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Purdy Swoft · 09-23-2011, 10:48 AM

Here is some info that I copied off the old ford barn a few years back. The minimum thickness of the head after milling to the MAX shouldn't exceed 240 . Original thickness of the unmilled head was 394 . Measure the thickness of the casting from the underside in the edges of the water jacket holes near the distributor, near stud holes 4 & 5 . This measurement will let you know how much has been removed in the past and how much to remove for the desired compression ratio. Stock ratio was 4.22 If you remove .025 the ratio changes to 4.40 , remove .050 and it jumps to 4.59 , remove .075 and the ratio will be 4.81 , remove .0100 and the max ratio will end up at 5.04. The 5.04 is not that far from the 5.2 ratio of the original police head with the large letter B . The design of the original combustion chamber won't be as efficient as the heart shaped combustion chamber on the police head or the other high CR heads that are now available. You could try smoothing the rough cast in the combustion chambers and measureing the CC in each chamber and trying to equalize. This on the other hand will slightly increase chamber size and lower CR slightly to the point that you may have to mill a little more to get back to the 5.04 reading. This is not my recommendation but was considered to be on the safe side of how much to mill. I have seen heads milled to the point that the counter bore was completely removed. What you have to be careful of is the height that the piston rises above deck height. The stock engine had the pistons rising about .032 above deck height. You may need to calculate the thickness of the head gasket when torqued minus the amount that the pistons rise above deck height. You need to maintain a distance of at least .030-.035 between the top of the piston and the top of the combustion chamber over the piston when the head is torqued down.

09-23-2011, 10:48 AM	#11
Purdy Swoft Senior Member Join Date: May 2010 Location: Alabama Posts: 8,099	Re: Milling a head for compression? Here is some info that I copied off the old ford barn a few years back. The minimum thickness of the head after milling to the MAX shouldn't exceed 240 . Original thickness of the unmilled head was 394 . Measure the thickness of the casting from the underside in the edges of the water jacket holes near the distributor, near stud holes 4 & 5 . This measurement will let you know how much has been removed in the past and how much to remove for the desired compression ratio. Stock ratio was 4.22 If you remove .025 the ratio changes to 4.40 , remove .050 and it jumps to 4.59 , remove .075 and the ratio will be 4.81 , remove .0100 and the max ratio will end up at 5.04. The 5.04 is not that far from the 5.2 ratio of the original police head with the large letter B . The design of the original combustion chamber won't be as efficient as the heart shaped combustion chamber on the police head or the other high CR heads that are now available. You could try smoothing the rough cast in the combustion chambers and measureing the CC in each chamber and trying to equalize. This on the other hand will slightly increase chamber size and lower CR slightly to the point that you may have to mill a little more to get back to the 5.04 reading. This is not my recommendation but was considered to be on the safe side of how much to mill. I have seen heads milled to the point that the counter bore was completely removed. What you have to be careful of is the height that the piston rises above deck height. The stock engine had the pistons rising about .032 above deck height. You may need to calculate the thickness of the head gasket when torqued minus the amount that the pistons rise above deck height. You need to maintain a distance of at least .030-.035 between the top of the piston and the top of the combustion chamber over the piston when the head is torqued down.