Breaking down a VM-68 into parts and narrating each step along the way.
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The thread for the screw to connect the m-16 lone star grip is 28 tpi.
The length of the cup seal in this VM-68 is 1.35 inches but you can typically use standard non piercing cup seals that are 1.285 inches.
I recommend drilling the port on the valve out when using the shorter length standard cup seal.