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How to CC a Head
by Tom Carnegie
A bit of history:
When my brother and I first got involved with the Montana 500, low heads were allowed. The rule was changed
in the fall of 1974 to only allow high heads. I believe the
thought was that low heads were getting too hard to find,
so why not only allow high heads, of which there is a
good supply. There also might have been some concern
that someone might come up with one of the very early
heads made for flat-top pistons that had a far smaller combustion chamber. As time went along, folks did some
tricky things under the heading of "no limit to the amount
of milling of the head." I have been told that one guy
poured a layer of babbitt into the combustion chamber to
increase compression. When he was sure that his car was
declared "legal" at the teardown, he proudly proclaimed
"look what I done!" As you mill a high head, the section
in the center is usually the first part to get weak. To overcome this weakness some folks took to internally (inside
the water jacket) bracing their heads with a piece of angle
iron held in place by an extra bolt. This bolt was then hidden by a temperature sender, or some such thing. The
brace was however discovered during teardown on one car
that was so equipped, and the motor (or rather the driver)
was disqualified. As time went along people got trickier.
The deck of the head and the combustion chamber were
sometimes manipulated to allow more milling or to make
the combustion chamber smaller. The club devised a contour gauge to try to stop the chamber manipulation.
No rule, nor any way to police the deck manipulation was
found or used. There was a lot of argument, or maybe
"discussion" is a better word, as to whether this manipulation was illegal or not, with many drivers insisting that it
was legal. Some folks also found out that certain heads,
either from Canada, or those made in US around 1917 had
thicker decks than the later ones. I (and probably others)
found that often heads would have a "core shift" when
they were cast, so the combustion chamber was noticeably
shallower on some heads than others. Also the deck, even
on early heads are often thicker on some heads than others, presumably for the same reason. So, if you had a
large supply of heads, as I have, you would go through
them all and find the one with the thickest deck and shallowest combustion chamber, and mill the heck out of it.
Even at that though, the heads would only last so long before they would fold up and die. The group in Spokane
got to talking about this and decided that the Montana 500
shouldn't be a contest about who can mill their head the
most, which is essentially what it had become. After a bit
of thought it was decided to restrict the milling of the head
to about .080". A high head starts out at about 300 CC's.
Milling it .080" to .100" puts the CC's at about 275. Then
someone said, "why not allow low heads?" A low head
typically starts out life at around 280 CC's, so just cleaning it up would put it at around 275. At teardown time as
the inspectors were CC'ing a head, there was quite a bit of
discussion about the proper method to be used. Should
you use the sparkplug or fill the sparkplug hole with a
pipe plug, or perhaps clay? Do you include carbon that
has accumulated in the combustion chamber or make an
allowance for it? Does the combustion chamber include
the head gasket or not? The rules were clarified.
Clay would be used to fill the sparkplug hole. Carbon, we
didn't address. The headgasket is not part of the chamber.
Rather than force people to get new heads to meet with
the clarified procedure, an additional 5 CC allowance was
subtracted from the original 275 CC limit. The new limit
became 270 CC's.
How to do it:
The method we use is to get a big syringe at the farm
store. The biggest we could find was 60 CC's. Get a
piece of Plexiglas approximately 18" X 7". Drill four
3/8" holes. (See photo 1.) Put a thin layer of grease
around each combustion chamber.
Use some modeling clay
to fill the sparkplug hole flush. Invert the head and
stabilize it so that the valve side is slightly lower. Place
the Plexiglas onto the deck, sealing it with the grease.
Proceed to fill the chambers with water from the syringe,
being careful to keep track of how much water you use.