How much can I take off my heads?

[PierreBelvedere]

So, I'm rebuilding my 273 heads.
Have the 68 heads and flat head pistons.
I'm currently porting the heads, and it hit me!

Machine them for higher compression!
I know a guy here that oves me a favour, and can do the job.
Need to drop them off tomorrow, so I need some numbers.

How much can I tell him to take off?
Will run on std gasket.

 

[IQ52]

I think the factory has recommended a maximum of .060", which will reduce the combustion chamber size about 11cc. Stop before that, if the deck surface becomes transparent! Remember too, that to keep the manifold fit you must mill the intake surface of the head .0095" for every .010" milled from the deck surface. That could make the valve cover gasket surface thin. With milling the head it will be necessary to recheck the piston to valve clearance. Check to be sure that milling the head that much won't interfere with the intake valve seat for the valve size you are using. Milling that much will change pushrod lengths also. Could open a big can of worms milling the head, so be careful.

 

[PierreBelvedere]

So if I mill 0,019 I don't have to mill the intake side?
Is that correct?

 

[Pops1967GTX]

Here's a chart from one of my engine books

 

[IQ52]

So if I mill 0,019 I don't have to mill the intake side?
Is that correct?

If you mill it .019", your intake manifold might fit okay. But ideally you should mill the intake side .018" if you mill the deck .019".

The chart that Pops1967GTX has provided is correct for big block heads ONLY and is not correct for small block heads.

 

[PierreBelvedere]

Thanks everybody, I'll mill them .019 for starters.
Have'nt gotten hold of an intake manifold yet.