milling your head?

We may earn a small commission from affiliate links and paid advertisements. Terms


Senior Member
since i have my head off right now anyways, i was thinking about milling my head down. thing is, i dunno much about do you determine how much you want to take off? i think this would be a good investment right now for later on down the road when i get the cam gears and a chance to tune it on a dyno with an afc..
well, it depends on few things.

What engine is this for ? it'll help alot for me to know this info. I'll be able to figure out a pretty good estimate of the compression ratio if you can supply this info. or you can do it yourself here's the formula you'll need

First off before you can even think of doin this you need to know some info on the current set up of the engine. What is the current compression of the engine, The thinckness of the current head gasket, What is the valve clearance ** This is a crtical dimension **

You need to be 100% sure you'll have adequate valve clearance after the head is milled or your going to put a valve into the top of you piston(s) after you put things back together and start it up for the first time.

You're also going to need to know the piston to deck clearance (whether or not your piston(s) at TDC are above, flush or below the deck hieght). If your piston to deck clearance is a negative value (below the deck hieght) you probably have nothing to worry about, but for the other cases milling the head will bring the pistons closer the the head and combustion chamber depending on how the head is cast you might run into problems and the
piston might actually hit the bottom of the head.

I know for sure valve clearance is a 100% must for making sure you don't fux0r things up. As for piston to deck clearance this value might not be as critical as valve clearance (I Thinkvalve clearance takes piston to deck hieght into concideration but don't quote me on that)

Basically you'll be changing the Volume of the Cylinder Head. Similar effects can be seen by using a thinner head gasket. both will raise compression. The cost of doing this will most likely outway the benefits you'll recieve. Money better spent elsewhere in building you engine or head.

For example:
Cylinder Head cc = 38.5 cc
Piston = Flat-top w/ 2 valve relief pockets that measure a total of 2.5cc's
Head Gasket = 76mm round and .762mm thick
Deck Clearance Volume = Depth below block deck @ tdc = .254mm
Ring Land Volume = Piston to Wall Clearance = .05mm, Height from top of piston to ring = 16mm

Your looking at a Compression Ratio of 9.19:1

If you can change the Cylinder head volume by 3cc's (which I'm not sure is or is not possible) keeping all other variables the same you'll end up with a Compression Ratio or 9.76:1

which is a net gain of .57 points in compression if it is possible to enough material from the head without causing harm to the engine set-up, is it cost effective , I'm not sure.
milling the head is only something you shuold do if its the head is warped...otherwise, don't touch it!

people think they can mill their vtec heads over and over and thats simply not the case. rule of thumb: only mill the head enough to make sure its straight...raising compression levels may increase power...but not by much...
the engine is a ZC, and i was just thinking that since i had the head off, it would be easy to do now. and i heard that it can be as cheap as $25? i'm pretty sure i read that here. but i'll try to look up all those specs..see what i can dig up on the net. thanks for the responses =]