What is the minium head thickness before I have to get a thicker head gasket. Mine is 4.869 inches
It isn’t really measured this way. You seal off a hemisphere chamber with a piece of plexi, and drip in fluid like ATF from a burette and make note of how many CCs of fluid it takes to fill. Then plug that into a spreadsheet and it gives you an optimum gasket thickness.
As Erica says ccing the head is your best approach.
Aim for no more than 9:1. We routinely use 8:1 pistons in our rebuilds to allow for head and block skimming over the years.
You really don’t want your compression over 175psi or so or you will start to run into a lot of trouble with pinging.
So then…I am assembling the rebuilt engine and I know the head was milled. Am I understanding that there was no minimum head size before you go to an oversize head gasket?
What would be the drop in compression if I use a thicker gasket, just to be safe?
I like Prestone anti freeze for that. Bright yellow green and much easier to clean up after
I used 91% Isopropyl Alcohol in a burette and a straight edge to determine volume. The IPA has no appreciable surface tension so no meniscus making it easy to see when the level reaches the straight edge spanning the hemisphere of interest. (The head needs to be absolutely level.) Every one of my hemispheres measured 99cc. Given that information and the 0.020" overbore on my block I calculated the needed Cometic head gasket thickness required to get 9:1 compression ratio. This is the only method I’m aware of to get an fairly accurate CR (other than assembling the motor, breaking it in, and then measuring the CR and adjusting the head gasket thickness from there )
If a picture is worth a 1,000 then a video is priceless
That is the how-to
I’ll let the smart guys explain how to convert the volume of the cylinder head into compression ratio (CR) and then select the proper head gasket (it may be thin, but it’s thickness needs to be accounted for and can be used to manipulate the final CR).
Take the calculated SWEPT volume of the combustion chamber, and ratio it with the calculated volume of the cylinder.
With anything but dead flat pistons, that is best calculated, volumetrically. To be completely accurate, the top ring should be replaced with an o ring, and the interface of the crown/bore sealed with heavy grease.
Fixed it for you
Appreciate the clarification, kind sir! I even clarified it better…