A crank-piston mechanism for a reciprocating double-acting piston-type gas compressor that allows for infinitely variable compression ratios during operations. The mechanism includes a gear pin crankshaft, a forward piston-connecting rod, a rearward piston-connecting rod, a forward crosshead, a rearward tubular crosshead, and a tandem piston assembly. The forward piston-connecting rod and the rearward piston-connecting rod are each rotatably connected to the gear pin crankshaft about two parallel but offset axes. On the opposite end, the forward piston-connecting rod and the rearward piston-connecting rod are connected to the forward crosshead and the rearward tubular crosshead, respectively. Wherein, the forward crosshead is slidably mounted within the rearward tubular crosshead. The forward crosshead is connected to a forward piston from the tandem piston assembly and the rearward tubular crosshead is connected to a rearward piston from the tandem piston assembly. The forward piston and the rearward piston are aligned for reciprocating motion.