After making the piston move down over 6 frames, make 6 more frames. Just copy the original six frames...in reverse order to make the piston go up again!
Now draw the crankshaft and piston rod. Each position is different and needs to be drawn separately. The crankshaft makes two complete revolutions during our 24 frame animation.
With half the frames partially finished, make 12 more frames. The crankshaft, piston, and piston rod will be in the same positions as in the first 12, so you can just copy those frames.
The main part of this animation is now complete. You will need to draw the movement of the valves, and the air and fuel mixture and the exhaust to complete the image.
The positions of the valves, fuel and exhaust, and the moment of explosion, all need to be hand-drawn as appropriate for the actual stages of the 4-cycle piston engine.