With the first, the Mini Centurion, I didn't dry fit the parts first (yes, stupid of me) and I had to apply excessive force to get it in there. I could easily have buggered the whole thing.

So for the Lil' Centauri I test fitted the ring; it wouldn't go at all, initially. I used the handle of my knife on the inside edge of the inner tube, and then the ring would go, but still only with excessive force. Knowing that the yellow glue I favor would "grab" at the least convenient time, I needed the ring to be looser. I turned it in a sheet of medium-coarse sandpaper to get it smaller... and as I noted, it took several dozen turns to get it small enough. The white paper seemed exceptionally hard (and it was a new sheet of 3M sandpaper).

I'm not a novice at this. As I said, I can adapt. I just got done assembling the engine mount for a Quest AGM from a few years ago, and I swear the floppy loose centering rings THEY used were made from a cereal box. I made 'em work anyway. (I had to, I don't have any metric centering rings in my parts box.) I used Semroc parts, specifically, a couple of AR-2050 rings, to support them.

So anyway, just something I thought I should mention.

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Once upon a better day... SAM #0076
My site: http://rocketry.gonnerman.org