Maximum Compression Ratio for Non-Posted G Heads Home » Keeping The Dream Alive! » Technical http://forums.y-blocksforever.com/Topic168950.aspx

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By 55blacktie - 8 days ago I intend to use non-posted G heads on my build. The heads have been milled .013. The block will be zero-decked. My calculated static compression ratio is 9.4:1; static is 8.04:1. Best composite gaskets will be used. Although I don't consider this high compression, will it be a problem with non-posted heads? By Ted - Last Week Great question but answers will be somewhat vague or ambiguous simply due to the number of variables involved.  The unposted ‘G’ cylinder heads have unsupported decks within them where the head gasket sealing ring resides.  This increases the chances for a head gasket failure should there be any flex in those head decks.  From the Ford shop manual, it was recommended to hold any milling of the 1957 heads to a maximum of 0.010”.  In doing that to a 312 with the ’57 camshaft, that makes the static compression ratio ~9.25:1 and the dynamic compression ratio ~7.85:1.  You can consider those your factory safe limits. It must be remembered that FYB cylinder heads prior to the ‘G’ heads also had non-posted heads but those heads are not as susceptible to excessive deck cutting as the ‘G’ heads are.  The reason for that is the earlier heads, take the ECZ-C as an example, had lower intake ports that were cast as part of the decks thus adding a significant amount of support to the deck portion of the head.  The ‘G’ heads had lower intake ports that were raised and subsequently had a gap in the casting between the lower port and the deck internally. All the heads cast at the Dearborn facility are of the unposted variety.  Late in the 1957 production run, the Cleveland facility started casting the G heads with the posts cast within them.  Earlier Cleveland heads are also unposted.  May 15th, 1957 comes to mind as the date that change at the Cleveland foundry took place but feel free to correct that if you have better information.  As a general rule, all small letter G heads are unposted but when dealing with the large letter G heads, you will need to verify if the heads are posted or not. But here’s where those other variables start coming into play.  Increase the intake duration of the camshaft or make the intake closing event later, then the DCR actually gets lower thus permitting some latitude in milling the heads more and still keep the combustion pressures the same as with the aforementioned factory ratio. The heads bolts and head gaskets are yet another consideration.  The factory grade six bolts were noted for not holding their torque or said another way, stretching over time.  Grade eight bolts do help with this.  The factory steel shim gaskets were also not very forgiving when dealing with cylinder head deck flex.  The newer crop of composition gaskets do help in this area but some piston to deck adjustments must be made due to those gaskets being thicker in regards to bringing the compression ratio back up.  Block deck milling and/or different wrist pin locations are a couple of ways to address this. Then there’s fuel quality.  Any detonation will shake or rattle the head gasket and bring to the forefront a head gasket failure.  The amount of head mill in this case is logarithmic in that the propensity for a head gasket failure increases with the square root of the amount of mill.  My personal experience with unposted ‘G’ heads is to limit the total amount of mill to 0.025”. Any other comments always welcome.         By 55blacktie - Last Week As I've already mentioned, I intend to use Best composite gaskets. I will also use ARP fasteners, but I'm debating on whether to use bolts or studs. Will studs be the better choice in my particular situation? I could also select a bigger cam that will lower the dynamic compression ratio, but it won't change the static compression ratio. With a zero deck and compressed thickness of the Best gaskets, my quench will be .046. I would prefer not to go upwards to .060 on the quench.  By Blind Willie - 7 days ago For what it's worth, I saw somewhere that Joe C had stated the small letter G heads were better for porting.   Now if I could find the reminder I sent to myself or the link I saved......  By KULTULZ - 7 days ago Wasn't there a TECH ARTICLE written sometime back describing  posting a cylinder head with a SS threaded rod or is just another one of my CRS hallucinations? It seems to me on a factory posted head, that the post itself would be subject to corrosion and failure after so many years. Are the MUMMERT ALUM HEADS posted? By Ted - Last Week KULTULZ (8/12/2025) Wasn't there a TECH ARTICLE written sometime back describing  posting a cylinder head with a SS threaded rod or is just another one of my CRS hallucinations? It seems to me on a factory posted head, that the post itself would be subject to corrosion and failure after so many years. Are the MUMMERT ALUM HEADS posted? Those posts added to the 'G' and newer iron heads are very substantial.  The amount of corrosion required to break down the posts is much more than what’s needed to break through elsewhere.  The castings are relatively thin at the intake and exhaust runners so that’s the area corrosion would be more of an issue. No idea on how the Mummert aluminum heads get around the posts but suspect that thicker decks alone would fix that problem.  I donated my cut up aluminum heads to the local racing museum but will take a look at them the next time I am there.  Zero head gasket issues in regards to sealing with the aluminum heads so however John fixed the problem, it is fixed. This link will take you to several posts on the subject. posting heads