In the course of milling cylinder heads for a specific decrease in combustion chamber volume, it becomes necessary to know exactly how much a cylinder head must be milled for a 1cc (cubic centimeter) reduction. While this value is useful for milling heads for a specific compression ratio increase, this value becomes increasingly more important when attempting to equalize combustion chamber cc’s over the length of the head due to a particular head having cylinder chambers that get progressively larger (or smaller depending upon your perspective) from one end of the head to the other. For a number of cylinder heads out there on the market, there are resources that can be accessed to obtain this information but for the non mainstream engines such as the Ford Y-Block, that information is vague if it’s to be found at all. To add to the confusion are the different combustion chamber shapes that were available on the Ford Y-Blocks over the course of its production run. With all this in mind, I’ll share the steps I use to determine how many thousandths of an inch a particular head must be cut to reduce its chamber volume by 1cc.
- Measure the distance or length around the edge of the combustion chamber on the head. (inch format)
- Take this measurement and divide by pi or 3.1416
- This result multiplied by itself or squared.
- This result multiplied by 0.7854 (this is the result of pi divided by 4)
- This result multiplied by 16.387 (this is the result of 2.54 cubed)
- Take the value one (1.0) and divide by the previous answer. This will be the amount to mill the cylinder head in an inch format to reduce the combustion chamber volume by 1 cc.
Or looks like this in a math formula: 1 / (((measured distance / 3.1416) squared) X 12.87) = inches cut for a 1 cc reduction.
The first order of business is to measure the actual length around the edge of the combustion chamber and this can be performed from at least two different approaches, either being equally effective. One method is to simply take a piece of wire or string and lay around the edge of the combustion chamber along its perimeter which will provide a measurement of the length around the chamber itself. Another method is to lay a piece of paper or light cardboard over the combustion chamber and rub the pattern of the combustion chambers edge onto the paper or cardboard. This chamber imprint can then be measured for its length with a number of different measuring devices of which a map route reader is both inexpensive and effective.
For the following example, a Ford truck 292 ‘CITE’ casting head is being used. On this particular head, the measurement around the length of the combustion chamber is 10.63”. Going through the aforementioned calculation steps, the final result is 0.00678” (rounded to 0.007”) and this would be the amount to mill the head for a one cc reduction.
But to take some of the work out of calculating the amount to mill for some of the various Ford Y-Block heads, here’s a chart with some known values.
|Cylinder head casting||Combustion chamber perimeter||Amount of cut for a 1 cc reduction in combustion volume|
Armed with this information, it’s now possible to mill a pair of heads of different or varying cc’s with a specific amount of cut for each head and ultimately having them equalized or all the combustion chambers at the same cc’s on the first cut.
Special thanks goes out to Tim McMaster and Carl Lynn for providing additional head tracings for some of the various Ford Y-Block head castings that were not on hand. Until next time, happy Y motoring. Ted Eaton
This article was originally published in The Y-Block Magazine, Issue #104, May-Jun 2011, Vol 18, No.3