how much should I mill this head?

[Anonymous]

With a 76 250 head on a 62 170.....I want a CR of around 9, maybe a shade less. I'm thinking .060 to .070 should do it? I'm kind of concerned about piston/valve clearance, is that a concern? I don't have the cam specs yet, how much lift/ duration can I get away with before I'm going to have to worry?

 

[barroom hero]

http://falconperformance.sundog.net/compcalculator.asp

this could help ya out if you know all your engine specs.

 

[CZLN6]

Howdy ER;

Assuming standard bore & Stroke, OEM steel shim head gasket and standard chamber volume of 52 ccs you '62 170 should be at 8.6:1CR.

The '76 200/250 head should have a chamber volume of 62 ccs. YOu will also have to compensate for the composite type head gaskets now available. If you can find a Napa, Victor it will measure .045" compressed thickness. You will have to mill the difference in thickness of the two gaskets (.020") and aprox .010" to get a reduction of about 2 CCs- that's .050" to get the 62 cc chamber down to 52ccs.

Assuming a Victor head gasket and a mill cut of .070" you should get a CR of 8.7:1. A cut of .080" will yeild 8.9:1, and a cut of .090" will get you aproximately 9.2:1. I would not recommend milling beyond .090" with this head. If you mill the head this much be sure to use hardened steel washers on the head bolts on reassembly to avoid bottoming out the bolts in their holes.

All of this is based on assumptions of dimensions, which is not a good idea. It is always best to measure and base computation on actual dimensions.

Your 170 will have flat top pistons, but very conservative cam specs. I doubt that you will have any valve to piston interference, but as always check for yourself. Stock cams are almost closed as pistons come to TDC.

A 170 should love the extra breathe.

Adios, David

 

[Anonymous]

Thanks for the numbers, David. I should have mentioned that it is bored .040, which will make the cr a tad bit lower, won't it? I do have the victor gasket, and as for the cam, I am having one ground for it as we speak, I just havent gotten the numbers from them yet. Thanks!

 

[CZLN6]

Howdy Back ER:

Overboring .040" will rais the CR slightly.

Adios, David

 

[CZLN6]

Howdy Again:

I refigured your CR using the new overbore dimension and it came out like this-

reducing the cc by 14/mill of .070"= 9.4:1
reducing the cc by 12/mill of .060"= 9.1:1
reducing the cc by 10/mill of .050"= 8.9:1

If I were in your shoes, I'd go .060" and spend some time cleaning up and polishing the chambers, camfer the edge of the bowls slightly. This will gain a slight amount of volume and will be more resistant to pre-ignition.

The Comp Cam catalog say a generality they use is that a cam under 230 degrees of duration at .050" lift will have sufficient p-to-v clearance about 98% of the time. It is best to check for your self on initial assembly.

Adios, David