Milling cylinder heads

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by: Cobalt327
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[edit] Overview

Cylinder heads are milled to raise compression or to correct the deck surface if it's warped or otherwise out of spec.

[edit] How much to mill

Generally speaking the less that's milled beyond what is needed to true the surfaces, the better. Care should be used when milling production heads because the castings are generally none too thick. Even so, builders have milled stock SBC heads all the way to where the intake seat is almost flush w/the deck surface, or to where the cooling passages around the spark plug are removed. Milling 0.125"-plus is not unusual, but this is usually only done where a race track/series has rules regarding what cylinder heads can be used. This is often "open chamber" SBC heads as was used in the smog years, the idea being the costs associated with engine building will be less using these commonly available heads.

The compression ratio can also be raised by using a thinner head gasket. Only when the thinnest practical head gasket is used should milling be considered. Not only are head gaskets cheaper than milling, but the decks will be thicker with less milling. As an example, the difference between a 0.041" thick head gasket and a 0.015" head gasket is almost 0.75:1 on a 350 SBC.

Warning Note: It should be taken into consideration that when the heads are milled or a thinner head gasket used, the valve train geometry changes, the piston to valve clearances will be closer, as well as the quench measurement will be reduced.

[edit] Correcting the intake flange

Whenever the block or heads are milled, often the intake flanges and the ends of the intake manifold (or the surfaces of the block, or a combination of them both) need to be milled to correct the port and bolt hole alignment.

[edit] Mill the heads or the intake manifold?

Often, builders opt to mill only the heads so any off the shelf intake manifold will fit the engine. In any event, it is better to keep all the modifications to either the cylinder head or intake manifold- one or the other. Otherwise, the heads and intake become basically inseparable.

On engines that show an amount to be removed from the ends in the table below, do a mock up before milling because in cases where the intake seals the lifter valley (like the SBC) there may be enough clearance without milling the ends. Always seal the ends using RTV whenever possible. On engines that use a separate valley cover, the ends need to be checked.

[edit] How much to mill the decks

Whether milling the intake mounting surfaces is necessary and how much needs to be removed depends on the angles involved and how much material was removed from the decks.

While there are math formulas for this, the table below is quick and easy to use.

Warning Note: The figures below are used when flat milling the decks.

All measurements are in inches.

Engine/Head Type

Amount to reduce chamber volume by 1cc

Amount to mill intake flanges for each 0.010" removed from heads or block

Amount to mill intake ends for each 0.010" removed from heads or block

SBC 64cc chamber 0.0055 0.0123 0.0173
SBC 76cc chamber 0.0059 0.0123 0.0173
BBC 109cc closed chamber 0.0065 0.0070 0.0193
BBC 118cc open chamber 0.0050 0.0070 0.0193
Ford 1965-'77 289-302 0.0060 0.0100 0.0143
Ford 1969-'77 351W 0.0055 0.0100 0.0143
Ford 1978-up 302/5.0L/351W
(except 1986 5.0 HO)
0.0045 0.0100 0.0143
Ford 351C 0.0050 0.0100 0.0143
Ford 385 Series with quench-type head


0.0100 0.0143
Ford 385 Series with open chamber head


0.0100 0.0143
Chrysler 273, 318ci "LA" with 57-61cc chamber 0.0053 0.0095 0.0144
Chrysler 340-360ci "LA" with 63-71cc chamber 0.0048 0.0095 0.0144
Chrysler B/RB with 66.5cc or 73.5cc chamber 0.0062 0.0123 Not required
Chrysler B/RB with 79/81/83cc chamber 0.0042 0.0123 Not required
Chrysler B/RB Max Wedge with 81 or 86cc chamber 0.0050 0.0120 0.0170
Olds 403-455 .0050 .0130 .0169
Olds 350 .0060 .0130 .0169
Pontiac 1965-'67 0.0061 0.0061 Not required
Pontiac 1968-'70 0.0052 0.0052 Not required
Pontiac 1971-'79 0.0047 0.0047 Not required

[edit] Another way to calculate the amount to remove

In cases where the head in question isn't listed or the list cannot be accessed, the amount of material to remove from the cylinder head deck to reach a specific volume can be estimated by using a graduated burette or even a large syringe.

Assuming the target chamber volume is known, a burette or syringe can be used to dispense that amount of fluid into the chamber. Then measure down to the surface of the fluid and that will be the amount of material that needs to be removed from the deck.

Using a fluid other than water will make this easier, since water has a relatively high surface tension. Mineral spirits or rubbing alcohol with a drop or two of food coloring works well for this.

[edit] CC'ing the chambers

Cc chamber.jpg

[edit] Resources

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