The 1951 AJS Porcupine.
During the winter of 1950, the AJS factory at Plumstead had been busy developing their 500cc Porcupine machine for the 1951 season.
The aim of the modifications was to produce “increased power, greater engine flexibility and an overall reduction in weight”.
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| The Porcupine without the spikes on the cylinder head. |
Going to the engine, the most notable change to the Porcupine for 1951 is the losing of its spikes. For ease of development reasons, separate cylinder heads with horizontal transverse fins replaced the former cooling spines. This enabled detachable cam boxes and spring chambers to be incorporated, whilst the 14mm spark plugs are centrally located. The only other change to the engine is the enclosing of the main oil pump into the bottom of the crankcase to which is bolted a long boat-shaped light alloy sump holding over a gallon of lubricant.
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| New central floatchamber supports dual mixing chambers fitted with horizontal jets |
An important modification was the use of a central float chamber. This enabled the carburettors to be mounted lower, giving a better sweep to the intake port. It was hoped that this modification would overcome the misfiring and uncertain running at low engine speeds.
The wheelbase was shortened by 1” for 1951 and weight was also shaved from the frame. Extra weight savings were made due to the absence if an oil tank and associated pipe work. The front forks were also shortened by 1” and a new alloy top yoke and two-piece sheet steel bottom yoke were used. Smaller wheels were fitted - 19 by 3.00 ins at the front, and 19 by 3.25 ins at the rear. The 10” front brake which was first used at the 1950 Belgian GP was refined slightly, but used for 1951. The rear brake remained unaltered.
New F.I.M. regulations meant than a new triangulated support for the longer rear mudguard was necessary. The seat was shorter than the 1950 design, and a zipped pocket was incorporated into it in order to hold two spare spark plugs.  |
| The 1951 AJS Porcupine |
