My inside diameter is roughly 8.75 inches.
Overall, I can't say that I'm overly thrilled with these pieces. Turns out I'd need four 153IRs to properly operate two bridges over both of my mainlines.
The above can be worked around. Perhaps more aggrevating (to me) is that the bases appear to be too small. I had a very hard time getting wires into the spring clips on the two that are next to each other.
Oh well. It was too cold outside to do anyting else today.