AJS Model 31

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The Plumstead-based Matchless/AJS factory announced it's first parallel twin model, the G9, and it's sister bike, the Model 20 AJS, in 1948.

It followed the established design of other manufacturers’ products with the exception of a third central main bearing to support the crankshaft. This engine design was modified, fine tuned and increased in capacity as production went on and the burgeoning USA market demanded more power.

The first enlargement was to 600cc by increasing the cylinder bore from 69mm to 72mm, but this still produced an engine that was smaller than the more popular BSA and Triumph products. To make the increase to 650cc the stroke of the engine was increased in 1958 to create the AJS Model 31 and its sister bike, the Matchless G12.

While this enlargement created the necessary rise in cubic capacity, it did show up some of the shortcomings of the design; these early 1958/9 models were prone to crank failure and a new design was needed. From 1960 the crankshaft was modified by a change from the original cast iron to a spheroidal graphite material. At the same time pistons became flat topped and the cylinder heads were re-designed to suit with angled cooling fins added.

As the original green log book shows, this AJS Model 31 was first registered in Three Bridges, West Sussex, in August 1961 and is thus of the improved post-1960 design. The vendor acquired the bike in 2009 from a gentleman who had owned it for the previous 10 years.

Looking very smart cosmetically, the machine is also said to be in good running order and has long been in regular light use, with various old MOTs going back to 1992. Taxed until May and MOTd until June 2015 with no advisories recorded it is ready for immediate use.