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Ariel Square Four Classic Motorcycle

The Ariel Square Four was a completely new design and had a spectacular impact on the motorcycle design industry in the 1930’s and then beyond. 

It was a tough, endurance workhorse capable of nearing 100 mph that stood the test of time. Let’s take a look at what made it so great.

Ed Turner and the Ariel Square Four

As with the 1938 Triumph 5T Speed Twin which I wrote about in a previous post, you simply cannot ignore the man behind the machine and with the Ariel Square Four, that man was Edward Turner. 

By 1925 after a short time in the Navy, Edward Turner had designed his first motorcycle engine and shortly after that he had built a design that would be named the Turner Special. 

The Square Four engine was originally designed in 1928 and it started out as a 500cc, before swiftly expanding to 601cc. 

In its essence it was two 2-cylinder engines with across-frame crankshafts that were geared together. They shared a common cylinder block and removable head and used an overhead camshaft. These early Square Four engines were called Model 4F, and were produced from 1931-1936.

British marque BSA showed no interest in the design, but Jack Sangster who controlled Ariel at the time saw the potential in the four-cylinder engine and offered Turner a role at Ariel.

Edward Turner worked under Val Page who was Ariel’s Chief Designer (a man who Turner would come to despise later on) and between them the design was developed, first being shown at Olympia in 1930. 

By 1936, Turner had moved on from Ariel to Triumph and following issues with the original models the Square Four had a redesign, designated Model 4G, it was now OHV and a displacement of 995cc. In production from 1936 to 1949.

Ariel Square Four History

1931 saw the release of the 500cc engine Model 4F which was quickly stretched to 601cc in 1932, the main reason for this was to accommodate the growing sidecar market in the UK.

The Model 4F was driven by a single chain on the right hand side of the engine. Its top speed was said to be 80mph.

The Square Four engine had its cylinders spaced at a 4.5” pitch to help with cooling, the problem was the space was too cramped and a reputation for overheating plagued the early models.

A further issue in the early days was that the bikes weren’t getting hot enough in the cold winter, it meant condensation formed in the cam chain cover and eventually into the engine oil. Water in the oil meant engine damage was sure to follow. 

The Great Depression was taking over the economy and Ariel was pulling back wherever it could, which meant standardising frames across all models. So, the crankcases had to be redesigned for the Square Four to fit into the frame. 

Cutting costs and recycling parts from other models was something that plagued the whole production of the Square Four, and later throughout many of the new British bikes that would later be manufactured. Recycling parts is cheaper than investing in research and development and unfortunately that would be the downfall of many British manufacturers over the years.    

The flaws with the Model 4F prompted a redesign in 1936 and the Square Four engine was revamped to a new displacement of 995cc and called Model 4G. The Model 4G would remain in production until 1949. It was a slow revving, torquey engine built for a sidecar and for tackling long distances.

The girder forks at the front remained but the back end was updated with the Anstey link plunger rear suspension system although this was an optional extra rather than standard. After the war the Square Fours reappeared with new telescopic forks at the front end.

The Anstey link plunger rear suspension system was designed and patented by Frank Anstey to overcome the problem of chain tension.

The Ariel Square Fours weren’t high performance but they were reliable, and that is why they sold so well. 

They had a one-piece iron cylinder block, with all four cylinders arranged in a square pattern and a one-piece removable cylinder head. The airflow however, still wasn’t amazing.

Another major change came in 1953 with a new cylinder head and manifolds which would become the Mark II, it ran from 1953-59. Both cylinders on each side prior shared just one exhaust pipe, after the change each cylinder got its own pipe. The Mark II benefited by being bumped up to 40 horsepower and a top speed of 100 mph.

Ariel Square Four Performance

The press took a shine to the Ariel Square Four from it’s display next to the Silver Hawk, on the Matchless stand at Olympia in 1930.

It had a solid reputation and was built to work which was what the market was demanding at the time, therefore the Ariel Square Four was received very well, even with its overheating issues. Ariel responding to the issues, kept customers on board, and the constant improvements didn’t go unnoticed.

The Ariel Square Four wasn’t built for speed but for practicality. However, it did win two endurance type titles: 

The 1932 601cc model covered 700 miles in just 700 minutes during the Maudes Trophy endurance test, followed by an 87 mph timed lap. Quite a feat for a bike from 1932. 

‘Gaffer’ Littledale landed 1st in the London to Lands End Trial in 1930.

Buying an original Ariel Square Four

The Ariel Square Four
This 1939 Ariel Square Four has done just 24,000 miles, comes with plenty of paper work is for sale in London, asking £15,000

There are some excellent Ariel Square Four motorcycles on the market. Car and Classic in the UK has a few good options for £15,000 – £25,000 and Ariel North America has a wide selection from $10,000 to $30,000. 

There are many factors to consider when buying an Ariel Square Four including how much of the bike is original vs restored. 

When browsing through some of the listings there was one example for £20,000 but it had been through a 100% restoration; then there was another for $22,000 that was 100% original even down to the rust and mounting brackets for the sidecar it used to pull. 

So, when considering buying an Ariel Square Four, think about what you want from the bike, how authentic do you want it to be or how much of a showpiece it is going to be? 

Remember the Square Four was built for its endurance and they were used heavily for long trips with luggage, passengers and side cars, so expecting to find a completely immaculate (none-restored) one today would be extremely lucky.

Another factor to add in is the model spanned three decades so there are variants within that, make sure you research the model you want. 

Early Model 4F bikes are the most expensive and are rarer to come on the market. Should you want an 4F model that is in working order you will need to ensure it is mechanically sound.

Restoring an original Ariel Square Four

As mentioned, for a good quality Ariel Square Four you could be looking at £15,000 or $10,000. So, a bike in not quite amazing condition that needs a bit of work could come up for a little less than this or certainly be sat at the low end of the price range. 

It might seem steep for a project, but it is safe to say the return you could get back makes up for it, if financial gain is your motive. 

Ariel Four Square parts are readily available and they are generally priced fairly. Some examples are as follows eBay UK and US:

  • £50 Gaskets
  • £200 Cylinder Head
  • £50 Tank
  • £100 Crankcases
  • $160 Oil Gauge Kit
  • $400 Cylinder Head
  • $400 Carb

There seems to be a bit of discrepancy in prices between the UK and the US with the latter being more pricey for parts, but even then they aren’t extortionate. 

Is an original Ariel Square Four a Good Investment? 

Yes, the Ariel Square Four could sit in the dictionary next to the definition of classic. It was one of the early revolutionary motorcycles that paved the way for many things to come, not only for the British marques but across the world. 

Prices will continue to slowly increase for these bikes, particularly bikes in original condition, it seems that even a bit of rust and bruised exhaust won’t devalue the Ariel giant. 

The Ariel Square Four is one of the coveted British classics and I can’t see that changing anytime soon, so if you were to invest, you will likely see a decent return down the line. 

As for a restoration project too, you can’t go wrong with investing in the Ariel Square Four, with immaculate restoration projects hitting the 20K mark if you get your donor bike for the right price there is money to be made.  


You cannot argue with the fact the Ariel Square Four was a feat of engineering from Edward Turner and later Val Page. By Ariel taking Turner up on his design meant that he would continue to build classic, standard setting motorcycles for decades afterwards. 

The Ariel Square Four is a monster machine, anyone with a taste of British classics would love to own one and the chances are you could keep riding it for many more years to come with a bit of care and attention. 

I saw an Ariel Square Four at a classic bike show a few years back and the owner could not have looked more dapper in his throwback gear proudly displaying his love for his ‘commuter’, I dare to think what his prized possession was. 

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