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Split-single engine
Split-single engine
Split-single engine
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Split-single engine used by the DBS Valveless car from 1908 to 1915. The first picture shows the moment of firing.

The split-single is a type of two-stroke internal combustion engine where two cylinders share a single combustion chamber.

The first production split-single engine was built in 1918, and the design was used on several motorcycles and cars until the mid-1950s, with Puch continuing with the design until 1970. During this time, the design was occasionally used for engines with four or more cylinders.[clarify][citation needed]

Operating princple

[edit]
Operating principle, with exhaust port left and intake right

The split-single uses a two-stroke cycle (i.e. where every downward stroke produces power) with the following phases:

  1. Pistons travel upwards, compressing the fuel-air mixture in both cylinders. A spark plug ignites the mixture (in the right side cylinder in the animation) when the pistons are near the top of the cylinders.
  2. Pressure from the ignited air-fuel mixture pushes both pistons downwards. Near the bottom of the travel, an exhaust port becomes exposed (at the rear of the left side cylinder in the animation), causing the exhaust gases to exit both cylinders.[1] At the same time, the intake port is exposed on the other cylinder, causing a fresh air-fuel mixture (which has been compressed in the crankcase by the downward movement of the pistons) to be drawn into the cylinder for the next cycle.

Characteristics

[edit]

The advantage of the split-single engine compared to a conventional two-stroke engine is that the split-single can give better exhaust scavenging while minimizing the loss of fresh fuel/air charge through the exhaust port. As a consequence, a split-single engine can deliver better economy, and may run better at small throttle openings. A disadvantage of the split-single is that, for only a marginal improvement over a single-cylinder engine, a split-single engine is larger, heavier and more expensive.[clarify] Since a manufacturer could produce a conventional two-cylinder engine at similar cost to a split-single engine, a two-cylinder engine is usually a more space- and cost-effective design.[2] Most split-single engines used a single combustion chamber (spanning both cylinders), with larger engines (such as with four-cylinders) continuing to share one combustion chamber per cylinder pair.[3]

Initial designs of split-single engines from 1905 to 1939 used a single Y-shaped or V-shaped connecting rod. Externally, these engines appeared very similar to a conventional single-cylinder two-stroke engine, with one exhaust, one carburetor in the usual place behind the cylinders, and one spark plug. The Puch GS 350 of 1938 and the Triumph BD 250 of 1939 used a conventional single rod per piston.

After World War II, more sophisticated internal mechanisms improved mechanical reliability and led to the carburetor being placed in front of the barrel, tucked under and to the side of the exhaust. An example of this arrangement was used on the 1953-1969 Puch 250 SGS.

Early engines using a "side-by-side" layout (with the carburetor in the "normal" place behind the cylinder) had similar lubrication and pollution problems to conventional two-stroke engines of the era,[citation needed] however the revised designs after World War II addressed these problems.[citation needed]

Pre-World War II examples

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Lucas

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The first split-single engine was the Lucas,[4] built in the UK in 1905.[citation needed] It used 2 separate crankshafts connected by gears to drive 2 separate pistons, so that the engine had perfect primary balance.

Garelli

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From 1911 to 1914, Italian engineer Adalberto Garelli patented a split single engine which used a single connecting rod and long wrist pin which passed through both pistons. Garelli Motorcycles was formed after World War I and produced a 350 cc (21 cu in) split-single motorcycle engine for road use and racing from 1918 to 1926.[5]

Trojan

[edit]

The Trojan two-stroke, as used from 1913 in the Trojan car in the UK, was independently invented but would now be described as a split-single. Photos of a 1927 "twin" model at the London Science Museum show the internals.[6] The "fore-and-aft" layout of the cylinders means that the V-shaped connecting rod has to flex slightly with each revolution. Unlike the German/Austrian motorcycle engines, this engine was water-cooled. The tax horsepower regulations in the United Kingdom resulted in a lower road tax for the Trojan compared with a conventional engine of similar displacement.[7]

Trojan also made another split-single engine later with the cylinders arranged in a 'V' formation. The unusual 'V6' design had two split-single sets of cylinders (4 cylinders total) on one bank of the V and two scavenge blower cylinders on the other bank of the V.[8][failed verification]

Puch

[edit]

After World War I ended, Austrian industry struggled to recover. Italian engineer Giovanni Marcellino arrived at the main factory of Puch with the instruction to wind up operations. Instead of liquidating the factory, he settled in the town and designed and began production of a new split-single engine which debuted in the 1923 Puch LM racing motorcycle.[9] Influenced by industrial opposed-piston engines, the Puch engine had asymmetric port timing and pistons arranged one behind the other (instead of the side-to-side arrangement used by Garelli). To avoid flexing of the connecting rod, the small-end bearing of the cooler intake piston was arranged to slide slightly fore-and-aft in the piston.[10] In 1931 Puch won the German Grand Prix with a supercharged split-single.[9] By 1935, a four-cylinder version of the Puch split-cylinder design produced 10 kW (14 hp) and was used in motorcycles.[11] Since 1938 Puch produced the GS 350 model, each piston has its own connection rod.

Triumph-Werke Nürnberg

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1939-1943 the model BD 250 with asymmetric port timing was produced. Each piston had its own connection rod.

Motor racing

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From 1931 until 1939, DKW racing motorcycles powered by split-single engines dominated the Lightweight and Junior racing classes.[12]

At the 1931 and 1932 Indianapolis 500, Leon Duray's competed with cars powered by the 16-cylinder Duray U16 engine using a split-cylinder design.[13]

In 1935, the Monaco-Trossi Grand Prix car was built with a 16-cylinder radial engine using a split-cylinder design.[14]

Post-World War II Examples

[edit]

Puch

[edit]

Puch's split-single production and racing were restarted in 1949,[citation needed] and a split-single engine was used in the Puch 125T model.[15]

The 1953-1969 Puch 250 SGS (sold in the United States by Sears as the "Allstate 250" or "Twingle") used with an improved system of one connecting rod hinged on the back of the other. These engines typically use the forward piston to control both intake and exhaust ports, with the interesting result that the carburettor is at the front of the engine, under and to the side of the exhaust. The rear piston controls the transfer port from the crankcase to the cylinder.[16] Increasingly, these models were fitted with an oil mixing pump, fed from a reservoir incorporated in the petrol tank. Some also have a twin-spark plug ignition system firing an almost figure-eight shaped combustion chamber. The improvements tamed, if not virtually eliminated, the previous problem of two-stroke plug fouling.[citation needed] A total of 38,584 Puch 250 SGS motorcycles were produced between 1953 and 1970.[17]

Puch ceased production of split-single engines around 1970.[18]

EMC Motorcycles

[edit]

EMC Motorcycles in the United Kingdom manufactured a 350 cc (21 cu in) split-single engine that was used in the 1947-1952 EMC 350.[19] After 1948 the engine also was fitted with an oil pump controlled by the throttle, which dispensed two-stroke oil into the fuel at a variable rate depending on throttle opening, instead of having to pre-mix oil in the fuel.[20]

Iso Autoveicoli

[edit]

The Italian manufacturer began producing a 236 cc (14 cu in) split-single engine in 1952 for their Iso Moto motorcycle. This engine was then used in the Iso Isetta bubble car from 1953 to 1956.[21]

Triumph-Werke Nürnberg

[edit]

Triumph-Werke Nürnberg (TWN) in Germany continued production of a split-single engines for their motorcycles in 1946, but not with asymmetric port timing as the pre-war BD 250. The 125 cc (8 cu in) TWN BDG 125 and 250 cc (15 cu in) TWN BDG 250 models, produced from 1946 to 1957,[22] used a Y-shaped connecting rod, so the pistons are "side-by-side", making the engine little different visually from a regular two-stroke. Other split-single models from TWN were the 1954-1957 TWN Cornet (200 cc with 12 volt electrics and optionally electric starter), the 1953-1957 TWN Boss (350 cc) with 2 carburetors and the 1954-1957 Contessa scooter (200 cc).[1] The bulbous shape of the exhaust of the Cornet and Boss is an anti-noise two-stroke TWN feature. All TWN motorcycle production ceased in 1957.

In 1953 TWN showed the model Duplex at a fair. This 250 cm3 engine was a 2-cylinder split-single with 4 pistons. This motorcycle was not produced.

Potential supercharger development

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Arnold Zoller's supercharged "Doppelkolben" engine

Phase shift between pistons also allows use of a supercharger, which may be effective for two-stroke diesel engines.[citation needed]

See also

[edit]
Wikimedia Commons has media related to Split-single engines.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Split-single engine

View on Grokipedia
from Grokipedia
A split-single engine, also known as a twingle, is a specialized type of two-stroke that employs two pistons operating within a single cylinder, divided by a fixed central deflector wall to separate the exhaust and transfer while sharing one and one transfer passage. This design enables the pistons—often connected via a Y-shaped or to separate synchronized crankshafts—to move with a slight phase offset, typically around 10-20 degrees, allowing the exhaust port to open before the transfer port for enhanced scavenging of exhaust gases and improved fuel-air mixture retention. The result is a compact configuration that combines the power and characteristics of a twin-cylinder with the simplicity and lighter weight of a single-cylinder unit, making it particularly suitable for motorcycles and small vehicles. The split-single engine's origins trace back to early 20th-century efforts to address scavenging inefficiencies in two-stroke designs, with the first known prototype developed by the Lucas company in the in 1905, featuring two separate crankshafts geared together. Italian engineer Alberto Garelli refined the concept, introducing a production version in 1913 for military motorcycles that achieved notable racing successes, including victories in the 1919 Milan-Naples race and the 1922 . By the 1930s, the design gained prominence through manufacturers such as , which produced the 250cc S4 engine that won the 1931 , and , whose split-single variants powered championship-winning Grand Prix machines in 1938 and 1939. Other notable implementations included the EMC 350cc model from 1946, developed by Dr. Joseph Ehrlich in Britain, delivering 18 horsepower at 4,000 rpm for postwar motorcycles. Despite its advantages in balance, reliability, and power delivery—such as reduced and lower production costs compared to four-stroke engines—the split-single fell out of favor by the mid-1950s due to the advent of Schnürle loop scavenging, which allowed simpler single-piston two-strokes to achieve comparable or superior performance without the added mechanical complexity of dual pistons and crankshafts. continued production into the 1970s for models like the 250 SGS scooter, but the design largely became obsolete in mainstream applications. Recent interest has revived the concept in research prototypes focusing on weight reduction for niche applications.

Design and Operation

Principle of Operation

The split-single engine is a specialized two-stroke characterized by two s operating within a single barrel divided into two parallel bores by a fixed central deflector wall to separate the exhaust and transfer ports while sharing one . This allows the pistons to compress and expand the fuel-air mixture in unison while managing and exhaust through distinct ports controlled by each piston. The engine's operational cycle follows the standard two-stroke sequence but leverages the dual-piston arrangement for enhanced gas flow management. Both pistons ascend simultaneously (or with a minor phase offset), drawing a fresh fuel-air into the shared through an and compressing the already present in the shared . At the top of the compression stroke, a centrally located ignites the , generating high-pressure gases that force both pistons downward during the power stroke. As the pistons descend, in phased designs the "exhaust" piston first uncovers the exhaust ports to begin expelling burnt gases, followed shortly by the "transfer" piston uncovering the transfer ports near the bottom of the cylinder wall, admitting the pressurized fresh charge from the into the to scavenge residual exhaust gases; in unphased designs, the ports open simultaneously. The cycle completes as the pistons rise again, sealing the ports and initiating the next compression phase, with timing typically set such that exhaust opens 10–20 degrees before transfer to optimize scavenging without excessive charge loss. Piston synchronization is achieved through a common crankshaft, with the two pistons linked by a single forked (Y- or V-shaped) connecting rod that branches to each wrist pin, ensuring parallel motion and balanced forces on the crankshaft. Early variants employed separate crankshafts geared together for precise phasing, allowing the exhaust-side piston to lead by a small angle (e.g., 10–15 degrees) to refine port overlap and gas dynamics. The pistons follow straight, parallel paths within their bores, with contoured crowns—often conical or deflector-shaped—directing the incoming charge toward the exhaust side for efficient sweeping of combustion residues, a feature unique to this configuration that minimizes the need for auxiliary baffles common in single-piston two-strokes.

Key Characteristics

The split-single engine, a variant of the two-stroke design, features two pistons operating within a single cylinder barrel, sharing a common combustion chamber and connected via synchronized crankshafts or a shared connecting rod assembly. This dual-piston configuration enables distinct control over intake and exhaust ports, with one piston typically managing transfer ports and the other exhaust ports, often offset by 10-20 degrees in timing for optimized gas flow. Central ignition placement in the combustion chamber, positioned between the two bores, promotes more uniform flame propagation compared to offset plugs in conventional single-cylinder two-strokes. A primary benefit lies in enhanced exhaust scavenging and fuel economy, achieved through the dual-piston action that facilitates directed scavenging patterns using the central deflector wall to guide the fresh charge across the , sweeping exhaust toward the exhaust while minimizing short-circuiting. This reduces short-circuiting of unburned mixture into the exhaust, allowing for leaner air-fuel ratios and better efficiency at partial openings than standard two-strokes, where scavenging relies on a single piston's symmetric timing. The and power delivery further emulate that of a twin-cylinder , providing smoother output and stronger low-end response due to the phased operations, while retaining the mechanical simplicity of a single-cylinder layout with fewer valves or cams. Common design elements include deflector pistons or fixed cylinder wall deflectors to guide charge flow and prevent direct exhaust contamination, alongside provisions for port timing adjustments via skirt length or crankshaft phasing. Supercharging potential is notable, as exemplified in Arnold Zoller's conceptual designs from the 1930s, which integrated timed offsets or auxiliary charging pistons to compress the , boosting power without external blowers in early iterations. However, these engines exhibit larger physical dimensions and higher weight relative to equivalent-displacement single- two-strokes, stemming from the elongated barrel accommodating dual bores and longer pistons (often with skirts 2.5 times the crown for stability). Manufacturing costs are elevated due to the need for precision gearing between crankshafts, dual sets, and complex machining. Operational challenges include uneven piston loading from asymmetrical gas pressures and thermal gradients, which can cause cylinder distortion and accelerated wear, necessitating robust materials like pistons. In shared-rod setups, such as Y-shaped connecting rods linking both pistons to a single , lubrication demands intensify, often requiring oil mixtures up to 15% to mitigate risks in the closely packed assembly, though this can compromise emissions and longevity.

Historical Applications

Pre-World War II Examples

The split-single engine first appeared in early 20th-century as an innovative variant of the two-stroke design, aimed at improving power delivery and scavenging efficiency in compact applications like motorcycles and light vehicles. Developed amid broader advancements in two-stroke technology during the , these engines gained traction in niche markets, particularly in , , , and the , where they powered both civilian and racing machines before 1945. One of the earliest implementations was the Lucas engine, designed in the United Kingdom in 1905 as the first known split-single configuration. This prototype featured two separate pistons and crankshafts geared together within a single cylinder barrel, marking an initial step in practical two-stroke experimentation. In Italy, engineer Adalberto Garelli patented a 350 cc split-single engine between 1911 and 1914, with production commencing after World War I for Garelli motorcycles. These engines powered civilian touring models and utility bikes from 1918 to 1926, delivering reliable performance for everyday use while also proving competitive in racing events, including victories in the Milan-Naples race from 1919 to 1926 and the 1922 French Grand Prix. The United Kingdom's Trojan vehicles adopted a similar design in 1913, employing a two-stroke engine with four cylinders arranged in pairs that shared common combustion chambers—a configuration retrospectively classified as split-single. This setup powered economical light cars and motorcycles, emphasizing simplicity and low-cost operation for urban transport. Austrian manufacturer Steyr-Daimler-Puch introduced its first split-single two-stroke motorcycle engine in 1923, mounted in a sturdy frame with a two-speed transmission. Initial 122 cc models targeted entry-level riders in European markets, followed by 250 cc and larger 500 cc variants that expanded the design's application in mid-sized bikes by the late 1920s and early 1930s. In , Triumph-Werke Nürnberg (TWN) developed split-single prototypes during the late , including the BD 250 model from 1939 to 1943. This 250 cc engine, with a bore of 45 mm and stroke of 78 mm, produced 12 horsepower at 3,800 rpm via control and a four-speed gearbox, undergoing pre-war testing that informed later production efforts. Split-single engines also featured prominently in pre-World War II motor racing, particularly in . German manufacturer utilized supercharged split-single designs in their racing motorcycles from 1931 to 1939, achieving dominance in lightweight and junior Grand Prix classes with engines developed in collaboration with Ing. Zoller, securing multiple championships through superior power and handling. The Duray U16 racer, entered by American driver Leon Duray for the 1931 and subsequent events through 1932, employed a supercharged 16-cylinder arranged in split-single pairs, though reliability issues limited its competitive success. In , the 1935 Monaco-Trossi Grand Prix car incorporated an air-cooled 16-cylinder radial in split-single configuration, boosted by dual Zoller superchargers to produce approximately 250 horsepower, though development challenges prevented widespread racing impact.

Post-World War II Examples

Following , the split-single engine experienced a revival in , particularly in , the , , and , where manufacturers adapted the for mass-produced motorcycles and microcars suited to the era's demand for economical urban and touring . This resurgence capitalized on the engine's inherent efficiency and simplicity, making it ideal for affordable vehicles in a recovering economy. In , AG restarted split-single production and racing in 1949, introducing the 125T as an entry-level scooter with a 125 cc engine delivering approximately 5.2 hp, targeted at everyday commuters seeking reliable, low-cost mobility. further expanded its lineup with the 250 SGS in 1953, featuring a 250 cc split-single engine that produced around 14 hp and emphasized long-distance comfort with its smooth power delivery and fuel economy. Over its production run through 1969, the 250 SGS achieved significant commercial success, with 38,584 units manufactured and sold worldwide, underscoring the design's appeal for post-war touring enthusiasts. The United Kingdom saw limited but notable adoption through EMC Motorcycles, which produced 350 cc split-single models from 1947 to 1952 for the domestic market. These tourers, outputting 16-18 hp depending on configuration, prioritized reliability and simplicity in their air-cooled two-stroke layout, appealing to riders valuing durable performance in everyday use amid Britain's post-war austerity. Italy's Iso Autoveicoli S.p.A. integrated the split-single into both two-wheeled and four-wheeled vehicles during the early . The Isomoto 125 scooter, launched around 1951 and produced through 1962, employed a 125 cc split-single engine derived from influences, offering of about 2.2 liters per 100 km and a top speed near 80 km/h for urban errands. Iso extended this powerplant to the from 1953 to 1955, enlarging it to 236 cc and yielding 9.5 hp; the compact engine's placement enabled the vehicle's signature front-hinged door design, maximizing interior space in a 2.3-meter-long body ideal for city navigation. Approximately 1,000 Iso Isettas were built, highlighting the split-single's role in innovative, space-efficient personal transport. In , Triumph-Werke Nürnberg (TWN) bridged pre-war developments with post-1946 production of split-single motorcycles, including 250 cc models like the Duplex and Contessa variants through 1957. The Duplex featured a 250 cc split-single for versatile road use, while the Contessa scooter variant used a 200 cc version, maintaining the design's economical two-stroke characteristics for reliable daily commuting. TWN's efforts reflected a broader European trend toward split-singles in affordable vehicles, with collective output across manufacturers surpassing 50,000 units by , including applications in microcars like the for enhancing urban mobility in densely populated areas.

Legacy and Developments

Reasons for Decline

The split-single engine's decline began in the automotive sector by the mid-1950s, as manufacturers shifted toward more reliable four-stroke designs that offered superior and easier adaptation to emerging performance demands, rendering the complex two-stroke variant obsolete for car applications. In motorcycles, the design persisted longer in niche European markets but faced mounting pressures from economic recovery, where cost-conscious consumers favored simpler and cheaper alternatives amid the broader industry boom. By the 1960s, stricter emissions regulations, such as those foreshadowing the U.S. Clean Air Act of 1970, accelerated the phase-out of two-stroke engines like the split-single, which struggled with poor exhaust control compared to four-strokes that provided better combustion efficiency and lower emissions. Four-stroke engines also demonstrated greater under high-mileage conditions, resisting the wear from oil-mixed systems inherent to two-strokes, including lubrication inconsistencies that led to premature piston and bearing failures in split-singles. Manufacturing complexities further eroded viability, as the split-single's dual-piston setup required heavier crankcases and asymmetrical components, driving up production costs without proportional power gains over emerging simpler two-stroke designs like those using Dr. Schnurle's reverse-flow scavenging. Japanese manufacturers, entering European and global markets with affordable, reliable four-stroke motorcycles in the , intensified competition, making the labor-intensive split-single uneconomical for . Specific operational drawbacks compounded these issues: uneven piston forces from differing intake and exhaust roles generated notable vibrations, exacerbated by non-simultaneous top dead center positions that hindered smooth operation. Lubrication challenges arose in high-mileage use due to the design's reliance on fuel-oil premix, which unevenly distributed across the shared , promoting carbon buildup and overheating. Scalability proved limited for larger displacements, as the expansive reduced compression ratios and efficiency, confining the engine to small-capacity applications below 250 cc. The market shift culminated in the discontinuation of key models, with Steyr-Daimler-Puch ceasing production of its final split-single, the 250 SGS, in 1970 after a run of 38,584 units since 1953, marking the end of widespread adoption by the early 1970s.

Potential Modern Applications

The split-single engine's design, exemplified by Arnold Zoller's 1930s "Doppelkolben" supercharged prototype—a 12-cylinder, 1,500 cc two-stroke unit fitted with twin Zoller vane-type superchargers—demonstrates inherent compatibility with due to the phase-shifted pistons that facilitate timed air delivery without valve interference. This configuration, which achieved boosted performance in racing applications like the via a similar 16-cylinder split-single setup, offers adaptability to contemporary systems such as turbocharging or modern superchargers for enhanced power density in compact engines. In niche markets, the split-single persists through hobbyist restorations of vintage models, including Steyr-Puch 175 SV and Iso motorcycles, where enthusiasts and professional services rebuild these engines for historical preservation and recreational riding, leveraging their unique scavenging efficiency for reliable operation. Such efforts highlight potential integration into electric-hybrid two-stroke systems, where the design's dual-piston layout could support low-emission range extenders in vehicles by combining with battery assistance to minimize pollutants. A 2020 prototype redesign, for instance, lightened the from 38 kg to 4.4 kg using aluminum while targeting 30-50 kW output for electrified mobility applications like portable generators or UAV . As of November 2025, no major production revivals of the split-single engine exist, though academic and industry interest continues in emissions reduction for two-strokes. The split-single's advantages align with modern constraints in weight-sensitive applications, delivering compact power—up to twice the performance of equivalent four-strokes at lower production costs—ideal for motorcycles and micro-mobility devices where space and limitations demand high without added complexity.

References

  1. https://magazine.cycleworld.com/article/1970/3/1/that-split-piston-thing
  2. https://www.cambridge.org/core/journals/proceedings-of-the-design-society-design-conference/article/design-modification-of-an-innovative-splitsingle-twostroke-engine/234246977C0E726D96B8862CCE43D0F2
  3. https://www.oldbikemag.com.au/splitting-single-1948-emc-single/
  4. https://cdn.imagearchive.com/homemodelenginemachinist/data/attach/73/73097-split-single.pdf
  5. https://www.odd-bike.com/2014/02/dkw-supercharged-two-strokes-force-fed.html
  6. https://historicvehicles.com.au/historic-motorcycle-brands/dkw/
  7. https://motorivista.com/garelli-racing-motorcycle/
  8. https://www.gracesguide.co.uk/Trojan
  9. https://www.classicmotorcycle.co.uk/a-to-z-classic-reference-puch-pv/
  10. https://cybermotorcycle.com/marques/twn/twn-bd250.htm
  11. https://thevintagent.com/2019/06/20/the-original-triple-dkws-singing-saw-racer/
  12. https://macsmotorcitygarage.com/a-most-remarkable-engine-the-duray-u16/
  13. https://oldmachinepress.com/2012/09/01/1935-monaco-trossi/
  14. https://magazine.cycleworld.com/article/1970/11/1/the-history-of-puch
  15. https://www.motorcyclespecs.co.za/model/Classic/allstate_sr_250.htm
  16. https://www.classicvelocity.com/2018/12/02/2018-11-29-puch-250-sgs/
  17. https://nationalmcmuseum.org/2019/07/05/1948-emc-350-mark-i/
  18. https://www.oldbikemag.com.au/1952-isomoto-125-split-personality/
  19. https://www.theautopian.com/the-full-history-of-the-most-famous-microcar-the-bmw-isetta/
  20. https://rmsothebys.com/auctions/bw13/lots/r206-1955-iso-isetta/
  21. https://www.motorcycleclassics.com/classic-german-motorcycles/1957-triumph-twn-bdg125l/
  22. https://cars.bonhams.com/auction/14264/lot/657/1956-twn-200cc-contessa-frame-no-554-545-engine-no-557-302/
  23. https://magazine.cycleworld.com/article/1967/11/1/the-history-of-japanese-motorcycles
  24. https://www.nytimes.com/2009/12/20/automobiles/20STROKE.html
  25. https://northwestautohouston.com/two-stroke-four-stroke-engines/
  26. https://www.bike-urious.com/no-reserve-1967-sears-allstate-puch-250-twingle/
  27. https://www.secretprojects.co.uk/threads/opposed-piston-engine.26408/page-3
  28. https://www.motorwestmotorcycles.com/restoration/Puch.php
  29. https://doi.org/10.1017/dsd.2020.102
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