Drain Cover Size Confusion

[joben1]

Hi,

I want to replace my round plastic drain covers with cast iron covers. I have measured the diameter to be 6.25" (160mm) but can only find cast iron covers that measure 6" (150mm), or 6.5" (165mm). I am measuring the external diameter, is this wrong? Very confused.

 

[Reg Man]

Don't think anyone makes them that small. Why not look at cast aluminium, you might find something.