brake backplate

Hi all,

As you may know I'm doing a disc brake conversion on my Elf. As it is a daily drive I don't really have the time to strip the front brakes to get a measurement so, does anyone know the dimension of the big hole in the brake backplate? It's the one that slots on the swivel housing.

It would save me a lot of time and messing if someone could give me the size. I have to make a caliper mounting bracket that mounts exactly the same way as the backplate and need to get the hole turned in the steel before I do anything else.

Thanks in anticipation.

I've found out, the hole is 3" diameter.