Today’s lead image is a 1939 Alfa Romeo 6C 2500, a car that I did some work on a few years back. It has a connection to the car I am currently working on, a 1957 Alfa Romeo 1900C Super Sprint Zagato, a car that bridged the pre-war to post-war engineering of the Italian automaker. My prior experience with the marque leaned more towards the pre-war models, such as the above 6C, the 6C 1750, 8C 2300 and 8C 2900.

Each of those models used a type of valve tappet that was integrated into the valve stem, with click-type adjustment to adjust the valve lash. The system that is also used on the Alfa 1900 was originally created and patented by engineer Marc Birkigt of Hispano-Suiza, circa 1915.

The Hispano-Suiza Design

Swiss-born Birkigt was a phenomenally creative man who had many automotive and industrial patents, spanning from braking mechanisms to automatic weapons. The aero engines that he designed in World War One were largely responsible for the Allies’ air superiority over the Germans after the middle of 1916.

His water-cooled 8A V8 engine was groundbreaking, with single overhead cams driven by bevel gears on tower shafts, and aluminum cylinder blocks using steel liners. The engine generated 150 horsepower with a weight of only 150 kilograms, one of the first to achieve that power to weight ratio. Mounted in the SPAD VII biplane, they gave Allied pilots the ability to match and surpass the German’s Mercedes-powered Albatros that had been dominating the skies over Europe.

A staggering number of these V8 engines were manufactured outside of the Barcelona works of Hispano-Suiza, with licenses going to manufacturers in France, England, and the USA. In smaller numbers, they were built by the Czechs, Italians, Japanese, Russians, and Swiss. Across all variants, almost 50,000 engines were turned out during and after the war.

After the war, the work that went into developing this engine was integrated into the new Hispano-Suiza H6 automobile engine. A vertical inline six-cylinder with the same features as the aero V8, including the unique valve operating mechanism.

The Hispano-Suiza design used a large diameter valve stem that was internally threaded, with a square notch cut at the top of the stem. This notch engaged with the valve spring retainer, which was free to float up and down the slot, but not to rotate. The top surface of the retainer had a ring of radial teeth to engage with the next part.

The threaded hollow stem carried the tappet, the flat round surface that the cam lobe swept across to open and close the valve. The male threaded post under the tappet allowed it to run up and down the stem as it turned to change the gap between the tappet and the lobe, known as valve lash. The bottom surface of the tappet had a mating ring of teeth that meshed with the top surface of the spring retainer.

The upward force from the valve spring kept the radial teeth meshed so the tappet could not turn, restrained by the notch at the top of the valve stem. To adjust the valve lash, the mechanic used a special tool that turned the tappet while a pin wrench held the spring retainer, and the parts would ratchet against one another, the teeth clicking as the tappet was turned. The number of teeth was engineered with the thread pitch to allow for .001″ change of lash with each click.

A truly brilliant design that worked wonderfully within the engine’s intended RPM range. The big aero V8 wasn’t intended to spin faster than 2,500 RPM and the H6 auto engine went a little faster, to about 3,000.

The size of the valves’ stems and the built-in adjusting mechanism meant that quite a lot of weight was jumping up and down with every revolution of the crankshaft. Metallurgy for valve springs in the period had not yet advanced far enough to control so much mass at higher speeds.

Another downside to the design was that the sideways force of the cam lobe against the tappet as it opens the valve is working against the valve guide, increasing friction and tending to make a round guide wear into an oval as the stem is pushed to the side.

The J12 Advances the Concept

When Birkigt designed the V-12 engine that was to replace the H6, he kept the ratcheting adjusting mechanism, but abandoned the overhead cam. Instead of the lobe sweeping across the tappet face, a rocker arm operated the valve from a centrally located cam with pushrods. This is often said to have been an effort to make a quieter engine for the big luxury car, but it also neatly eliminated the issue with side loading of the valve stem.

The J12 valve stems also added a fine spline to locate the spring retainer instead of the simple notch.

Alfa Romeo Enters the Picture

The connection to Alfa Romeo came from a Hungarian-Italian engineer who would become one of the most renowned designers in the automotive world.

Vittorio Jano worked for FIAT in Turin from 1911 to 1923. Everyone involved with engineering automobiles after the Great War had studied the “Hisso” V8, and with 20 factories making them in several countries, there were a lot of the engines lying around to inspect.

Jano had climbed the ladder at FIAT and worked on some of the most advanced engines to come out of Italy. When he was recruited to lead Alfa Romeo’s engineering department (at the suggestion of Enzo Ferrari, who was managing the Alfa racing effort), he came with all of that knowledge and he put it into the 1924 P2 Grand Prix car that was his masterpiece. A supercharged two liter straight eight with twin cams, it also had threaded valve stems, an evolution of Birkigt’s 8A and H6 design. The P2 claimed the inaugural Automobile World Championship in 1925.

It was developed further into the P3, also called the Tipo B, which was competitive for almost a decade. Tazio Nuvolari’s 1935 win in Germany at the Nürburgring in atrocious wet conditions is widely regarded as one of the greatest drives of all time.

At Alfa, Jano addressed the valve’s weight issue by making the stems a much smaller diameter and using a male thread, with the shorter tappet containing the female thread. This weight reduction, plus advancements in spring design, allowed engine speed to easily double to at least 6,000 RPM.

However, the smaller diameter stem did make the valve susceptible to failure, the threading operation introducing a number of stress risers into the part. The slotting operation, which cut two notches into the thread to locate the spring retainer, also added potential fracture points. So imagine everything going swimmingly, the engine sweetly revving to the limit, until BANG! A valve stem breaks, with a holed piston in the best scenario and a ruined monobloc in the worst case.

Despite these issues, it was good enough for Alfa to stick with it for all of their pre-war models, the 6C 1750, the 8C 2300 and the 8C 2900, all legendary performers.

The fine chaps over @leydon_restorations posted some photos of the Alfa Romeo version a while back that illustrate the adjusting tool in use.

Maserati Comes to a Similar Conclusion

Maserati was following a different path in that era. They largely used bucket tappets to open the valves, like the Millers that were developed in the ’20s along the lines of the 1913 Peugeot. Alfieri Maserati died in 1932 and his brothers sold the business to Adolfo Orsi in 1937. When Maserati’s ownership changed, the engineering mindset also changed, and finger followers came into use, like those Bugatti used in the Type 35s.

In 1952, Gioachino Colombo came to Maserati from Alfa Romeo. Colombo was responsible for a number of world-beating engines in his career, with the Alfa 158 and Ferrari V-12 among them. But he had worked at Alfa alongside a more senior Jano on the P2 project thirty years earlier, when he was just getting started.

That period must have had something to do with his decision in 1953 to use the ratcheting valve mechanism on the road car version of the A6 engine. The A6 GCS, the competition model, had gear driven cams and used finger followers to open the valves. But the version that went into the A6G 2000, meant to be driven to the country club or down the coast to the beach – that engine got chain drive and the second-to-last version of Hispano-Suiza’s valve mechanism from the World War One aero engine.

The Final Click

The last engine to use the concept was the Alfa Romeo 1900, which carried on with the click-type valve adjusters into 1958.

Jano had left Alfa for Lancia in 1937 and the post-war Alfa 1900 engine is credited to the engineer Giuseppe Busso, who had previously worked with Colombo at Ferrari on the V-12 project. So the 1900 engine, which was launched in 1950, predated Colombo’s clicker-valved Maserati A6 by three years!

As mentioned in the intro, the 1900 engine was a bridge between the pre-war and post-war Alfa Romeo engines. Busso kept the concept of the 8C’s threaded valve stems, but reverted to Hispano’s layout, with larger diameter stems being internally threaded, and the tappet having the male thread. The two-liter four-cylinder was intended to top out at 5,500 RPM, and the valve springs were able to handle the larger valve mass, the larger stems giving some added reliability.

Busso was also responsible for the next generation Alfa sports car engine, the 1,300 cc Tipo 750 as used in the Giulietta. That engine used bucket tappets, like the vast majority of future DOHC engines from all manufacturers. The exception being the finger follower, which has made something of a comeback in recent decades.

The Legacy of a Genius

A Swiss design that originally existed to help shoot wooden biplanes out of the sky was turned into a feature of an exotic series of Spanish road cars, and then reinvented by the Italians to win Grand Prix races and a world championship. Marc Birkigt, Vittorio Jano, Gioachino Colombo, and Giuseppe Busso each contributed to this 43 year span of engineering excellence, a true golden era worth remembering and sharing with a new generation of enthusiasts.

Have you been unlucky enough to have broken one of these valves? Ever run into this valve design on any other engines? (Excluding the Stutz version, which was quite a different concept.) Let us know in the comments below…