GT40 irons? GT40P's? GT40X's? GT40Y's? or GT40Z's?
Unless you know (for certain) the complete history of the heads you're referencing, you should measure the volume of the chamber for any serious calculating you're gonna be doing. Others can tell you what size they might be --- but if they've been cut or modified in any way -- you have to measure to know for sure.
From what I remember the GT40 irons that were purchased thru FMS vendors were approx 64cc. The GT40 irons that were used on the 93-5 Cobras and so on were the same casting but factory milled to around 60-61 cc's to keep compression around 9.2:1 or close to it anyhow. I'm not sure what the GT40P heads usually CC at but I'd guess they are also around 61cc.
What Michael Yount said is right, it's not difficult to cc the chambers on heads, just get a graduated cylinder measured in cubic centimeters and use a piece of plexiglass with a small hole. Use wheel bearing grease or something similar to seal the plexiglass plate to the chamber and pour the liquid from the graduated cylinder in until its full and then take note of what you have poured in. The graduated cylinder will have increments of cc's that will tell you how much liquid is left in the cylinder. As long as you took note of how many cc's of liquid you started with, simple math will tell you what you poured into the combustion chamber.
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