A 7-cylinder radial aircraft engine on a display stand, 34 cm across the propeller. Turn the crank at the back and everything inside moves the way it does in the real thing: the crankpin drives one master rod, six link rods ride on knuckle pins around its big end, and all seven pistons slide up and down in their finned cylinders while the three-blade propeller turns. The crankcase splits in two and clamps the cylinders, so you can take it apart again and show how it works. Everything is held with M3 screws, and every part prints without supports.
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The front half of the crankcase with the nose case and a 40 mm long front bearing for the crankshaft. Seven half-sockets hold the cylinder spigots, the 14 holes in the nose take the pushrod tubes, and the seven M3 × 50 bolts go in from the front.
The other half of the crankcase. Together the halves clamp the collar on each cylinder spigot.
Finned barrel and head in one piece, with a 28.6 mm bore, rocker boxes, an exhaust stub and two sockets for the pushrod tubes. The spigot at the bottom has a collar that locks it into the crankcase.
Ø28 mm, with two ring grooves. The wrist pin is an M3 × 20 screw: its head sinks into one side and the nut drops into a slot on the other.
Runs on the crankpin and drives the top piston. The two knuckle plates are bolted to it.
One plate on each side of the master rod. Six M3 × 12 screws through both plates are the knuckle pins for the link rods; two more fix the plates to the master rod. The hex pockets take a screw head or a nut.
Connects a knuckle pin on the master rod to one of the other six pistons. 46 mm between the holes.
Front crank web with its counterweight, the front journal and the hex shaft for the propeller.
Rear crank web, rear journal and the hex for the crank arm.
Ø14 mm pin with hex ends that key into both webs. One M3 × 30 screw through its centre holds the crankshaft together.
Two per cylinder, from the head to the nose case. Just for looks: they plug into the sockets on both ends.
Half of the hub. The blade roots lie in its three sockets; it slides onto the hex shaft.
The other half. Three M3 × 20 screws clamp the blades between the halves, and one M3 × 10 holds the hub on the shaft.
Twisted blade with a curved airfoil, 158 mm long. The round root has a collar that the hub halves clamp.
The nose cone. Its lip presses into the groove in the front of the hub.
Plate behind the crankcase with the rear bearing for the crankshaft, and the leg down to the base. The seven crankcase bolts end in nuts in its back.
200 × 190 mm foot. The leg stands in a shallow socket and two M3 × 16 screws hold it from underneath.
Fits on the hex at the back of the crankshaft and is held by an M3 × 10 with a washer.
Turns freely on an M3 × 35 screw in the crank arm.
For a 0.4 mm nozzle. Filament amounts are estimates for these settings.
Nothing needs supports. Print the blades with a brim. Clean any elephant's foot off the pistons and the crankpin so they run smoothly; the pistons have 0.3 mm play in the bores and the bearings 0.2 mm.
Put the master rod's small end into one piston and a link rod into each of the other six. Push an M3 × 20 through from the round pocket, drop a nut into the slot on the other side and tighten.
Lay one knuckle plate down with its hex pockets underneath and a nut in each pocket. Put the master rod on it, then the six link rods on the outer holes, in order, and the second plate on top. Screw eight M3 × 12 down through it into the nuts.
Push the crankpin through the master rod's big end. Put a nut in the pocket of the front web, slide both webs onto the hex ends with the counterweights on the same side, and screw them together with the M3 × 30 from the rear web.
Stand the front half on its nose in a mug, split face up, and push the seven M3 × 50 bolts up through it from below. Lower the crankshaft's front journal into the bearing. Grease the bores, slide a cylinder over each piston and lay its spigot in the socket.
Lower the rear half over the bolts and the rear journal, then the engine mount. Put a nut in each pocket at the back of the mount and tighten the bolts evenly. Turn the crankshaft: all seven pistons should move freely.
Put a drop of glue on the long pin of each tube, push it deep into a hole in the nose case, swing it into line and slide it up into the socket under the head.
Lay the blade roots in the rear hub half, put the front half on top and clamp them with three M3 × 20 and nuts. Slide the hub onto the hex shaft, fix it with an M3 × 10 and a washer, and press the spinner on.
Stand the leg in the socket on the base and screw two M3 × 16 up through the base into it. Fit the crank arm on the hex at the back with an M3 × 10 and a washer, and the grip with the M3 × 35 and a nut. Turn the crank.
The engine has small parts and screws: keep them away from small children. Keep fingers out of the propeller while someone turns the crank.
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Put your own photo in a phone case, lamp shade or lithophane, right in your browser.
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