Re: head machining

[Randy Wilson <randy@taylor.infi.net>]

>
>
>Randy Wilson:
>> > BTW, with the Mays head design, you would have to mill a very large
>> > amount off the bottom to have an impact on the compression.
>
>The Mays head design should be the MOST CRITICAL in this regard.  Since the 
>compression ratio is relatively high, that means the combustion chamber is 
>relatively small -- hence changes resulting from milling make a larger 
>percentage impact.

not necessarily.


>
>> From:          "pcsolutions" <pcsolutions@mindspring.com>
>> I would also imagine that if the head in question was machined to a
>> such an extent to affect compression ration that drastically, it
>> would be impossible to adjust the timing chain slack.
>
>I dunno what you guys are thinking.  The volume of a 5.3 V-12 cylinder is 442 
>cc.  With an 11.5:1 compression ratio, that means the volume of the 
>combustion chamber is about 42cc.  If you mill 20 thou off the mating 
>surface, this volume will decrease by 3.23cc -- increasing the compression 
>ratio to 12.4:1.  

Your calculations are totally correct if you assume the combustion chamber
area at the deck is identical to the bore (90mm). This is not the case
with the Mays design. The combustion chamber, the part in the head, is a
very small diameter *deep* pocket which holds the exhaust valve and spark
plug. It's not quite round, 42mm diameter with a bit of a bulge for the
plug well, and a bump as part of the swirl path. This is what I based 
my statement upon. A .020 cut (which is a massive amount for just trueing) 
would bring the compression up to about 11.7. 





>This is a significant change, and anyone doing such engine 
>work should know this.  And if the block is warped and one cylinder gets 
>shaved more than another to make it flat again, the compression ratios can be 
>significantly different from cylinder to cylinder.  cc'ing the head is 
>mandatory when doing such work.

Anyone that does this sort of work for a living should know that if
you have that much warpage in the head (I assume you meant head; .020
in an open deck block is major problems) requires checking the cam journals
for trueness. We had a discussion on the straightening oven a few weeks
ago. If the cam area is still straight, then the main casting did not 
shift, just the deck.



   Randy K. Wilson
     randy@taylor.infi.net

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References:

• Re: head machining
• From: "Kirbert" <palmk@mailer.gadcomm.net>

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