I found a thread in this forum that suggested making a reamer by flattening out the end of an old GP and sharpening the edges.

Sounds do-able to me but my question is: How wide is the hole that I'm reaming? As wide as the threads on the GP?

I'd hate to flatten the old plug too much and damage/get it stuck in the hole. Any ideas? Better yet, how about a photo of the chamber where the GP hangs out!

Thanks guys.