The Monkey Motion Valve Gear On A Hornsby-Ackroyd Oil Engine

The Hornsby-Ackroyd oil engine occupies an important place in the history of practical internal-combustion machinery. Developed in Grantham, Lincolnshire, by Herbert Akroyd Stuart and manufactured commercially by Richard Hornsby & Sons, it offered farmers, workshops and small industries a dependable alternative to steam power. Its hot-bulb ignition system allowed heavy oil to be used where a boiler, coal supply and trained stoker would have been impractical.

Among the most recognisable features on some Hornsby-Ackroyd engines is the so-called “monkey motion” valve gear. The nickname refers to the lively appearance of the external linkage as it transfers movement from the camshaft to the exhaust-valve mechanism. Although the parts are straightforward, their proportions, adjustment and lubrication are central to the engine’s correct operation. Understanding this small mechanism helps explain how an early oil engine breathed, fired and governed its working cycle.

A British Answer To Small-Scale Power

Akroyd Stuart’s early engines appeared during a period when industrial users wanted power without the expense and complexity of a steam plant. A steam engine required a boiler, regular water treatment, fuel handling and compliance with increasingly important boiler safety rules. The hot-bulb oil engine avoided the boiler altogether. It still needed heat for starting, usually supplied by a paraffin or petrol blow lamp, but once running it maintained the necessary temperature through combustion and exhaust heat.

The Hornsby-Ackroyd design used a separate vaporiser or hot bulb connected to the cylinder head. Fuel entered this heated chamber, where it vaporised before meeting air and igniting under compression. This was different from the high-pressure direct injection systems that later became common in diesel engines. The arrangement tolerated relatively low-grade liquid fuel and suited agricultural work, pumping, drainage, generators and workshop machinery.

For Australian owners, the appeal is easy to understand. A stationary engine in a Queensland sugar district, a Victorian farm shed or a New South Wales sawmill could provide useful shaft power without relying on a town electricity supply. The same practical logic explains why surviving examples still appear in machinery collections, although moving a large cast-iron engine across Australia can cost more than expected because of freight distance, lifting equipment and regional access.

What The Monkey Motion Actually Does

The engine follows a four-stroke cycle: induction, compression, power and exhaust. The crankshaft turns twice for each complete cycle, while the camshaft turns once. A cam on that slower shaft provides the timed lift needed to open the exhaust valve. The inlet valve on many engines is atmospheric or automatically operated, opening when cylinder pressure falls below outside pressure during the induction stroke.

The monkey motion is the mechanical link between the cam and the exhaust valve. A cam follower or roller rises as the cam lobe passes beneath it. That movement is carried through a collection of levers, pins and a rocking member to lift the exhaust-valve stem. The linkage changes the direction and amount of movement, allowing the camshaft to remain in a convenient position while the valve sits above the cylinder.

Its exposed action gave the mechanism its memorable name. When the engine is turning slowly, the lever seems to reach, withdraw and rock in a quick, almost animated sequence. The movement is purposeful rather than decorative: valve timing depends on the exact relationship between cam profile, lever geometry, tappet clearance and crankshaft position.

Different Hornsby-Ackroyd models and production periods used variations in detail. Some engines have visibly different brackets, rollers, springs or governor connections. Owners should therefore compare the mechanism with the correct parts drawing or a known original engine rather than assume that every linkage should be adjusted to the same dimensions. A missing pin or substituted bolt can alter the effective centre distance enough to affect valve lift.

Timing, Clearance And Wear

A healthy valve gear should move freely without excessive side play. The pins and bushes in the monkey motion carry repeated loads, and years of inadequate oiling can produce oval holes, worn rollers and loose pivots. Wear increases lost motion. The cam may still rise normally, yet the exhaust valve opens late, lifts insufficiently or fails to close cleanly. Symptoms can include hard starting, weak running, smoky exhaust and an engine that refuses to take its load.

Valve timing must be checked with the crankshaft and camshaft correctly related. On a four-stroke engine, the camshaft’s half-speed relationship is essential. Before dismantling, it is sensible to photograph timing marks, note the position of every washer and measure clearances. A restorer should identify whether a clearance is intended for a cold engine, a hot engine or a particular operating condition. Guessing from a modern petrol-engine practice can create trouble.

The exhaust valve works in a severe environment. It sees hot gases, deposits from oil fuel and repeated impact from the operating gear. A valve that sticks in its guide can overload the linkage, while a weak or incorrect spring can prevent reliable closure. The rocker or lifting lever must meet the valve stem squarely; a contact point that runs at an angle will gradually peen the stem end and introduce further inaccuracies.

Lubrication is equally important. Small oil cups and holes may look insignificant, yet they serve the pins that allow the linkage to articulate. Old lubricant can harden into a waxy residue, blocking passages while giving the appearance that a joint is well greased. During a careful overhaul, each pivot should be cleaned, inspected and measured. Replacement bushes are preferable to simply fitting larger pins into worn castings, because accurate geometry matters as much as reduced looseness.

Reading The Mechanism During Restoration

Before applying paint or dismantling sound parts, clean the valve gear sufficiently to read its history. Original wear patterns can show where a roller ran, which face belonged against a shoulder and whether a previous owner fitted a temporary repair. Cast marks, stamped numbers and the shape of a bracket may help identify the engine variant. A collection of clear photographs is valuable, especially when the engine has to be stored for months while parts are made.

Restoration does not require every working surface to look new. A sound original pin with modest polishing may be better than a poorly made replacement. Conversely, a cracked lever, deeply grooved roller or badly elongated pivot deserves attention even if it appears attractive after painting. Functional preservation should guide cosmetic work. The engine needs to demonstrate its design rather than become a static object whose moving parts are merely decorative.

The same disciplined approach applies to the rest of the machine. Anyone new to stationary-engine work can learn useful methods from this beginner’s steam restoration guide, especially the emphasis on recording parts, resisting unnecessary dismantling and testing each stage. A Hornsby-Ackroyd oil engine has different combustion and valve arrangements, but the principles of patient inspection and evidence-based repair are closely related.

Australian conditions add their own considerations. Engines stored in open-sided sheds may suffer from condensation during cool nights, while coastal collections face salt-laden air. Dust from unsealed floors can mix with oil around the valve gear to form abrasive paste. In a dry inland district, a cloth cover protects against grit; near Melbourne or Sydney, ventilation is just as important as protection, because trapped moisture can attack bright steel and machined castings.

Keeping An Historic Engine Safe And Useful

Starting a hot-bulb engine is a controlled procedure, not a theatrical gesture. The fuel system, cooling water, lubrication points, flywheel key, guards and exhaust arrangements all deserve inspection before the first turn. The hot bulb must reach the correct working temperature, but a flame should never be left unattended near spilled fuel. A fire extinguisher suitable for flammable liquids should be immediately accessible.

The large flywheel stores considerable energy. Loose clothing, rags, jewellery and long hair must be kept away from the rotating assembly, and spectators should remain behind a proper barrier. Australian state and territory Work Health and Safety laws place duties on people managing workplaces and public demonstrations. A private collection may have different obligations from a museum or rally, but safe guarding, supervision and documented procedures remain essential.

At public events such as the Lake Goldsmith Steam Rally in Victoria or machinery gatherings around Toowoomba, an engine may be viewed by children and visitors unfamiliar with exposed machinery. Operators need clear exclusion zones, stable flooring and a method for stopping the engine quickly. Noise, exhaust emissions and fuel storage should be considered alongside the historical display. A well-run demonstration preserves public confidence in old engines and protects the machines from careless handling.

There is also a strong case for maintaining local repair skills. Specialist foundries, machinists and agricultural engineers are fewer than they were, and an Australian owner may need to send a small component interstate or commission a part from a workshop in Adelaide, Brisbane or regional Victoria. The survival of this knowledge depends on accurate drawings, measured samples and patient apprenticeships. Interest in American craft manufacturing reflects a similar belief that practical making skills remain valuable even in an age of disposable equipment.

Practical Recommendations For Owners And Collectors

The monkey motion deserves attention because it makes the Hornsby-Ackroyd’s operating principles visible. The camshaft, follower, levers and exhaust valve form a compact lesson in mechanical timing, while the hot bulb reveals how engineers once adapted combustion to available fuels and manufacturing limits. Its movement is a reminder that reliable power often came from carefully proportioned components rather than complicated electronics.

For Australian collectors, preserving such an engine means more than keeping a rare object polished. It means protecting evidence of agricultural, industrial and engineering history, from the British factories that built the machines to the Australian farms and workshops that put them to work. Record the mechanism, repair it sympathetically, run it safely when appropriate, and share its story while the knowledge remains recoverable.