principle of design set in stone a
formula that would apply to any valve lift,
provided the engine builder set the "installed geometry" for the net valve lift
of the engine, a mandatory task with any rocker. Finally, a standard! And in
1974, the phrase Jim kept using was coined: "Mid-Lift" becoming the terminology
to a Patent application in 1978, that was filed in 1980 and issued in 1982. The
MID-LIFT Patent: #4,365,785.
How could this be, how could an
industry of multi-million dollar cam companies make such an oversight? When
you're 20 years old, it's hard to appreciate the value of such a discovery,
especially if you don't know how it was missed. Finding the answer to this
confounding question became necessary. Within a year the answer would be known.
In 1958, Harland Sharp, an
avid automotive enthusiast and hard working, innovative machinist, took a
standard OEM stamped steel "shoe" type rocker arm and laid it on its side to
trace its silhouette on a piece of paper. Knowing he wanted to place a roller
tip on the end, he traced a roughly .600" diameter roller, placing the bottom of
the roller's diameter on the same horizon as the shoe's contact pad of the
original rocker arm. This was the mistake. He marked a center to this circle for
the location of the roller pin, then using a scribe, he transferred his
silhouette to a block of aluminum where he proceeded to machine his roller tip
rocker body. Several years later, Harvey Crane asked Harland to make roller
rocker arms for his company: CRANE CAMS. However, within a short period of time,
Crane's excellent national marketing outstripped Harland's production
capability, forcing CRANE to use other sources, eventually making them in-house.
Having already established an identity with Harland's roller rocker arms, CRANE
simply kept the same design basics which Sharp had been using, and the mistake
was copied. During the 1960's, several of today's top cam companies began making
their own line of rocker arms, and although many would no doubt argue that their
design was unique to their own ideas, the fact of the matter is: that same
mistake appears to have been copied throughout the industry... and except for
some copy-cats pretending they have a "better geometry," it continues to
this day. True story!
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THE ERROR
What was it?...
In much the same way the designing of a flat tappet cam is measured differently than the roller
tappet cam, so
too is the principle of design with a roller tip rocker arm. For on a "shoe"
rocker arm, it is the bottom contact pad that has traditionally been used for angular measurement
with the axis of the fulcrum (trunnion or shaft), but with a "roller tip" it is the axis of the
roller that is used for angular motion. By placing the bottom of the roller on
the same horizon as the shoe, the measurement was off by half the diameter of
the .600" roller, or .300". In angular terms on the small block rocker arm, this works
out to more than 12 degrees of error. To set this in perspective, the rocker arm
only moves 24 degrees or so for a valve lift of .600" making the rocker arm
wrong by 50% of its motion, regardless of the pushrod length or the rocker's
height to the valve tip!
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Line "A"
represents the contact pad of the rocker,
traditionally used for rocker design under the "1/3
RULE."
But with roller tappets or roller rockers it is the axis of the
roller which matters, not the outer diameter, which is what Harland used, as
shown. This moved the roller axis up by half of its .625"
diameter. This extra .312" error had the effect of moving the
push rod "cup" UP (shown as "C") by more than 12 degrees.
The rocker arm
only moves about 24 degrees, so no matter what pushrod length you choose, only one
side of the rocker arm can be set for minimal back and forth
motion, while the other end suffers. "B" represents this increased
angle with "C" while the dashed line beneath "C" shows where MID-LIFT geometry should be.
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Note:
The late Harland Sharp's background with
racing interests dated back to the 1950's, when many of today's largest and
most well known companies were first beginning. The hand written notes and
drawings from Harland, during the 1970's when he made custom designs for Jim
are still kept as a reminder to the simple legacy of this history.
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MILLER PRODUCTS GROUP
1777 Blount Rd. #501
Pompano Beach, FL 33069 USA
954-978-2171
"MID-LIFT" & "PRO-SHAFT" are ® Registered Trademarks
of MILLER ENGINEERING INC; Copyright © MMIII - MMX JM Miller
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