Ariel Twins 1948 to 1960

The singles (NH & VH) have a long history but the parallel twins (KH & FH) are really “me too” bikes, created just after the war in response to a growing market that had been created by other marques. Ariel was a latecomer to the marketplace for twins and, although the first Ariel twins had a distinctive appearance, there were few surprises in either their internals or in their cycle parts. There were two models announced initially in 1947 for the 1948 season, the “cooking” KG Deluxe and the slightly more sporting KH Red Hunter. They were both 500cc OHV parallel twins with a 360deg crank. The mechanical differences between them were slight and the frames and cycle parts were borrowed from the singles. Although not ground breaking, they attracted favourable comments from road-testers when they were launched.

The KH engine had a plain timing side main bearing and a roller drive side main bearing, in common with the contemporary Triumph and BSA twins. Where it differed from them was in the construction of the crank shaft, which was a one-piece casting with a bolt-on flywheel between the crank pins and counter-balancing webs outside of the big-end journals. Ariel believed that this would provide a stiffer crank with lower production costs. The connecting rods were alloy with a bronze bush at the small end and split big-ends with shell bearings.

The crank case was made in two halves, split vertically, in common with its competitors of the time. There were two camshafts, one at the front of the mouth of the crank case and the other at the rear. The cams were at the ends of the camshafts, which minimised flex and meant that the push-rods were at the outer ends of the iron cylinder block – an unusual feature that was adopted to provide free flow of cooling air between the cylinders. The cylinder head was a single iron casting with integral rocker boxes with alloy cover plates. The two inlet valves shared a single inlet port (and a single Amal 276 carburettor) that entered at the rear of the head. The push-rods were operated via inverted mushroom tappets that ran in sleeves set into the cylinder casting.

The cam-shafts were driven by duplex half-time sprockets via a duplex chain from a sprocket keyed directly onto the crank-shaft. A chain tensioner was provided. The timing gear was covered by a triangular alloy casting at the non-drive end of the crankshaft. Unlike the singles, there was no oil pump under the timing side cover. The oil pump was of the spur-gear type, driven off the rear camshaft and situated at the bottom of the crank case. There were two separate pump chambers, one for oil delivery, initially to the plain timing-side main bearing, and the other to scavenge the dry sump and return the oil to the tank. Oil passed through drillings in the crank-shaft to the big-ends and the drive-side main bearing. An external pipe delivered oil to the four rocker boxes. The oil ran down the push-rod tubes, past the followers and onto the cams. Lubrication of the timing gear was via oil mist from the crank case.

Each cam sprocket had a gear attached to it to drive the electrical ancillaries. The rear one drove the magneto and the front one drove the 6V dynamo. Unlike the singles, a centrifugal auto-advance mechanism was fitted to the magneto.

The transmission was pretty much identical to the singles of that model year, with a face-cam shock absorber and a chain, running in an oil bath, that drove the dry clutch. The crank-case design was the same as the singles, including the domed inspection cover for the clutch. The gearbox was a Burman CP four-speed box.

The Ariel telescopic forks provided the front suspension and the rear was either rigid or, optionally, used the Ansty link plunger spring units. The cycle parts were identical to the singles but the frame had a slight modification at the bottom of the front down-tube to give clearance for the Lucas dynamo.

There were a just few minor modifications for the 1949 model year, the most obvious of which were the addition of finned exhaust clamps and a polished front brake plate. A more practical improvements were the fitting of a slightly larger and more powerful dynamo and a change to the BA gearbox, as a part of a change to the gearing.

Changes for the 1950 season mirrored the changes to the singles, including some new options, such as a “quickly detachable” rear wheel and a Burgess air filter. The twin also acquired the Lucas headlamp shell with a light switch and ammeter. For 1951, like the singles, the tank-top instrument panel disappeared and the speedometer moved to the top fork yoke.

The KG Deluxe model was dropped for 1952 so the KH Red Hunter was the sole representative of the twin range for a while. During the year, the Burman GB gearbox was substituted for the BA box. In 1953, a competition model was introduced with an all-alloy engine with iron cylinder liners. It was called the KHA. It shared a frame and cycle parts with the KH. A dual seat was offered as an option on the KHA. However, the KHA didn't last long, since it was dropped in the following year.

1954 saw a number of other changes as well. A new 650cc twin was introduced using a superficially disguised version of the popular and reliable BSA A10 engine. The new model was designated FH and named Huntmaster and shared a frame and cycle parts with the KH. The new twin rail cradle frames of both twins were equipped with the same swing arm rear suspension and dual seat as the singles. The general design of the two engines was very similar, except that the push-rods were all operated from a single can-shaft at the rear of the cylinder block and the oil pump was situated at the bottom of the timing chest and driven by a worm gear off of the crank-shaft.

The oil tank and tool box were redesigned to suit the new frame and the Burgess air filter became standard. The fuel tank acquired chrome flutes on the upper sides and a wide chrome strip adorned each lower side.

For 1955 both twins received the Amal Monoblock carburettor and some other detail changes were made. In 1956, the fully enclosed rear chain case (FERC) was offered as an option. Also in this year, the full-width hubs that the singles used were fitted to KH and FH twins, which had a similar effect on their braking efficiency. The twins were also blessed with the ugly and irritating head lamp cowl and the small instrument panel on the top fork yoke. In this year, the KH was named Fieldmaster. Like the singles, the tank was modified so that the chrome flutes became detachable so that the whole tank was painted instead of chromed.

In 1957, yet another new tank was introduced without the flutes and with the new bulbous shape and single-bolt fixing as used on the singles. Other changes were also exactly as on the singles. The KH was not listed for 1958 and so the FH became the only Ariel twin.

Other than some new colour scheme options, the FH continued through 1958 and 1959 with no further changes. It survived into 1960 but only in its US export guise as the Cyclone. This was styled to appeal to the American market but, broadly speaking, didn't.