Hairpin valve springs
The Hairpin Valve-Spring Return System (HVRS) was patented by Sunbeam for its motorcycle engines, in 1925. Engine designers employed different methods for valve control, including coils, hairpins and, later, desmodromic (no springs at all).
1925 Sunbeam M10 Sprint with hairpin valve springs
Most engines of the early post-WWI period were side-valve types, with coil springs acting as the return method for reseating the valves. The springs were compressed by camshaft and tappet action, forcing the valve to open and then the springs extended as the tappets dropped down.
The side-valve engine limitations were exposed by comparison with more exclusive and expensive overhead-valve (OHV) engines – particularly overhead-camshaft (OHC) engines.
Overhead-valve engines had much better combustion chamber shape – wedge or hemispherical – and that guaranteed more efficient use of the fuel. This efficiency allowed higher engine revs that produced more power.
The problem was that the valve springs of the time lacked modern metallurgical manufacturing techniques and the coil springs that handled side-valve engine speeds were often found wanting in overhead configurations. Double and even triple coil springs were arranged inside one another to share the loads, but valve spring breakages were common.
Ricardo side valve engine
Valve spring breakage in a lower-stressed side-valve engine was far less likely and, if it happened, the valve would sit on its seat, thanks to gravity and any broken spring pieces would fall off the outside of the engine, or, in the case of enclosed-spring designs, hopefully find their way into the sump and cause no problem.
However, a broken spring in an overhead-valve layout meant that the valve retainer might let go and the valve could fall into the active cylinder, causing catastrophic engine failure.
Sunbeam’s solution was to use a pair of externally-mounted coil springs, with extended upper and lower ends, on either side of each valve stem. The lower ends of each spring were clamped to the head and the upper ends of each spring, to a retainer on the valve stem.
Single-overhead camshaft Norton International Bronze Head with hairpin valve springs
Unlike the axially-arranged coil spring on a conventional OHV layout that put torsional forces into the spring, the Sunbeam ‘hairpin’ design imposed only bending forces on the springs, so they were more reliable.
Another positive for hairpin springs was the fact that the springs didn’t shroud the valve stems, like concentric coil springs did. On motorcycle engines with exposed valve gear that meant more cooling airflow around the forward-mounted exhaust valve stems.
Felt sheet for oil splash absorption
On engines with exposed valve gear, it was common for cloth or felt sheets to be inserted around the springs, to absorb oil splash.
The downsides of hairpin springs were the expense of making these hand-formed springs, compared with the machine manufacture of simple coils, plus the additional bulk hairpin springs created.
However, by the 1930s, most British racing motor-cycle engines had changed over from coil-spring to hairpin springs.
Norton SOHC Manx hairpin valve springs – Racing Norton UK
After WWII, the hairpin spring valve continued in use on motorcycles – notably Manx Nortons until 1962 – and also was adopted by some performance car engine makers.
Colombo’s original Ferrari V12 had hairpin valve springs
When Gioachino Colombo designed V12 engines for Enzo Ferrari immediately after WWII he employed hairpin spring valves and Aurelio Lampredi continued the layout in the Ferrari Type 500, four-cylinder, two-litre engine of 1952 – 1953.
Hairpin valve springs clearly visible in this BRM V16 drawing
BRM’s famous 1947 V16, 1.5-litre GrandPrix engine also had hairpin valve springs.
However, by the early 1950s, the quality of concentric coil valve springs had greatly improved, making it hard to justify the expense and bulk of hairpin springs. Two Grand Prix race car engines designed originally in 1953 with hairpin springs – the 900V8 2.5-litre Lancia D50 and the Coventry Climax FPE – were both re-designed with concentric coil valve springs.
Ducati 125 GP engine with hairpin valve springs – Phil Aynsley
By 1956, Fabio Taglioni, Ducati engineer, who employed hairpin valve springs on racing engines, was well down the track of avoiding both types of valve springs, releasing a desmodromic valve system for the Ducati 125 Grand Prix bike.
Ducati desmodromic valve actuation schematic
As far as we can ascertain, the 1958 Constructors’ Championship winning Vanwall in-line, four-cylinder, 2.5-litre engine, with its Norton carburation and combustion chamber heritage, was the last racing car engine to use hairpin valve springs.
Vanwall’s Norton-based engine with hairpin valve springs