So am planning on getting a rear beam from a MK6 ST to replace my beam from my zetec s as the ST rear beam has discs whereas the ZS is drums. One question I have is will the discs fit over the wheels? I have the standard ZS 16inch wheels. Also might replace the discs for an focus ST or mondeo ST so need to know if these will fit. I'm planning on doing this probably next year as first I need the injectors sorted out. 

If your wheels fit over the front discs then they will certainly fit over the rear. As to the larger discs (and callipers?); that will depend very much on the type of wheels you have. They will for under some 16" but not under others unless you use spacers. There's some online calculators that will work that our for you if your wheels are common. 

Thanks ThaiFiesta but are the ST rear discs bigger than the ZS front discs? The cars I've had had rear drums so don't really know about rear discs compared to fronts 

258mm Zetec front

253mm ST150 rear

I'm not sure how sophisticated the ABS is and whether it will encounter problems with brake bias. If so an easy fix would be to fit ST150 front discs and callipers also. 

Thanks again. Looks like it won't be any problem then however if I do think of changing them to focus ST or mondeo ST I might have to see if it fits over the wheels without spacers. For some reason I wouldn't put spacers on as I don't find it safe.