I have a set of small letter unposted G heads that have been heavily angle milled (0.175”) and these have an extra post installed in each head. The additional posts were added such that they go through the pads under the rocker stands and between the rocker stand bolts. Because these heads are currently installed on an engine, I don’t have pictures of this particular post installation. But there are some pics demonstrating a similar installation in the thread that is linked below. I’ll add that the adding of any extra posts needs to be done before any head milling is done so any preloading or stressing outwards of the decks can be relieved at the time of milling.

And here’s a direct link to the post that Vic refers us to.

Compression ratios and keeping detonation at bay is the key on the G heads when attempting to mill the unposted varieties more than 0.025”.  The 1957 shop manual only recommends 0.010” for cleanup on the G heads so the factory engineers were cognizant of what they had.   When milling the unposted G heads, 9½:1 should be considered the limit for the static compression ratio while also keeping ignition timimg curve attributes reasonable.  That means using a timing light on an accurate damper and not ‘power timing’.  With this in mind, then getting away with more than the customary maximum 0.025” mill on the unposted G heads may be doable.  I’ll let you know more on that as the ported and unposted G heads that are going to be used for the brunt of the exhaust testing have already been milled 0.055”.  Consider this a test within a test.

I’ll add that the ECZ-G heads are in a class by themselves when it comes to milling.  The ECL-B and the ECG-A heads would be the 1955 292 heads while the 182HP Special heads for the 272 engines have these same casting numbers.  But as member 314 mentions, the 182HP heads are identified by the 0.866” thick measuring pads on those.  The ECZ-C heads (1956) as well as the earlier 272 heads are cast differently internally and are more robust when it comes to milling than the G heads.  Most efforts at heavier milling from a performance standpoint typically concentrates on the G heads simply because they are in the large valve class and have the smallest combustion chambers available versus what was available on the earlier intermediate valve sized heads.  An unposted G head unfortunately has a pair of deck surfaces within the water jacket area that are internally unsupported which is the root of the problem.  The posted heads addressed this problem specifically.

I tried posting this last night but it wouldn't go.  I'll try again.

On the surface of the head for #3 and #6 cylinders, just outside the fire ring of the head gasket chamber opening, drill a 5/16 hole perpendicular to the head surface.  Tap it 3/8-16 and screw a piece of 3/8 allthread firmly into the opposite wall of the head.  Cut the allthread off a few thousandths long, pein it, and fill off the high spots.  The gasket will seal any coolant that tries to get past the allthread.

JJJ

Not sure of your description.  There is no need to drill through the top of the head with my solution.  Drill through the machined surface, screw in the all thread until it bottoms snugly against the top of the head.  The spotface would be a good idea, I have always slightly rounded the allthread so it would seek a solid seat.  The rest of your proceedure looks good to me.  I have found that the #3 and #6 chambers require this, the others have intake ports that support the chambers.

JJJ

Here’s a picture showing the locations of where the factory added the posts to the late ’57 and newer heads. The dotted lines shows the actual post locations while the arrows simply point to the water holes that are used to 'feel' for the posts. Like John, when adding posts to what was originally non posted heads, I’ve only seen it done to the #3 and #6 cylinders but adding the posts to the #1 & #8 cylinders would also be recommended. Although I've only observed this being performed by coming in from the top, adding posts from the deck side does sound less complicated but the decks are likely on the thin side to adequately thread for this. Maybe Vic will comment on if he added his posts from the top or the bottom?

And here’s the link to the thread that particular picture comes from along with some more detail.

http://forums.y-blocksforever.com/Topic6711-3-1.aspx

http://forums.y-blocksforever.com/Topic6711.aspx