Volkswagen wasn't the first to squeeze the two banks of a V engine together to fit under one cylinder head, but they were the first to wonder what they would get if they put two of THOSE together with a common crank. The result was an engine with four banks of cylinders, which they called a "W" engine (a name that had already been taken by a 3-bank layout, Wikipedia argues the double-vee or WR might describe it better). VW tiptoed in, bringing W8 and W12 engines to market first, but I'm skipping ahead to the ultimate version, the W16. You might recognize this as the layout used by the Bugatti Veyron.
W8/W12 versions now available for download! I will be adapting this new (crossplane!) crankshaft design to fit my VR engine models.
If you downloaded the W16 in the first few days and you're having trouble printing the crank, download the updated file, or the remixed one over there.-> Sorry!
Pick an engine. Print one crankshaft and two sets of everything else, using a high infill setting (unless you're printing the W8, then you just need one crank, two block halves and one set of "W16" pistons and rods). Sand or hone the parts until the pistons slide easily in their bores, rods spin on their crank pins, and the crankshaft actually fits together. Use 3mm filament as wrist pins for the pistons and rods. Slide one pair of piston/rod assemblies onto each crank pin (note that the pistons are not symmetrical, the lower edges of the piston crowns should face the center of the block. Each pair of piston sharing a crank pin should be facing the same way, if that makes sense.), then assemble the crankshaft minus the end pins. It's a pain, but I've added reference photos. Glue if necessary. Slide the pistons into each half of the block, then line up the crank holes. Press one of those end pins into the hand crank, then use pliers to press them into the ends of the crankshaft.