Friday, 15 April 2011

AJS Porcupine losing its spikes.

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”.
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.

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


  1. Tell me, did they ever actually cast up a silver cylinder head, or was that just an option?

  2. Well it has always been a bit of a rumour whether they actually did it. I am from Sheffield, and I did hear from a good source that a head was actually cast in silver by a firm in Sheffield. I have no written evidence to support this.

  3. I saw one in a well known private collection and it was the works Supercharged model . I am told only 2 of thes were ever made. It was a really quite odd looking but looked a real special tool.

  4. According to PEI who worked at AMC around 1942 on the E90S as the Porcupine was then known they only considered using silver. To make it castable it needed to be alloyed with copper and so much had to be used that the heat conductivity would be greatly reduced. The cost of each casting was estimated at £350 and a lot more than the budget would stand.
    See p.296 of Phil Irving's autobiography...
    A MUST read....
    Dennis Quinlan.

    1. Thanks for the reply Dennis. I haven't read Irving's book, but I bet it would be a good one. I'll have to look out for a copy.
      Regarding the silver porcupine head, being a Sheffield lad, I actually heard it on good authority that one silver head was cast in Sheffield. However, upon realising how much it actually weighed, it was quickly thrown back into the melting pot!