How much to mill off my head?

Hi, so I got around to cc ing my replacement head and the two chambers I measured were at 59cc or pretty close. We think the head was milled once. I can tell the machinist to take a light cut unless there is a good reason to mill more. This is a weekend driver Bronco on regular fuel. It is the small 1 1/2" carb to match what I have. I really just need a valve job and don't need to get carried away. How much to mill? Thanks

Depends on the cam you're running. If your dynamic compression ratio drops too far, you won't be happy. I'm trying to keep mine close to the limit for pump gas, which would be round 8.1-8.4 (according to the sources I've come across). I would think that as long as you stay in the 7.5-8.0 range, you'll be happy. Since you're sticking with the 1.5" carb, it doesn't make much sense to mill too much.

If you send me the specifics of your build, I can compute where you're static and dynamic compression ratio will end up. I've actually been thinking about publishing my scripts somewhere here for people to use. They run in a freeware version of some math software programs. It's not a pretty interface, but I trust it. Too many of the online ones approximate stuff and lead to errors...

It is critical that you know what your specific build is to know how large
your combustion chambers should be.

Your signature suggests you are running a 1966 170/200 engine.

The stock combustion chamber of a 1966 200 is around 53cc, for a CR of 8.7.
Utilizing a compression calculator:
Stock 1966 200, 3.68"bore, 0.025" steel gasket, 0.030 recess, 8cc piston dish,
53cc combustion chamber = 8.7CR

The same stock 200 with a combustion chamber of 59cc = 8.1CR

The same 200 with a 59cc head + 0.050" head gasket = 7.7CR

Mill the block - you will increase the CR.
Use flat top pistons - you will increase the CR.
Use a thicker head gasket - you will decrease the CR.

The numbers above are the calculated static CR.
The dynamic CR will be determined by the cam you are running, that's what cr_bobcat will help you calculate.

Hope this helps.

Hi, thanks for looking into this for me. The short story is I rebuilt a 200 about 25 years ago. .030 over, stock rebuild very mild Wolverine blue racer cam (solid) and now I have a burnt valve on #6 so I am cleaning up a 69 J code head to put on. I have a broken ear on the old head and I want to swap the head in one day. We knew the replacement head was done once and the chambers went from about 62 cc to 59 cc. I think the head should get a light cut just to surface it unless some one can say take .0?0 off it and you will have ?? cc chambers for a ? 9 to 1 compression ratio. These are just examples of what I am looking for. This is a regular gas weekend Bronco and I'm just doing a plain old valve job. Thanks.

I have an m code C9 head. I had 0.070" shaved and went from 60-61 to 48 cc. Note though, my new valves sit a little more proud than the stock ones did so there is some chamber cc difference involved with the installation and not just the result of milling.

By the way, if you fill in your build specific info per this template, I can run it for you.


///////////////////////////////////////////////////////////////////////////
Engine Build Parameters
Title: Steve's Build w/ Clay Smith 264/274-110* Cam
Engine: Ford 200 I6
///////////////////////////////////////////////////////////////////////////
Cylinder Bore (in): 3.68
Amount Over-Bore (in): 0.00
Number of Cylinders: 6
Connecting Rod Length (in): 4.715
Stroke (in): 3.126
Piston Dish Volume (cc): 6.5
Piston Compression Height (in): 1.500
Centerline to Deck (in): 7.808
Deck Milling (in): 0.00
Gasket Diameter (in): 3.81
Gasket Crush Thickness (in): 0.035
Combustion Chamber Volume (cc): 48
Camshaft Intake Duration (deg): 264
Camshaft Exhaust Duration (deg): 274
Lobe Separation Angle (deg): 110
Camshaft Intake Lift (in): 0.294
Camshaft Exhaust Lift (in): 0.300
Ground-in Advance (deg): 0
Installation Advance (deg): 4
Rocker Arm Ratio: 1.65
Intake Valve Diameter (in): 1.76
Exhaust Valve Diameter (in): 1.50


The OUTPUT will look like this:

================================================
INPUT - BUILD PARAMETERS
================================================
Block Specifications:
Cylinder Bore = 3.68 in
Centerline to Deck Height = 7.808 in
Rod Length = 4.715 in
Stroke = 3.126 in
Piston Dish Volume = 6.5 cc
Piston Compression Height = 1.5 in
Cylinder Head Specifications:
Combustion Chamber Volume = 48 cc
Intake Valve Diameter = 1.76 in
Exhaust Valve Diameter = 1.5 in
Rocker Arm Ratio = 1.65:1
Cylinder Head Gasket Dimensions:
Gasket Diameter = 3.81 in
Gasket Crush Height = 0.035 in
Camshaft Specifications and Events:
Intake Duration = 264 deg
Exhaust Duration = 274 deg
Lobe Separation Angle = 110 deg
Cam Intake Lift = 0.294 in
Cam Exhaust Lift = 0.3 in
Ground-in Advance = 0 deg
Camshaft Installation Advance = 4 deg
Intake Opening Angle = 26 deg BTDC
Intake Closing Angle = 58 deg ABDC
Exhaust Opening Angle = 67 deg BBDC
Exhaust Closing Angle = 23 deg ATDC
Intake Valve Lift = 0.4851 in
Exhaust Valve Lift = 0.495 in
Valve Overlap = 49 deg
================================================
OUTPUT
================================================
Engine Displacement = 199.49 in^3
Rod to Stroke Ratio = 1.51:1
Rod Angle = 19.36 deg
Resulting Deck Height = 0.03 in
Squish Distance = 0.065 in
Static Compression Ratio = 9.22:1
Dynamic Compression Ratio = 7.78:1