For
too long, ".100"" longer, and ".200"" longer than stock valves have been
the normal mode of thinking for engine builders trying to gain extra
valve spring clearance. But with the myriads of head manufacturers,
models, and changes, it is crazy to fit rocker arms to all these
combinations. Since head preparation was the main business Jim Miller
was in for 15 years, while developing MID-LIFT Geometry, is was a
natural question - and answer for him. Standardize to the valve spring
pads. These used to be sacred ground anyways. Back in the "old days" of
"292" Chevy head castings, it was a very expensive mistake to cut
through the spring pads for extra spring clearance. Because everyone has
become so used to just ordering longer valves, few people really cut the
valve spring pads anyway, and it is a dimension that remains relatively
constant from head model to model, of each manufacturer. That doesn't
mean the same intended design from two different manufacturers will be
the same, and probably won't. But chances are that the same part numbers
you bought this year will have the same specs next year, if you're using
the same manufacturer; like AFR, Brodix, Dart, Edelbrock, and so on.
So in knowing what the
various valve spring retainers needed to be; knowing what the various
valve springs coil bind clearance were; knowing what the optimum valve
tip length was; and knowing what the typical spring cups, shims and
other related components all stacked up to... all served as a great
foundation for deciding on what a "standard" for small block, full
competition, and big block, full competition needed to be. This is the
advantage that MILLER has over most rocker manufacturers, hands on
cylinder head development background. It may be a few years back, but
the "standards" of good head preparation haven't changed.
For small block
engines, that will see valve lifts up to .750", 2.250" is
what the designed VTH is. For big block engines, running valve lifts
of .850", then this is increased by another .100" to 2.350".
Needless to say, on the crazy valve lifts of .900"+, another .100" is
used. But these two main standards have always worked since 1980, when
Jim finally locked them in for heads he was preparing.
^
NOTE: The above is for high valve lift, professional racing;
MANY small block applications are fine with a 2.150" VTH, to avoid
excessive "shimming" beneath the valve spring cup. The 2.150" VTH may
better be suited for installed spring heights of under 2.00", where
valve lifts will be .650" or less, OR... valve spring pressures are
under 175 lbs, as with hydraulic roller applications. In either of these
examples installed spring heights over 2.00" are not likely. But for a
standard which precision geometry rocker arms are designed to, the above
2.250" is chosen to centrally place the roller pattern upon the valve,
and easily accommodates shifting this pattern .020" across the valve's
centerline for this short VTH, or behind the valve centerline for an
even higher VHT, such as 2.350".
Valves are made to
specific lengths that are derived from the spring pads of the heads they
are going to fit in. There is no mass production of this, unless the
measurements are closely controlled on all components. That means valve
seat depths from the deck, spring pad heights from the deck, and spring
pad heights from the deck. These "gauge" points are represented by the
red circles in the above illustration. On the small block Chevrolet, the
red circle at the lower left corner of the above illustration is the
DOWEL point, which is where all critical dimensions originate.
The other critical
dimension that is available and used two ways, is the valve tip
length. Ford uses .310" while Chevrolet used. 250". Many
aftermarket valves come standard with the Chevrolet tip length, unless
you specify otherwise. We highly recommend the .300" (.310") length.
When ordering your valves, you must be careful about how you give your
dimensions. Good valve manufacturers will know how to interpret your
needs, but why shoot in the dark. Valve specs refer to "overall lengths"
which include dimensions you may want to allow for, or not allow for. If
measuring your valve length from the valve SEAT of the HEAD, then you
need to ADD the MARGIN width and the BEVEL height on the end of the
valve head. As explained in
the next illustration, if using a more common (to aftermarket valves)
.250" tip length is preferred, then the 2.250" (Small Block) and 2.350"
(Big Block) dimensions can be reduced .050" with adequate clearances in
all but the most radical cases. But retainer selection to get the
desired amount of valve tip above the retainer will have to be carefully
selected. See next illustration:
VALVE TIP LENGTH.
^
