Right got the arb off the car -- got all the decades of hammerite off it-- found it had some form of sleeving ??
Removed it and yep its 13mm without the sleeving ??

Got some bushes and its fine

So why was it sleeved - there were some very fine spot welds ?? --- answers on a postcard cause I rallied for many years and buit and looked after many cars and never, never seen an ARB sleeved
The sleeving was well done - professionally I would say but not its entire length
The sleeve was about 2mm thick