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The 164 shop manual that I have, it is a copy with no title page, gives the allowable thickness of the cylinder head with a range from 4.91x inches from the base of the head to the center of the cam. Measuring my front head, I get a much larger dimension. Is this the right number? What I am after is how much to mill a stock head to end up with a 10.3 or 4 compression ratio, but I can't tell if my head has already been milled or not. I did search for threads but couldn't find this.
 

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The thickness in the manual is the minimum thickness. CC your head and with the basic compression calculators you can get in the ball park.
 

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But do you really want a 10.3 - 4? I think even premium would be unsufficient for that much compression...
 

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I have run as much as 11.5 on a 2.5 without issue. When you get that high you have to be watchful of engine temps and pinging. I would not recommend that much today.
 
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