Steel Barrels for BSA C15 & B40 Trials Engines

When venturing into the intricate world of classic motorcycle engine building, particularly for trials competition, the choice of components can make all the difference. A common query among enthusiasts revolves around the practicality and performance of using a steel barrel on BSA C15 and B40 engines. The good news is, yes, a steel barrel can indeed be effectively utilised and even bored out to accommodate larger pistons, enhancing capacity and performance. While alloy barrels are often considered for their weight-saving benefits, extensive research and practical builds have shown that a steel barrel, when bored out to accept, for example, a Triumph piston to achieve capacities like 268-270cc, and with some strategic drilling in the finning, shows barely any difference in weight on the scales compared to an alloy equivalent. This makes the iron barrel a robust and often safer bet for significant bore-outs.

The Enduring Debate: Steel vs. Alloy Barrels

For many years, the discussion around cylinder barrels for trials engines has often leaned towards alloy for its perceived advantages. However, for the BSA C15 and B40, the steel barrel presents a compelling case, especially when considering modifications for increased capacity. As noted by experienced builders, an iron barrel can be safely bored out to dimensions such as 69.25 mm. This capability allows for greater flexibility in piston selection and engine enlargement without compromising structural integrity. The initial thought process might gravitate towards alloy for its modern appeal and reduced mass, but the practical reality reveals that a well-prepared steel barrel can perform just as admirably, often with an added layer of durability for the rigours of trials. The critical factor is the ability to bore it out to the desired specification, which the steel barrel readily offers.

Unleashing Power: The Long Stroke Conversion

One of the most significant modifications for boosting the performance of a BSA C15 or B40 for trials is the long stroke conversion. This involves a fundamental change to the engine's internals to increase the piston's travel, thereby enhancing torque and low-end grunt – crucial for navigating challenging trials sections. The key to this conversion often lies in adopting a BSA B44 flywheel assembly. This particular setup delivers an impressive additional 20 mm of stroke, pushing the standard 70 mm stroke of the C15/B40 to a more potent 90 mm, akin to the B44/B50 engines. This alteration dramatically changes the engine's characteristics, providing a much-desired "plonk" and tractability at low RPMs.

Implementing this long-stroke requires careful consideration of other components. Primarily, a "slipper type" piston is typically recommended. This design, with its shorter skirt, is often hoped to allow the conversion without needing to alter the C15 barrel length. However, a slight easing of the piston skirt may still be necessary to prevent it from fouling the flywheels at bottom dead centre. For the crankcases, using C15 G-type or B25 units is essential to ensure the correct barrel stud spacing. While the concept seems straightforward, real-world application often introduces unexpected complexities, which, for many builders, is part of the challenge and enjoyment of custom engine work.

Piston Precision: The Heart of the Long Stroke

Choosing the right piston is paramount for a successful long-stroke conversion. Early builds experimented with various options. For instance, a Gilera piston was used in one notable build, but it's no longer readily available. Another successful approach involved a piston from an early Honda Fireblade, though these are typically 70 mm wide, sometimes narrower preferences apply, such as 69 mm. A highly effective solution, championed in various builds, is the 69.025 mm piston from a Yamaha 250YP Majesty scooter. These pistons, often sourced from specialists like Jim Nicholson at Grampian Motors, are a fantastic fit, with Grampian Motors even having their own manufacturing facility in Taiwan (Mitaka) for such parts.

The critical factor with piston selection for a long-stroke engine is the stack height. To achieve the 20 mm clearance difference required at both the top and bottom of the stroke, the piston from the centre of the gudgeon (wrist) pin needs to be 20 mm less in measurement top and bottom than, for example, a standard B25 piston that runs a 70 mm stroke. If necessary, the top dead centre height can be adjusted by fitting a small spacer under the barrel. However, this spacer must be minimal, as excessive height would necessitate lengthening push-rods, push-rod tunnels, and barrel studs, potentially altering frame head steady fixings. This precise measurement and adjustment highlight the detailed engineering involved in these high-performance trials engine builds.

Building the Ultimate Trials Engine: A Philosophy

The overarching goal when building a BSA quarter-litre (or slightly larger) engine for trials is to create a power unit that excels in low-speed control and torque delivery, rather than outright horsepower. This means focusing on characteristics like a smooth, tractable power delivery, often referred to as a good 'plonk'. The engine should be capable of pulling away from a standing start in a higher gear and climbing steep banks with ease. This philosophy guides choices from crankshafts to camshafts, ensuring every component contributes to the desired trials performance. The evolution of these builds, spanning over a decade, demonstrates a continuous quest for perfection, adapting to part availability and incorporating lessons learned from each iteration.

Ignition Systems: Sparking Performance

For trials engines, a reliable and effective ignition system is crucial. Several options exist, each with its proponents. PVL systems are widely popular due to their simplicity and performance. Electrex-World systems, often likened to PVL and originating from "Horsepower" units in the Netherlands, offer similar reliability and effectiveness. However, despite their modern efficiency, some builders still express a preference for the Boyer Bransden Trial ignition unit. While slightly heavier due to a larger rotor, which may aid in achieving that desirable 'plonk' sensation, it often provides a more free-running feel and is fuelled by the standard Lucas alternator. This avoids the complexity of modern electronic units with multiple mapping settings, keeping the classic feel. Furthermore, some enthusiasts still opt for total discharge coil ignition, particularly when paired with new, tiny lithium batteries that hold charge for extended periods, a setup frequently seen in classic scrambles bikes.

Gearing & Carburation: Fine-Tuning the Drive

The drivetrain and fuel system are critical for trials performance. For gearing, while wide-ratio gears were once advantageous for road work in past trials eras, today's pocket-sized courses often make standard or close-ratio gears equally effective. Common sprocket setups include an 18-tooth engine sprocket, a 13-tooth gearbox sprocket, and rear wheel sprockets ranging from 56 to 65 teeth, depending on rider preference and terrain.

Carburation is another area ripe for optimisation. Amal or Wassell concentric type carbs, typically between 22-26 mm, are standard choices. The newer Amalminum carbs, though more expensive, offer better resistance to ethanol found in modern pump fuels. However, a more definitive solution to ethanol-related issues is the use of Aspen Fuel. Although costly, it entirely eliminates carburation problems and other issues associated with pump fuel. A more economical and proven alternative, especially for smaller engines, is the Japanese-made Mikuni VM22 carb, commonly found on pit bikes and highly regarded by Triumph Tiger Cub owners for its performance and reliability.

Compression, Flywheels, and Valve Timing

Determining the optimal compression ratio is often a matter of personal preference and experimentation. The 'works BSA boys' and Triumph teams historically favoured high-compression pistons, claiming they smoothed out the motor. However, this was often a compromise given the extensive road work in trials of that era, so modern trials may benefit from different approaches. Similarly, flywheel mass significantly impacts engine characteristics. Contrary to some beliefs, B25 flywheels are actually heavier and possess greater inertia than the standard BSA C15 items. This makes them a preferred choice for some tuners looking to enhance the engine's 'plonk' and low-speed tractability.

Valve and cam timing also play a crucial role. For trials, long dwell periods or excessively high lift are generally not advantageous after the initial opening and shutting. Therefore, the standard road cam can often be perfectly suitable. If using a B25 as a base, with its larger cam followers, opting for the softest available is recommended. An early C15 Dizzy cam, with radiused followers and minor modifications, can also be effective. The point at which a valve opens, whether defined by zero tappet clearance or a specific lift, is less critical than how effectively the gases flow. Gas-flowed heads and valves will naturally achieve earlier and more efficient gas escape, aligning with the performance needs of a trials motorcycle.

Exhaust & Lubrication: Essential Details

For BSA C15 and B40 trials engines, the exhaust system design is geared towards quick gas expulsion with minimal restriction. Typically, this involves sharp, short pipes leading into a resonator expansion box with light silencing. While a long megaphone-type system with a slight reverse cone might offer ultimate torque for four-stroke competition engines, for trials, the priority is to get the gases out efficiently and quietly. Elaborate expansion boxes or reversed-flow silencers are less critical than simple packing to deaden sound waves. Ensuring optimal lubrication is much simpler: keep the clean oil clean, and change it frequently. For B25 type engines, fitting a canister-type oil filter is highly recommended, as all engines benefit from filtration, regardless of how often they run.

The Evolution of Hybrid Engine Builds

The world of classic trials engine building is constantly evolving, with builders continually experimenting with hybrid configurations to achieve superior performance. One such example involves basing an engine on a 1967 BSA C25 (often destined for the USA) combined with a 1970 B25 crank for its improved connecting rod and an updated roller bearing on the drive side, replacing the earlier C25's dual ball bearings. Gearing might come from a B40 GB engine, while a B25 cylinder barrel, shorn to C15 shape, and a standard B25 piston are used. The cylinder head often features a small inlet port BSA C15 design, with steel polished inlet guides and bronze exhaust guides from a B25. Such builds require meticulous sourcing of parts, sometimes from unexpected places, and even custom fabrication for unique components like spring retaining washers.

Another compelling build involved B25 crankcases and a B25 crankshaft assembly, paired with an ex-works Dizzy camshaft (modified for fitment and equipped with an alloy timing wheel). This was combined with various barrel and head combinations, new road cluster gears, a Pete Kirby belt drive clutch conversion, a Mikuni VM22 carb, and a short silencer header unit. For ignition, the Boyer Bransden unit was favoured. These hybrid approaches demonstrate the ingenuity and dedication required to continually push the boundaries of classic trials engine performance, often navigating challenges like parts availability and unexpected complications in the process.

Frequently Asked Questions About Trials Engine Building

Q: What's the best ignition system for a BSA trials engine?

A: PVL and Electrex-World systems are reliable and effective. However, some prefer the Boyer Bransden Trial ignition unit for its 'plonk' and free-running feel, despite being slightly heavier. Total discharge coil ignition with modern lithium batteries is also a viable option for classic scrambles bikes.

Q: Are wide-ratio gears necessary for modern trials?

A: For today's smaller, pocket-sized courses, standard or close-ratio gears are often just as effective. Wide-ratio gears primarily offered an advantage for road work in past trials eras.

Q: What are the recommended carburetors and fuel for trials?

A: Amal or Wassell concentric carbs (22-26 mm) are common. The new Amalminum carb combats ethanol better, but using Aspen Fuel is the most effective solution for ethanol problems, albeit expensive. The Mikuni VM22 carb is a proven, cheaper alternative.

Q: Should I use high compression for my trials engine?

A: The 'works BSA boys' used high compression to smooth out motors, but it's a matter of personal preference and what suits your riding style and the specific trials course. Experimentation is key.

Q: Are B25 flywheels better than C15 for trials?

A: Yes, B25 flywheels are actually heavier and have more inertia than C15 flywheels, making them a preferred choice for some tuners looking to enhance low-end torque and 'plonk'.

Q: What's the ideal exhaust system for a trials bike?

A: Short pipes and a resonator expansion box with minimal silencing are generally preferred for BSA C15/B40 trials engines. The goal is quick gas expulsion with little fuss and restriction, rather than a long megaphone system for ultimate torque.

Q: How important is oil filtration for these engines?

A: Extremely important. The primary goal is to keep the oil clean and change it often. For B25 type engines, fitting a canister-type filter is highly recommended, as all engines benefit from some form of filtration.

In conclusion, building a BSA C15 or B40 engine for trials is a journey of meticulous planning, part sourcing, and expert execution. The use of a steel barrel is not only feasible but can be highly effective, especially when bored out for increased capacity and paired with a long-stroke conversion using B44 flywheels. Attention to detail in piston selection, ignition, gearing, and lubrication will culminate in a robust, tractable engine perfectly suited for the demands of classic trials. The continuous exploration of hybrid builds and the sharing of knowledge among enthusiasts ensure that these iconic machines continue to evolve and compete at the highest level.

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