25/02/2020
When we talk about the heart of many vintage vehicles and older small engines, the term 'side valve engine' often comes up. While more modern designs have largely superseded them, these venerable powerplants hold a significant place in automotive history and continue to power countless classic cars, motorcycles, and utility machines. Understanding their unique design, operational principles, and the specific considerations for their maintenance and tuning is key for any enthusiast or mechanic working with these enduring pieces of engineering.
A side valve engine, often referred to as an 'L-head' or 'flathead' engine, is a type of internal combustion engine distinguished by the placement of its intake and exhaust valves. Unlike the more common overhead valve (OHV) or overhead camshaft (OHC) designs, where valves are located in the cylinder head, a side valve engine houses both valves within the cylinder block, adjacent to the cylinder bore. This configuration gives the cylinder head a relatively flat bottom, hence the 'flathead' moniker. The combustion chamber is then formed by the space between the piston crown at Top Dead Centre (TDC) and the underside of the cylinder head, along with the area above the valves.
The Anatomy of a Small Engine: Where the Side Valve Fits
To truly appreciate the side valve design, it's helpful to understand the core components of a typical small engine, as these are the environments where side valve technology often thrived. These low-powered internal combustion engines, found in everything from generators and lawnmowers to concrete mixers and snowblowers, are built with specific parts working in harmony.
- The Cylinder: Often called 'the bore', this is the precisely machined inner wall where the piston travels. Its incredibly smooth surface is crucial for the efficient operation of the piston and its rings. In side valve engines, the cylinder block is where the valves are also seated.
- The Cylinder Block: This fundamental, often aluminium, component forms the main housing for most of the engine's internal parts. It's cast in a mould to achieve its precise form. Many small engines use a single cylinder, though multi-cylinder configurations (in-line, opposed, V) exist. The exterior of the block often features aluminium alloy fins to dissipate heat, though liquid-cooled engines may lack these.
- The Cylinder Head: In a side valve engine, the cylinder head is relatively simple. It bolts to the top of the cylinder block with a head gasket and houses the spark plug. Crucially, the side valve design means that *both valves are housed on one side* of the engine, within the block itself, not the head. This contrasts with overhead valve designs where valves are in the head, or opposed valve designs where intake and exhaust valves sit on opposite sides of the cylinder.
- The Pistons: Crafted from cast steel or aluminium, the piston reciprocates within the cylinder. Its top part is called the head. The piston connects to the connecting rod via a pin, secured by retaining clips. Pistons typically have one to three rings: top rings for compression, and a bottom oil ring (found only in four-stroke engines) to manage lubrication and prevent rotation.
- The Connecting Rod: This vital link connects the piston to the crankshaft, translating the piston's linear motion into rotational energy. It attaches to the piston via a wrist pin and can be a single or two-piece unit.
- The Head Gasket: Situated between the cylinder block and the cylinder head, this gasket creates a critical seal, holding pressure within the combustion chamber. It must withstand high temperatures and, in liquid-cooled engines, prevent coolant from entering the combustion area.
- The Crankcase: This houses the crankshaft, which rotates to convert the piston's up-and-down motion into circular motion. The crankshaft features heavy counterweights for balance and typically sits at a 90-degree angle to the cylinder. Engine types vary by crankshaft orientation: horizontal (cars, some lawn tractors), vertical (lawnmowers, outboard marine engines), and multi-positional (chainsaws).
- Bearings: Available in various styles (ball, roller, needle) and sizes, bearings support engine parts, ensuring smooth operation. They are designed for corrosion and scoring resistance and may or may not require external lubrication, sometimes featuring small holes for internal lubrication.
- The Camshaft: This component is responsible for operating the intake and exhaust valves. Unlike two-stroke engines which lack one, four-stroke side valve engines utilise a camshaft with one lobe per valve. As the camshaft rotates, it lifts the valves via push rods (if applicable) or directly, driven by gears, a chain, or a belt.
- The Valves: Made of high-grade steel, the intake and exhaust valves are located within the cylinder block. They seal to control airflow into and out of the engine. The intake valve is typically larger than the exhaust valve to facilitate better engine breathing.
- Valve Spring: These springs keep the valves closed, ensuring a tight seal, maintaining pressure on the camshaft, and preventing 'valve float' at higher RPMs.
- Muffler: A metal component bolted or threaded onto the engine, designed to reduce exhaust noise.
- Starters: Whether electric or rope rewind, starters turn the engine at a fast speed to initiate the combustion process, drawing fuel into the cylinder and creating a spark. Common in chainsaws and lawnmowers (rope rewind) or cars and lawn tractors (electric).
- Carburettor: Essential for mixing fuel with air in the correct ratio for efficient combustion.
- Speed Governor: In small engines, governors (air vane or mechanical) regulate engine speed by varying the throttle position to maintain specific RPMs, preventing over-revving.
- Transmission: Used in machines like lawn tractors and snow throwers, transmissions convert the engine's output speed, creating gear reduction to increase torque output.
- Lubrication Systems: Moving engine parts require constant lubrication, often achieved through splash lubrication or pressurised systems.
Advantages and Disadvantages of Side Valve Engines
The side valve design, while simple, comes with its own set of pros and cons that explain its historical prevalence and eventual decline in mainstream automotive use.
| Advantage | Disadvantage |
|---|---|
| Simplicity: Fewer moving parts in the cylinder head, reducing manufacturing cost and complexity. No pushrods or rocker arms needed if the camshaft is directly below the valves. | Poor Breathing: The 'L' shaped path for gases (into the cylinder, then over the valves, then out) is less efficient than straight-through overhead designs, limiting airflow at higher RPMs. This restricts power output. |
| Reliability: Simpler design often means fewer points of failure and easier maintenance for basic tasks. | Lower Compression Ratios: The shape of the combustion chamber, dictated by the valve placement, typically leads to lower compression ratios, which negatively impacts thermal efficiency and power. |
| Lower Centre of Gravity: The compact cylinder head can contribute to a lower engine profile. | Heat Retention: The exhaust gases have to make a sharp turn to exit, leading to more heat being trapped in the block, which can cause overheating issues, especially in performance applications. |
| Quiet Operation: The enclosed valve train generally results in quieter mechanical operation compared to exposed overhead valve mechanisms. | Tuning Challenges: Significant performance gains are difficult to achieve due to inherent design limitations, making them less suitable for high-performance tuning compared to OHV engines. |
Tuning Side Valve Engines: A Classic Conundrum
For enthusiasts of classic cars, particularly those with vintage Ford 10hp or 8hp side valve engines, the quest for more power is a common, yet often frustrating, endeavour. As noted by experienced tuners, side valve engines cannot be treated in the same manner as overhead valve (OHV) engines when it comes to tuning, which often leads to unexpected problems like torque loss if the wrong approach is taken.
Many attempts at tuning involve modifications common to OHV engines, such as re-seating oversize inlet valves, porting, tuning the exhaust, cross-drilling/lightening/balancing the crank and flywheel, fitting a smaller clutch, upgrading oil pumps, and installing twin carburettors (e.g., twin 1.1/4 SU's). Skimming the cylinder head to increase compression is also a common modification. While these changes can yield some power increases (e.g., 47bhp at the flywheel for a Ford 10hp), the fundamental torque characteristics of the side valve design often remain a bottleneck, especially when paired with original gearboxes like the Morgan's three-speed unit.
Alternative Tuning Routes and Their Limitations
Historically, certain period conversions existed to bypass the side valve's inherent limitations. The Willment IOE (Inlet Over Exhaust) head conversion or the Elva OHIV (Overhead Inlet Valve) conversion were popular, effectively turning the engine into a hybrid design. Another common trick was fitting a Ford 8hp head onto a 10hp engine to slightly increase compression. However, for those involved in racing, these conversions often push the vehicle out of its historical class, making them unsuitable.
This highlights the unique challenge: how to extract more power from a side valve engine without fundamentally altering its class-defining design. The consensus among experts is that it requires a deep understanding of side valve mechanics and often necessitates finding a truly skilled machine shop that specialises in these older engines. Simple bolt-on modifications, while helpful, rarely solve the core issues. Gearing alternatives can sometimes offer a way to better utilise the available torque, especially with the limited ratios of vintage gearboxes.
Rebuilding a 1950 Ford Side Valve Engine: E93A and E83W
The 1950s Ford side valve engines, such as the E93A and the 10hp E83W (common to all upright 1950s side valve Fords), are robust but, like any engine of their age, eventually require a comprehensive rebuild. This process is meticulous and crucial for ensuring many more years of reliable service.
A typical engine rebuild involves several key stages:
- Stripping and Inspection: The engine is fully disassembled. Initial inspection often reveals surface rust in the bores and a tarry 'gloop' in the valve gallery. The components are then sent to an engine specialist for thorough assessment of wear.
- Bore and Crankshaft Assessment: Often, bores are found to be in good condition, requiring only honing rather than a full re-bore. Similarly, big ends and mains might be perfectly adequate, needing no attention beyond cleaning. The camshaft usually benefits from a light dressing.
- Small End Remediation: Wear in the small ends (where the connecting rod attaches to the piston pin) is a common issue and is typically cured by re-bushing.
- Cleaning and Re-facing: The engine block and other components undergo a thorough cleaning in a specialist bath. The block, head, clutch, and flywheel are all re-faced to ensure perfectly flat mating surfaces, which is critical for sealing.
- Valve Train Overhaul: A crucial step for side valve engines is the fitting of new hardened valve inserts. This is vital for durability, especially with modern fuels. New piston rings, valves, guides, and springs are installed. Adjustable tappets are often fitted to allow for precise valve clearance adjustment.
- Crankshaft Polishing: The crankshaft is polished to ensure smooth bearing surfaces.
- Fasteners and Bearings: New head studs and nuts are fitted. Critically, new cap nuts are used on both the mains and big ends due to the high stresses they endure. Mains caps are typically wired for security, while big end caps use locknuts.
- Oil System Overhaul: The oil pump is completely stripped for cleaning, ensuring unimpeded oil flow. A popular modern modification involves fitting a contemporary spin-on oil filter into a housing that visually resembles the original canister, offering superior filtration and easier, cheaper replacement without altering the engine's original aesthetic.
A completely overhauled and reconditioned side valve engine, though an investment in time and resources, promises trouble-free running and can look as pristine as the day it rolled off the assembly line, ready to power its classic vehicle for generations to come.
Frequently Asked Questions About Side Valve Engines
- What is the main difference between a side valve and an overhead valve engine?
- The main difference lies in valve placement. Side valve engines have their valves (intake and exhaust) located in the cylinder block, next to the cylinder. Overhead valve (OHV) engines have their valves located in the cylinder head, above the cylinder.
- Why are side valve engines less common today?
- Side valve engines were largely superseded by OHV designs due to their inherent limitations, primarily poor breathing efficiency at higher RPMs, which limits power output and fuel economy. OHV designs allow for better airflow and higher compression ratios.
- Are side valve engines difficult to maintain?
- Basic maintenance on a side valve engine is often simpler due to fewer complex components in the cylinder head. However, more advanced repairs or performance tuning can be challenging due to their unique design and the decreasing availability of specialist knowledge and parts.
- Can I tune a side valve engine for more power?
- While some performance gains are possible through modifications like skimming the head, porting, and carburettor upgrades, side valve engines have inherent design limitations that make significant power increases difficult compared to OHV engines. They respond differently to tuning, and careful consideration is needed to avoid issues like torque loss.
- What classic cars commonly used side valve engines?
- Many early to mid-20th-century vehicles, particularly those from Ford (e.g., Ford Model T, Model A, Ford Popular, Ford Anglia 103E, Ford Prefect), used side valve engines. They were also prevalent in various small utility engines for garden machinery and generators.
- What are hardened valve inserts, and why are they important for a side valve rebuild?
- Hardened valve inserts are durable rings fitted into the valve seats in the cylinder block. They are crucial during a rebuild, especially for classic engines, because modern unleaded fuels lack the lead compounds that historically lubricated and protected valve seats. Hardened inserts prevent excessive wear and recession of the valve seats.
The side valve engine, a testament to early automotive ingenuity, might seem rudimentary by today's standards, but its enduring presence in classic vehicles and small machinery speaks volumes about its robustness and simplicity. While tuning them presents unique challenges that demand specialised knowledge and a nuanced approach, the satisfaction of coaxing more performance from these historical powerplants is a reward in itself. For enthusiasts and mechanics alike, the side valve engine remains a fascinating chapter in the ongoing story of internal combustion.
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