Printed on both an Ultimaker 2+ Extended and an Ultimaker 3 at .1mm with 100% infill for "Chassis Left.stl", "Chassis Right.stl", "Knob and Axle.stl" and "Spring.stl", 50% infill for the remaining components. Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Study "Assembly.stl". I assembled my Spring Motor Rolling Chassis Version 2 as follows: 1) Slide "Knob and Axle.stl" into "Chassis Left.stl", then press "Spring.stl" and "Pawl.stl" onto "Knob and Axle.stl". 2) Slide "Gear Pawl.stl" onto "Knob and Axle.stl". 3) Slide "Gear Spur.stl" onto the axle on "Chassis Left.stl". 4) Press "Chassis Right.stl" onto "Chassis Left.stl". 5) Press one "Axle.stl" onto one "Wheel.stl", then slide into position through "Chassis Left.stl" and "Chassis Right.stl", and secure with remaining "Wheel.stl". Make sure the wheels and axle spin freely. 6) Press "Gear Spur Small.stl" onto the remaining "Axle.stl" in the position shown. 7) Insert the small spur gear and axle assembly into the chassis assembly as shown, then press one each of "Wheel Rear.stl" onto each end of the axle. Make sure the wheels rotated freely. 8) Install one each of #18 o-ring onto "Wheel Front.stl", then one each of #111 o_ring onto "Wheel Rear.stl". 9) Apply light machine oil onto all moving contact surfaces.