I didn't have access to 3mm filament for petropixel's W16 engine, so I designed some wrist pins instead. These are 2.6mm x 14mm and slid in perfectly. A slight amount of tension to stay in place but not fall out, or lock up the piston from rotating with the rod. Attached is a single wrist pin, in case one of them fails in during printing you can print smaller quantities later, rather than only 16. Created using Solidworks. .10mm layer height and 100% infill.Just printed using the High setting on a MakerBot Replicator 2. .10mm layer height and 100% infill. Thats it!