I have sonic tested the unposted head deck surfaces on G heads and although I don’t recall the exact values, there’s not much there.  But the thickness is not so much the problem as are the large areas of unsupported deck surfaces that allows the decks to flex or distort when conditions permit.  There are two such areas on the unposted G heads.  The posting of the heads alleviated that particular problem without a complete redesign of the deck surfaces.  The service manual suggests limiting the mill on the 1957 heads (unposted G heads) to 0.010” but 0.025” is the generally accepted safe limit for these heads.  Posted heads have been milled much more heavily without issues.

 

On a side note, I am getting close to doing the exhaust test on the dyno and that test will be performed after testing two different pairs of ported iron heads.  The first pair of ported iron heads to be tested is a set of unposted G’s that have been angle milled 0.175” (not a typo) and were ran like this successfully on a circle tracker back in the Sixties.  This pair of heads has had homemade posts added to the heads.  Calculations show that the compression ratio on what was a 9.2:1 engine with mildly milled G heads is going to jump to 10.7:1 with the change to the angle milled heads.  Although as many as four sets of headers will be tested on the angle milled heads, the more detailed header test will be performed on the second test of ported heads which are milled and ported to more conventional standards.

 

And here is another link to a past thread discussing head milling.

http://www.y-blocksforever.com/forums/Topic27800-3-1.aspx

Lorena, Texas (South of Waco)

The lowest CC heads I have seen are ECR-D or C and the combustion chamber lterally had near no step to the deck surface, I will CC them some time. They were off a well drilling rig with the sodium filled exhaust valves. When I went to get the motor the jerks had already pulled it with a pulp loader, They #*@# the carb up, generator, starter, exhaust manifolds and scratched the surface of one head at the valve cover surface and bent a valve cracking the guide.

I was some mad at them, it was a total complete 4bbl engine and they destroyd nearly everything on it, I still got my $35 bucks out of it though, I gave them $75 for two engines they were going to scrap.

55 Vicky & customline

58 Rack Dump, 55 F350 yard truck, 57 F100

59 & 61 P 400's, 58 F100 custom cab, 69 F100, 79 F150, 82 F600 ramp truck, 90 mustang conv 7 up, 94 Mustang, Should I continue?

Thanks for all the infomation, Maybe i need to rethink things.

Gary Ryan San Antonio.TX.

Here is a link to a discussion on this subject.

http://www.y-blocksforever.com/forums/Topic16300-3-1.aspx  

In it Ted says for unposted G heads .010" is factory recomendation but it is generaly accepted that .025" is the max. Mike

1956 Fairlane Victoria (ORREO)

 

Overlooking Beautiful Rimrock AZ

Ted Eaton will provide definitive information when he gets time. 

Unposted G heads with too much compression will flex just enough to allow combustion gasses to get by the head gaskets and into the cooling system.  So not only are you dealing with how much can be milled re cracking but also all other combinations that increase CR.

You gotta have the right tools and know how to use 'em.

TC - Austin, Texas

I chopped 0.05" with no problem, but those heads were -113.


Hyvinkää, FI

My 113 heads are milled .030-.035.  The machine shop didnt keep track of the first milling, so therefore the range.  With that much off, and significant block decking, you may need milling on head intake flanges.  I had the block decked .028, and intake flanges .046.  .046 off the intake side removes the top step in the flange.  The static CR ended up between 9.1 and 9.2.  .060 over 292.

Mike, located in the Siskiyou mountains, Southern, OR 292 powered 1946 Ford 1/2 ton, '62 Mercury Meteor, '55 Country Squire (parting out), '64 Falcon, '54 Ford 600 tractor.

Gary, i dont know that thickness you mention as it would vary between castings, to my knowledge they dont crack on the flat surface but in the valve seat area. The amount that can be machined safely is more a matter of small amount .030" ? and be able to align intake holes with head or more and have to machine intake side of head to align bolts. Others can add thier expierences but certainly varies with head, block and intake combinations.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

The information that i am looking for is the thickness of the head surface to the water jacket, like between the combustion chambers.  How much can be milled off before you run into problems of cracking?

Gary Ryan San Antonio.TX.

Though the typically reported volume is higher, my unposted Cleveland (large letter) heads had not been previously milled, and after a .005" clean-up cut, the chambers averaged 65-66 c.c.s.

6 VOLTS/POS. GRD. NW INDIANA