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!  ^

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!

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.  ^