Milling a 420 head!

OPmini

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Hey guys! I recently bought a mill and was thinking about milling my predator 420's head down a few thousand along with a thinner head gasket. Any thoughts on this? Comments, concerns, and tips welcome! I just would like any helpful info on what I might be getting myself into before I start milling the head down or doing some extreme porting. I'd like info such as: pros/cons, or generally helpful tips. Thanks!
 

rockman96

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At minimum I would clay the piston to see how much valve clearance I had, and go from there. Make sure you have the cam/piston/rod you are going to run installed. If you are just wanting to bump the compression a bit its easy to accomplish this way.
 

vpd66

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It all about compression ratio. You need to cc the head, know the head gasket thickness, measure your piston deck height, measure the bore, and measure the stroke. With this info you use a compression ratio calculator (there are several online) to determine what your current ratio is. Then depending on what fuel your planning on using you determine a targeted compression ratio. For 91 octane pump gas I'd suggest to stay around 9.5-1 compression ratio. Then you can play with the head gasket thickness or the head cc (milling the head down) to get to your targeted compression ratio. There is no magic number on how much to mill off the head. The correct way to do it is to take measurements and do the math.
 

65ShelbyClone

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You should also technically shorten the pushrods by however much you mill the head in order to maintain valvetrain geometry.

I think you'll have to mill the head an awful lot before the compression ratio goes out of pump gas territory.
 

vpd66

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You should also technically shorten the pushrods by however much you mill the head in order to maintain valvetrain geometry.

I think you'll have to mill the head an awful lot before the compression ratio goes out of pump gas territory.
You will never know unless you do the math. I guess my intent of the post is to say there is no magic amount to mill off the head. First off you need a targeted compression ratio. Then you need to see what your actual compression is. Then you can decide what is needed to get to your targeted compression ratio. Blindly milling "x" amount off the cylinder head and installing a thinner head gasket is really shooting in the dark! Yes you are correct that if you do mill the head you should check the valve train geometry.
 
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