The main reason you'd want to do this is that uprated dampers tend to be more widely available for the mk1b than for the mk1a. In my case I have a set of mk1b struts fitted with Konis and converted to coilovers that I wanted to fit to the sprint car to allow to me to experiment with different spring rates and ride height adjustment.The main differences between the two struts are highlighted in the images below
(standard parts pictured to prevent confusion), essentially the mk1b uses a slightly larger diameter damper, requiring a larger diameter tube which leads to the two lugs which clamp on to the rear upright being slightly further spaced apart. The mk1b rear upright is also correspondingly wider. In addition to this the mk1b uses M15 bolts to attach the strut to the upright whereas the mk1a uses M12.
It had been suggested in he past that it would be possible to fit a mk1b rear upright to a mk1a in conjunction with the later strut.
I tried this and ran into problems with the rear tie rods. I found that I needed a custom length tie rod to make this work as neither mk1a or mk1b ones could be made to fit. This would also leave the tie rods shorter than those fitted to either type of setup and I was worried about there being adverse effects on the bump steer characteristics if I went with the hybrid setup. Instead I tried a different approach...I designed a spacer that would allow the mk1b strut to bolt to the mk1a upright, as well as adjusting for the difference in width between the upright and the strut the spacer also allows the mk1a 12mm bolt to be positively located within the 15mm holes in the strut casing.
I called in a favour from the guys in the machine shop at work and had a set made up:
Fitted the rear coilovers today
(along with some RCAs and TwosRUs adjustable tie rods) and everything went together ok, will be a couple of months before the car hits the road again though as it's lacking an engine atm.
