Better Half VW

The Milholland V-twin engine, properly called the Better Half VW, represents one of the most successful lightweight four-stroke powerplants in the homebuilt aircraft world. Designed by Leonard Milholland of Brookshire, Texas, and first introduced in 1993, this two-cylinder conversion takes a fundamentally different approach to creating a small aircraft engine from Volkswagen components. Unlike earlier half-VW conversions that required cutting the engine case in half, Milholland’s innovation preserves the full crankcase, an approach that has proven simpler, more reliable, and ultimately more successful than the cut-case alternatives that preceded it.

The Better Half VW conversion produces approximately 30 horsepower at 3400 rpm with standard 92mm cylinders, though builders can increase power to 37 horsepower or more by using larger bore cylinders. The engine’s displacement is 920cc with the 92mm cylinders on the stock 69mm crankshaft, though various strokes from 69mm to 86mm can be used depending on builder preferences. The engine weighs approximately 88 to 90 pounds when properly built, making it ideal for ultralight aircraft that must comply with FAR Part 103 regulations. Milholland’s own Legal Eagle ultralight, which has an empty weight of just 244 pounds, was specifically designed around this engine and demonstrates its suitability for the lightest end of the homebuilt spectrum.

The Better Half conversion differs dramatically from cut-case approaches in its construction methodology. Rather than sawing the VW crankcase in half and fabricating an aluminum plate to seal the rear, Milholland’s design leaves the case intact and simply blanks off the two rear cylinders. An eighth-inch aluminum plate bolts over each rear cylinder opening, and the rear pistons and connecting rods are removed entirely from the crankshaft. The crankshaft itself remains unmodified, though only the front two rod journals carry pistons. The camshaft similarly remains uncut, with only the front two lobes actuating valves.

This full-case approach offers several compelling advantages that early builders of cut-case engines discovered through hard experience. The VW crankcase uses the rear mounting bosses originally designed for the car’s transmission as engine mount points. When the case is cut in half, these critical mounting provisions are severed, requiring builders to fabricate new mount points and add them back to the modified case. Similarly, the oil drain hole at the bottom of the case is bisected when the case is cut, creating sealing challenges that must be addressed with additional fabrication work. The rear bearing support and case rigidity are also compromised by cutting, necessitating careful reinforcement to prevent case distortion under load.

More significantly, full-case engines run substantially cooler than their cut-case counterparts and exhibit far less crankcase pressure. Cut-case conversions often require oil coolers to manage temperatures, and many builders found it necessary to add reed valves to handle excessive crankcase breathing. The full-case design eliminates both problems. With the unused rear cylinder openings properly sealed and the case volume maintained, the engine achieves adequate internal breathing and oil temperatures remain within acceptable limits using only the inherent cooling capacity of the VW’s air-cooled architecture. When Leonard Milholland discovered these characteristics after building both cut-case and full-case engines, he adopted the slogan “Don’t Cut That Case” for his advertising and calling cards, a message that resonated with builders who had struggled with the complexities of cut-case conversions.

For builders who do choose to pursue the cut-case approach, the process is demanding and requires considerable machine shop capability. The case must be carefully measured and marked to ensure the cut line falls in the correct location, typically just behind the rear main bearing. The cut itself is usually accomplished with a band saw or milling machine, though some builders have used reciprocating saws or abrasive cutoff wheels. After cutting, the interior surfaces must be carefully cleaned of all metal chips and debris, then inspected for cracks or damage caused by cutting stresses. A flat plate, typically fabricated from quarter-inch aluminum, must be precisely machined to match the case profile and bolt pattern, then drilled and tapped to provide mounting holes. This plate seals the open rear of the case where the rear cylinders would normally reside.

The crankshaft presents another challenge in cut-case conversions. The stock four-cylinder crankshaft must be cut to remove the rear two rod journals and then carefully rebalanced to compensate for the missing mass. Some builders weld a plug onto the cut end of the crankshaft to restore the original length and provide a mounting surface for the flywheel or starter ring gear. The welding must be done with appropriate preheating and controlled cooling to prevent cracking or distortion of the hardened steel crankshaft. After welding, the crankshaft requires dynamic balancing on specialized equipment to ensure smooth operation at aircraft engine speeds. Similarly, the camshaft must be cut to length and the rear lobes removed, though this is somewhat less critical since the camshaft does not carry the same loads as the crankshaft.

The cylinder heads must also be modified for half-VW service. The stock VW Type 1 heads are cast as a single piece serving two cylinders on each side of the engine. For a two-cylinder conversion using the front cylinders only, the rear portion of each head must be cut away and sealed. Builders typically make a cut just behind the rear combustion chamber, then machine a flat surface and install a sealing plate or weld the opening closed. Some builders opt instead for aftermarket heads designed specifically for two-cylinder conversions, which eliminates the need for this modification but adds to the overall cost. Other builders use heads from larger-displacement VW engines that have been machined to accept 92mm or 94mm cylinders, increasing the engine’s power potential.

Understanding how VW engines came to dominate the lightweight homebuilt aircraft market requires looking back to the engine’s automotive origins. The Volkswagen air-cooled engine was designed in the 1930s under the direction of Ferdinand Porsche as the powerplant for Adolf Hitler’s “people’s car,” intended to motorize Germany. The original engine displaced just 985cc and produced 25 horsepower from its horizontally opposed four-cylinder configuration. This flat-four layout, with two cylinders on each side of the crankcase opposed at 180 degrees, offered inherent balance and compact dimensions. More importantly, the air-cooled design eliminated the weight and complexity of a liquid cooling system, making the engine both lighter and simpler than contemporary water-cooled designs.

After World War II, Volkswagen production resumed under British occupation and the Beetle rapidly became a global phenomenon. The engine grew progressively larger through the 1950s and 1960s as Volkswagen sought more power to satisfy customer demands. The 1192cc version appeared in 1954 rated at 30 horsepower, followed by the 1285cc model in 1965 with 40 horsepower, the 1493cc variant in 1967 with 44 horsepower, and finally the 1584cc engine in 1970 producing approximately 50 horsepower. Later industrial and performance versions reached 1600cc, 1700cc, 1835cc, and even 2180cc displacements through combinations of increased bore and longer-stroke crankshafts. The VW’s modular design, with separate cylinders that could be bored oversize and various available crankshaft strokes, made it remarkably adaptable to different displacement requirements.

These characteristics caught the attention of European homebuilders almost immediately after the war. In France, designers Edouard Joly and Jean Délémontez created the Jodel D.9 Bébé in 1946, a tiny single-seat wooden monoplane that first flew in January 1948. The Bébé was designed to use whatever small engines builders could obtain, including the 25-horsepower Poinsard two-cylinder and the 26-horsepower VW flat-four. Similarly, Roger Druine developed his D.31 Turbulent ultralight around 1952, another single-seat design that became one of the most popular homebuilt aircraft in Europe. The Turbulent was specifically designed for a 30-horsepower VW engine, and the French conversion house Ardem, along with British manufacturer Rollason Aircraft, developed the Ardem 4CO2 conversion specifically for these applications. This was a modified 1192cc VW producing 31 horsepower with dual ignition, and it became the standard powerplant for hundreds of Turbulent and Bébé builders throughout Europe.

American homebuilders were slower to adopt VW power, but by the 1960s the engine’s potential was becoming clear. One early proponent was Bill Hughey, who experimented extensively with VW conversions and advocated for their use in light aircraft through articles and presentations. The VW’s direct-drive capability was particularly attractive, as the engine’s relatively low operating speeds meant the crankshaft could turn a propeller at efficient rpm without requiring a reduction gearbox. This eliminated the weight, complexity, and expense of gear reduction units that plagued many other automotive conversions. Additionally, the enormous VW parts infrastructure meant that components were widely available and inexpensive compared to certified aircraft engine parts.

The development of half-VW engines emerged from the desire for even lighter, less expensive powerplants suitable for single-seat ultralights and minimum aircraft. Experimenters in the 1970s recognized that a VW engine with half the cylinders would produce roughly half the power at half the weight, creating an ideal powerplant for the smallest homebuilts. Details of the earliest examples are scarce, but cut-case half-VW engines were certainly flying by the mid-1970s. One important example was Dave Carr’s Carr Twin conversion, introduced in January 1975 through an article in EAA Sport Aviation magazine. Carr’s design won the John Livingston Award for outstanding contribution to low-cost flying and the Stan Dzik Memorial Award for outstanding design, bringing national attention to the half-VW concept.

The cut-case approach dominated early half-VW designs because it seemed logical to remove the unused half of the engine entirely, reducing weight and creating a more compact package. Plans for cut-case conversions were sold at EAA AirVenture Oshkosh and through various suppliers. However, builders soon discovered the complications inherent in the cut-case method. The additional fabrication work required to replace mounting provisions, seal the cut case, add oil coolers, and install breather systems often negated any weight advantage. More problematically, many cut-case engines suffered from reliability issues related to inadequate cooling, excessive vibration from poorly balanced crankshafts, and oil leakage from imperfect case sealing. Some builders reported engines that “shook so badly” they never completed more than test runs, while others struggled with chronic overheating despite added oil cooling capacity.

Leonard Milholland entered this environment in the early 1990s after building one of the cut-case designs himself. His experience mirrored that of other builders. Everything he cut off the case had to be put back on through fabrication and added components. The engine ran hot even with an oil cooler. Crankcase pressure required reed valves to manage breathing. When he built a second engine using an uncut case with simple aluminum blanking plates over the unused cylinder openings, the results were dramatically better. The engine ran cool, exhibited minimal case pressure, and weighed essentially the same as the cut version once all the added components on the cut-case engine were factored in. Milholland recognized that other builders were struggling with the same issues and began selling plans for his “Better Half VW” conversion in 1993, deliberately positioning it as an alternative to cut-case designs.

The Better Half plans provide detailed instructions for converting several VW engine models based on cylinder bore size. Most conversions use the 92mm bore, which was standard on many later VW engines. With the stock 69mm stroke crankshaft, this produces the O-56 designation in Milholland’s nomenclature, with 56 cubic inches of displacement or 920cc metric. Builders can opt for longer-stroke crankshafts to increase displacement without changing bore size. The 78mm stroke creates an O-63 at 1037cc, the 82mm stroke yields an O-67 at 1083cc, and the 86mm stroke produces an O-70 at 1137cc. Some builders bore the cylinders to 94mm, creating an O-66 variant with 78mm stroke and 1083cc displacement. The variety of available bore and stroke combinations allows builders to tailor engine specifications to their airframe requirements, with displacements ranging from roughly 900cc to 1150cc and power outputs from 28 to 37 horsepower depending on configuration and tuning.

A significant evolution in Milholland’s design came in 2002 when he modified the plans to specify dual Mikuni carburetors instead of a single carburetor. The Mikuni is a Japanese-manufactured slide-valve carburetor originally designed for motorcycles, and it proved ideally suited to aircraft use due to its simple construction, reliable operation at any attitude, and precise fuel metering. Using two carbs, one feeding each cylinder, provides better fuel distribution than a single carburetor and allows individual cylinder tuning. Milholland reported that “switching to dual carburetors was like a different engine” with noticeably improved power delivery and smoother operation. The Better Half now produces a reliable 30 horsepower at 3400 rpm with a 54-inch diameter propeller, consuming approximately two gallons per hour. This power level has proven entirely adequate for ultralight aircraft, with Legal Eagle examples demonstrating cruise speeds around 50 mph, top speeds near 60 mph, and climb rates of 300 feet per minute at gross weight.

Reliability and maintenance requirements have proven excellent for properly built Better Half engines. Milholland himself has flown one engine for over 25 years with good compression and strong performance throughout. A dyno test of this engine confirmed it still produced the claimed 30 horsepower despite decades of service. Maintenance is straightforward, with oil changes every 25 hours or six months and periodic valve adjustment checks. Milholland sets valve clearances at 0.010 inch for both intake and exhaust, a mild setting that minimizes valve train wear. Ignition timing is set at top dead center for easy starting, and the distributor or magneto points require occasional inspection and adjustment. The simplicity of the two-cylinder configuration means there are fewer parts to maintain compared to four-cylinder engines, and the robust VW architecture handles aircraft duty cycles well. Several Better Half engines have accumulated nearly 1000 hours in service, demonstrating durability comparable to other VW conversions.

The current state of VW conversions for homebuilt aircraft reflects decades of development and refinement. The Better Half VW, along with competing designs from Hummel and others, has established the half-VW as a legitimate and practical powerplant option for the smallest homebuilt aircraft. Several thousand sets of Better Half plans have been sold, and examples are flying throughout the world on ultralights, motor gliders, and light sport aircraft. The full-case approach pioneered by Milholland has substantially supplanted cut-case designs in new construction, though some builders still pursue cut-case conversions for their slightly reduced weight and compact dimensions. Modern understanding of VW aircraft conversion requirements, combined with improved parts availability through the aftermarket, has made these engines more reliable and longer-lasting than early examples. For builders seeking four-stroke power in the 30 to 40 horsepower range at minimum cost and weight, the half-VW remains an attractive option.

The magnesium crankcases used in most VW engines offer a significant weight advantage over aluminum alternatives, typically saving about 17 pounds. It can be argued that aluminum cases provide greater durability and are better suited to high-performance applications. The AS41 magnesium alloy cases most commonly used contain approximately four percent aluminum and one percent silicon in a magnesium base, providing adequate strength for stock and mildly modified engines while maintaining low weight. For aircraft use where every pound matters, the magnesium case is almost universal unless the builder is creating an extremely high-output engine with displacement well over 2000cc. Cases manufactured from the mid-1960s onward feature dual pressure relief valves in the oil system, an important improvement over single-relief cases that provides more consistent oil pressure to all bearing surfaces and better protection for the oil cooler against excessive pressure spikes. The dual-relief system uses spring-loaded plungers at both ends of the main oil gallery, allowing pressure regulation throughout the entire lubrication circuit rather than only at the pump end.

The Better Half VW and other half-VW conversions represent the culmination of nearly 70 years of VW aircraft engine development, distilling lessons learned from thousands of conversions and millions of flight hours into proven designs that serve the lightest end of the homebuilt spectrum. Leonard Milholland’s insight that the full-case approach offered advantages over cut-case designs has proven correct, and his Better Half has become one of the most successful ultralight engines ever developed. For builders constructing aircraft under FAR Part 103 regulations or simply seeking the lightest, least expensive four-stroke powerplant available, the half-VW remains unmatched in its combination of low cost, light weight, and adequate power for single-seat sport flying. The continuing availability of VW parts and conversion components ensures that these engines will remain a viable option for experimental aircraft builders for years to come.