This is a simple inline 4 internal combustion engine model with the side cut away, designed with generic cam and crank profiles. The marks on camshaft, crankshaft, and gears are for timing to show the approximate location of cam lobe profiles vs. piston position in typical 4-stroke fashion.

Quantity of parts required with orientation and whether supports are required: 1 - Block (print upside down) 5 - Camshaft Carriage (lay flat) 1 - Camshaft (print sideways with support) 4 - Connecting Rod (lay flat) 1 - Crankshaft (elevate from print bed 2mm or so with support) 1 - Gear_Camshaft (timing marks facing up) 1 - Gear_Crankshaft (timing marks facing up) 1 - Gear_Idler (timing marks facing up) 1 - Head (lay flat) 1 - Idler Cap (elevate from print bed 2mm or so with support) 10 - Joining Pin (lay flat) 4 - Knuckle (lay flat) 1 - Oil Pan (lay flat) 4 - Piston (lay flat) 4 - Wrist Pin (lay flat)

While finishing is not strictly required, especially if your settings are good - it prints in Elegoo Rapid PLA+ just fine with 0.15mm layers & 8 count brim - smoother rotation can be achieved by doing the following: Lightly sand/file out any rough edges on the camshaft & crankshaft bearing surfaces, paying attention to the very top and bottom surfaces of the part, as well as their bearing pockets on the head & block. Be sure all brim trimmings are removed, this can get tedious.

Assembly - read all instructions first before assembling:

• Insert camshaft into gear_camshaft while following the timing marks - they should line-up. Light filing may be required on the square hub, I've found they're a nice press fit. Set aside.
• Assemble the crankshaft in the same manner as camshaft.
• With the crankshaft in hand, place the open end of a connecting rod into one of the rod bearings. Install knuckle onto the connecting rod by seating one side and pressing it firmly on the other side, it should slightly bend over and snap in place. Repeat with the other three connecting rods.
• Place a piston over a connecting rod and insert a wrist pin, don't worry about how loose it is, the cylinder walls will keep them in place. Repeat with the other three pistons.
• With the block upside down, lower the crankshaft assembly until the middle two pistons are inserted into their cylinders, continue lowering until the outer pistons are inserted. Make sure the gear is on the right side - the same side as the protrusion in the block. Temporarily place the oil pan on the bottom of the block and give the bottom end assembly it a spin, it should rotate freely with the pistons moving up and down without binding. If it does bind, remove oil pan and file down the affected areas on the bearing pockets/journals.
• Insert five joining pins into the oil pan and lower the block onto it.
• Insert five joining pins into the top of the block and lower the head onto it.
• Place the camshaft assembly onto the head. Snap five camshaft carriages into their respective locations, installation is similar to the connecting rod knuckles.
• Rotate the camshaft and crankshaft until the timing marks are pointing roughly towards each other.
• Place the gear_idler onto the post of the block's right side, making all timing marks line-up.
• Screw in the idler cap.

Some notes, even smoother operation can be achieved by applying a drop of 3-in-1 oil to all the bearing surfaces. Once you're satisfied with the smoothness of motion, slightly separate the block & oil pan and apply a small amount of super glue to the joining pins, repeat with the head.

Viola!