Milling heads?
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Road Master
Joined: Aug 2008
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Milling heads?
Is there a generic formula that tell you what compression change you get from milling heads. I've heard for every .06 milled it drops your cc 1 point. Is that true? I'm just looking for for info I'm not taking my heads off , just trying to expand my knowledge base
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Joined: Oct 2010
Posts: 94
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From: New Orleans, LA
Think about the shape of the combustion chamber, whether bathtub shaped, hemi, or other. When you start milling the bottom, the first few thousandths will reduce the volume of the chamber at a different rate than further cutting.
Then there are all of the other components above the milling operation that move as well, in this case everything above the head gasket, like valves, rocker arms, etc.
Just milling heads for a compression ratio increase without radically altering other things has a fairly low limit. .010 or .020, not that big of a deal, .100 and you may have undesirable results. It is done all the time,but doesn't always yield good results.
Captsluggo
Then there are all of the other components above the milling operation that move as well, in this case everything above the head gasket, like valves, rocker arms, etc.
Just milling heads for a compression ratio increase without radically altering other things has a fairly low limit. .010 or .020, not that big of a deal, .100 and you may have undesirable results. It is done all the time,but doesn't always yield good results.
Captsluggo
Ultimate HDF Member
Joined: Aug 2008
Posts: 5,413
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From: eastern oklahoma
you have to be aware of piston to valve clearances also. that along with what's called the quench area. there are a lot of mistakes that can be made if you're not aware of what's required. sometimes it's best to leave stuff like that to the professionals. big boyz heads is a great source for information, as they have lots of stuff to read on this topic in their website. in there, you can see how it all works, it's really helpful when trying to make decisions on the topic of heads, milling, etc.
Extreme HDF Member
Joined: Sep 2010
Posts: 14,744
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From: Phoenix '53, '88, '09 Big Twins
Police evos were milled, my 88 FXRP had the heads milled .060" at the factory yielding 9.6:1 compression.
I had to be careful to figure this into solid manifold refits, pushrods etc.
It's a stinkin' fast bike, but much of that would be the light weight, under 450 with all the cop **** stripped off it
MK
I had to be careful to figure this into solid manifold refits, pushrods etc.
It's a stinkin' fast bike, but much of that would be the light weight, under 450 with all the cop **** stripped off it
MK
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