Tech question please. 
I know the original slotted rear discs on K series failed and were swapped for solid discs but I see on photos of my old (?’86) K100RS that the rear disc has slots that do not look standard but are oblique/circumferential (see my wanted advert on this forum!) and I wondered if this was a specific Australian modification, an option or something else. 
All replies welcomed,
J

Im running slotted never had a failure. Just use what you prefer remember in the UK its more DAMP than DUST 

The rear disk on my 86 K100RT went out of true and if you put a dial gauge on the surface there was an area that was out by approx 40 thousands. What would happen is that you would never have anything for rear brakes. If the wheel was stopped the rear brake would work fine, but once it was rolling the ever changing thickness wouldn't let the caliper/rear master adjust.  I know the previous owner rebuilt the rear master and I ended up putting in a new rear master before I found the problem.

That disk was the original disk and it had the slots. I went with a Brembo aftermarket rear disk for the K100 and it has the slots as well. Most solid disks would be found on the 70's bikes, mostly Japanese that I'm familiar with. They went away from the solid disks and everything know has slots/holes to aid in water drainage.

cycleman wrote: Most solid disks would be found on the 70's bikes, mostly Japanese that I'm familiar with. 

Solid rear discs were OEM on Bricks, too. They can be found offered in BMW parts fiches and on model photos in advertising brochures in the Tech page here.

Aftermarket EBC discs have holes but not slots. Original discs changed to solid rear discs.

My 83 RS had the original slotted disc same as the front discs, it eventually cracked. They had been recalled for this issue and the slotted discs changed out because of this cracking issue but mine hadn't been changed so I fitted the EBC one.

Thanks for that.

Just to clarify the rear disc cracking happened suddenly, no advance notice. I had all the information on those early slotted discs and checked it regularly but in my case it just appeared suddenly. Parked bike, a little later noticed the crack and the 2 sides offset by maybe 2mm so it had not been there riding it. Rode home using front brake only. The slotted disc I took off is identical to the front discs. I just put in EBC disc instead, problem solved.

Just got my new EBC, to be installed when the snow thaws. The OEM had both: dropped below min thickness and warped in the gap between 57k miles to 63k miles.

OEM fronts are on the way out too but 2021 looks like it will be a low mileage year, so I’ll just wait for the next MOT.

Just installed the EBC disc on my '85 RS. It costs a little more but the quality is great.

Snowman wrote:Just installed the EBC disc on my '85 RS. It costs a little more but the quality is great.

Yeah, I put EBC discs on front & rear and have been delighted with the quality