On older Cyklon 1st stages, the standard thread size of all ports were 1/8.
Today, all LP ports are 3/8 and all HP ports are 7/16 sized.
If you have an older style 1st stage, there are adaptors that can be used, so you can attach standard sized hoses to your 1st stage.
Male 1/8 --> female 3/8 thread (Article number 2415)
Male 1/8 --> female 7/16 thread (Article number 2910)