Machine Design

Gulliver's Engines

Shrinking full-scale engines to pocket size is no small feat.

Senior Editor

Click here to see and hear these engines run.

A running quarter-scale model of a 1937 Harley Knucklehead V-Twin. An electric motor in the base starts the engine. The fuel tank sits where the generator would in the full-sized version. For size reference, adjusting the pushrods takes a pair of 5 /32-in. open-end wrenches. The overhead-valve Knucklehead engine was built from 1936 to 1947 and is the progenitor of today's Harley aircooled V-Twins.

The Aurora V8 engine-block tool is built in several sections to prevent back locks and to let a wax form be removed in one piece without damaging it. Above: a wax positive of an Aurora cylinder head and a finished investment casting. Above: Nancy Warner holds an Aurora engine block. Also shown are a crankshaft, camshaft, connecting rod and piston, valve, distributor cap, and fuel-injection manifold.

The quarter-scale 1913 Gnome Monosoupape (single-valve) rotary is 9-in. diameter and spins a 24-10 prop at 2,400 rpm. The Gnome is unusual in that its cylinders rotate with the prop while the crankshaft remains fixed. Nine separate cams stacked together actuate poppet exhaust valves. Air, oil, and fuel flow though a hollow crankshaft. Induction is through ports in the cylinder bases. Extra inlet air enters the exhaust valves because the valves remain open 90° after top-dead-center of the intake stroke. Both full and quarter-scale Gnomes have no throttle — they run wide open at all times. (A “blip switch” in the aircraft let a pilot momentarily interrupt the ignition for landing.) Gnomes were noted for having the highest horsepower-to-weight ratio of anything flying at the time. The nine-cylinder engine produced 160 hp and gave the Nieuport 28 fighter a top speed of 122 mph. Gyroscopic forces induced by the rotating cylinder heads made flying the plane “interesting.” Castor oil was used in a total-loss oiling system, which bathed the plane and pilot in the hot, sticky stuff. The model uses the same oiling system.

Valve split keepers in Aurora engines are extreme scale, says Replica's Wally Warner.

The 1998 IRL Aurora V8 in quarter-scale runs on methanol, just like the real thing. In fact, all of Replica's engines run on methanol because gasoline tends to foul the tiny spark plugs. Methanol also helps cool the engines.

The quarter-scale water-cooled OX-5 OHV V8 engine displaces 7.0 cu in. and turns 3,000 rpm. The original 503-cu-in. engine produced 90 hp at 1,400 rpm. OX-5s during World War I powered Curtiss JN-4 (Jenny) trainers to a top speed of 75 mph and made possible the barnstorming craze that followed.

The 1940 Offenhauser four-cylinder engine on which this quarter-scale model was built powered a midget racer. The water-cooled miniature displaces 1.6 cu in. (0.750-in. bore 0.938-in. stroke). Offenhauser engines dominated oval-track racing for over 30 years.

Replica Engines, located appropriately enough in Gulliver, Mich., turns out astonishingly realistic miniature aircraft and automobile engines. The fact that the tiny mills look identical to the real things would satisfy most model aficionados. But there's a twist. They run.

What admittedly has become an obsession for Replica co-owner Wally Warner began when he scoped a magazine ad for radial engines that power radio-controlled flying models. He tried buying one but the manufacturer had gone under. Wally eventually bought the failed company and he and his son Scott started making five and seven-cylinder radials. Later, they focused their talents on running models of actual production engines. Their first project: a one-quarter-scale nine-cylinder Gnome rotary.

Gnome engines in various forms powered several World War I aircraft including the venerable Nieuport 28 fighter. The Gnome is unusual in that its cylinders rotate with the prop while the crankshaft remains fixed. Fortunately, both the Smithsonian and Experimental Aircraft Association museums had a wealth of information about Gnomes. More importantly, someone provided Replica with a complete engine to reverse engineer. "We need examples of full-scale parts or we can't do the work," Wally explains. "Detail drawings aren't available in most cases."

Such was the case with follow-on engines including the OX-5 OHV V8, another World War I-era aircraft powerplant. "A guy had a garage full of OX-5 parts that he let us borrow," says Wally. Likewise, an automobile-museum owner lent Replica two vintage race cars; one with a 60-hp Ford flathead V8, and the other with a four-cylinder Offenhauser. Quarter-scale versions of a Harley-Davidson Knucklehead V-Twin and Replica's most ambitious project to date — an Indy Racing League Aurora V8 — as well began with actual hardware.

GM hooked up Replica with race-engine developer Katech Inc., Clinton Township, Mich. Katech supplied a complete IRL Aurora engine, but no drawings. "Re-verse engineering the Aurora has taken several years," Scott says. "We're still working on it."

Step one in the shrinking process is to take careful measurements of the full-sized engine components. This painstaking task is done mostly by hand with calipers, snap gages, gage pins, and micrometers, and with a tabletop CMM. Large, highly complex parts such as the front cover of the Aurora V8 are farmed out for 3D digitizing. Scott converts the dimensions into AutoCAD drawings and, where applicable, CAM files and NC toolpaths.

Replica also uses a 1940s vintage pantograph machine to shrink some cast parts. Here's how it works: A handheld probe connected to one arm of the pantograph touches a full-sized part. The adjacent arm — adjusted for the desired scale factor and fitted with a rotating cutting tool — shapes the scaled part from a block of engineering plastic.

The resulting plastic miniature part becomes a "positive" around which a special resin is poured to build a two-piece, temporary mold tool. When the resin cures, the tool is separated, leaving a cavity that mirrors the part surface. Molten wax injected into the cavity produces a form for a lost-wax investment casting. The cast-aluminum Aurora V8 intake manifold, water pump, valve covers, and oil sump all are made with resin tools, which is fine for comparatively simple parts and short production runs.

Highly complex parts such as the engine block, however, need permanent, metal tooling. A combination of EDM and NC milling shaped the aluminum engine-block tool. The tool is built in several sections to prevent back locks and to let a wax form be removed in one piece without damaging it. Tolerances are held to tenths of a thousandth inch, so tight that an air film sticks together individual tool sections. Replica finish-machines the rough investment castings in a four-axis NC mill. A honing machine hones the cylinders and line bores the crankcases. Fixtures for these operations also are held to extremely tight dimensional and repeatability tolerances.

But if you think it’s simply a matter of shrinking parts to quarter-scale and assembling them, think again. Some parts don’t scale well. Case in point: GM in the interest of saving weight made the cast-iron cylinder liners in full-sized Aurora V8s just 0.030-in. thick. Obviously, scaling that number down fourfold wasn’t an option and necessitated internal-design changes in the model engines.

GM also removed excess material from the engine castings. "Casting shrink rate in this case becomes a driving factor because there isn't much material to begin with," explains Wally. "The Aurora front covers, for instance, never came back (from the caster) the same twice." It turns out casting tolerances vary widely with metal pour temperature. Higher temperatures boost shrink rate, while lower ones reduce it. Large, thin-wall parts such as the front cover tend to shrink a lot.

Casting problems plagued the Ford flathead as well. "The Ford exhaust ports go directly into the engine block,” explains Scott. “The cores exiting the intake manifold deck that run down the sides and out are extremely small diameter. Just one foundry reluctantly agreed to do the work. Each casting costs over $100 and scrap rate is high.”

Other miniature parts are prohibitively expensive or impractical to produce by methods used for full-scale engines. Quarter-scale Aurora crankshafts and camshafts, for example, start as investment-cast 4140 steel that is then heat treated for proper strength and hardness. Forging makes their full-scale counterparts, which is too costly for short production runs of model parts. Finish grinding is done the same way as on actual hardware, save the addition of a pantograph-follower mechanism that scales the features. The follower runs over lobe surfaces of a full-scale camshaft while a grinding head indexed to it faithfully reproduces the profiles on the miniature lobes. The same machine, with a different setup, grinds the crankshaft rod and main journals.

Replica prides itself on the fact that all its engines are oil-pressure lubricated, as opposed to some running models that rely on oil blow-by to lubricate bottom ends. Oiling-system components are incredibly tiny, as you'd expect. For example, the Harley Knucklehead oil pump uses a 0.125-in.-diameter drive shaft with a 0.0625-in. square machined on its end to mate with a pump gear. A broaching operation cuts the tiny square hole in the gear, no easy task.

In the Aurora V8, oil feeds through the engine block to the crankshaft and to four overhead camshafts. Hollow camshafts let oil pass through and feed cambearing surfaces cast into the heads. Oil then siphons to the bottom end via a "nonscale" portion of the oil pump. The full-scale Aurora oil pump has two pressure sections, two scavenger sections, and a centrifuge in the middle that expels air bubbles from the oil to improve lubrication. The centrifuge was omitted in the scaled-down version to reduce complexity. Oil passages at quarter scale are on the order of 0.063-in. diameter, too small for adequate oil flow, so holes are made a little larger.

"Basically, we tweak the internal workings to make the miniature engines run, which isn't a problem because you don't see these modifications," explains Scott. "Externally, the engines are dead on, except for the ignition systems. Electricity and electrical insulation don't scale down."

Ignition distributors are built slightly larger to prevent high-voltage arcing across terminals. Engines originally equipped with magnetos, such as the Offenhauser, instead use an electronic-distributor ignition. "Magneto" rotors contain magnets that trip a Hall-effect switch. The switch pulses a high-voltage coil in the engine-mounting stand. The distributor routes the high voltage to spark plugs as it would in a full-scale engine.

Speaking of spark plugs, Replica makes those, too, mostly out of necessity because of high costs specialty suppliers charge for them. An NC lathe turns the steel shells. A fine wire run through the ceramic insulator connects to a center electrode. Anaerobic adhesive seals the electrode assembly, and a ceramic adhesive bonds the insulator to the shell. Leaks can be a problem. "Scrap rate initially was about 50%," Wally says. "We've since learned a few things about making miniature plugs and now it's about 10%."

Supply an engine with air, fuel, and spark at the proper time and it should run, right? Well, if it has adequate compression, yes. And that can be a problem for engines with valves just slightly larger than straight pins. Valves are hardened and precision-centerless ground, and the seats are cut and lapped. But forget about visibly checking for proper valve seating. "We put oil on the valve head and chamber and draw a vacuum on the port. Oil bypassing the seat indicates a leak that needs repair," explains Wally. "It's an extremely time-consuming process."

Other less-obvious gremlins can prevent the miniature engines from starting. The Harley Knucklehead wouldn't run at first because blow-by about equaled compression pressure, this despite having two compression rings on the silicon-aluminum pistons. A modification to the crankcase-venting system relieved the pressure and the engine ran perfectly.

The Gnome ignition system was a particular challenge. High voltage travels from a ring fitted with copper buttons. Timing adjusts by rotating the ring relative to the pick-up electrodes located on the engine case, and by changing button diameter. Errant timing causes high voltage to jump the buttons to ground without firing the plugs. Just a narrow range of timing settings let the motor run. It was a tedious process of trial-and-error to get it right. Ditto for the Aurora V8 fuel jetting. No tuning guidelines are available for the full-scale engines' electronically controlled fuel injection, let alone for one-off mechanical fuel injectors on the miniature version.

Eventually, all the engines run before they ship. Which begs the question: Who buys them? "Mostly collectors and some modelers," says Wally. The market is fairly limited. Few people are capable of building scale cars and planes that match the level of engine detail. "Selling price is another issue," Wally adds. "Too high and people won't buy them. But there must also be some level of exclusivity, so few of any particular engine get built. This, of course, prevents us from realizing any significant savings from economies of scale."

Limited as the market may be, production runs of the Ford flathead V8 and OX-5 models both are sold out. And a dozen of the 100 Aurora engines to be built have been delivered. So what's next on the miniature drawing board? Scott says look for a quarter-scale Rolls-Royce Merlin V12 (P-51 fighter) engine.

Displacement: 3.8 cu in.
Bore: 0.915 in.
Stroke: 0.723 in.
"V" angle: 90°
Valvetrain: Dual overhead cams
Valves per cylinder: 4
Compression ratio: 15:1
Lubrication system: Dry sump
Fuel: Methanol
Fuel system: Mechanical injection
Block material: Aluminum
Head material: Aluminum
Cylinder liners: Cast iron

Want to see and hear these engines run? Click here to view video clips.

Replica Engines,

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