Some of my bikes have nuts with "teeth" on the back of the chainrings that bite into the ring and you snug the front. Or use the correct nut tool, which I did not. It spins in the hole and a large common in the little groove wont get it started. The nut is near toast.

My other bikes have different size allens front to back so a much better system in my opine.

Any Ideas how to get that nut off, that I may replace it with a new one and perhaps get a proper tool as well?

Thanks!