Motor Mount
or, "Rockets are fun!"
Well this was an interesting adventure. As before, I am following the instructions (which are the original Estes instructions, I believe, with actual words to go along with the pictures, it's glorious).
Step 1: centering rings
The two centering rings are the wound-paper type, with the one in the rear split to accommodate the motor hook. Standard stuff. I glued each one on, sticking one of the pieces of BT50 around the split ring to ensure it stays tight against core tube.
Step 2: Ummm.....
I was trying to see if I had missed something in the instructions. Nope! It turns out that the original kit did not use one of those retainer rings, it specifed cutting a piece of paper (or maybe cardstock) and gluing it around the hook.

I went with good old electrical tape instead, one wrap close to the rear and one over the front of the hook.
Sharp-eyed readers will note something missing from that step 13: a thrust ring/motor block. I double- and triple-checked the instructions but confirmed it was not there. I'm not too fond of letting the top of the hook handle this task on its own, even though it would probably suffice for the handful of flights this rocket is likely to see. Normally I would insert the hook and then push the motor block in from above, pushing it against the top of the hook. But given the very long length of the BT20, that was not an option. So pushed it in from the rear with a motor, trying very hard to position it *just* above the slot for the hook. I think I got it pretty good.

Honestly, that picture doesn't prove anything... but the in any event the motor is not going anywhere.
Step 3: WTF
I tested the fit first... snug but seemed OK. Then I applied Elmer's Glue-All to the front ring and the rear inside of the BT50 and slid it on. It proved very difficult and I couldn't quite figure out what was happening. Then I looked in front and saw this (sorry for the fuzzy pic, I was rushing):
What I was witnessing was that the ring had delaminated, and the outer portion had pushed out of the way. After a moment of consideration I concluded that there was going to be no way to fix that, so I cut off the outer tube, discarded it, and regrouped.
Has that ever happened to anyone before? It seemed cuckoo-pants to me.
Fortunately, there was another tube in the kit of the exact same size that was already spiral-filled and primed, so I was ready to go with a replacement. I glued the front ring back together (remember the inner part was glued quite solidly to the core tube, so I couldn't just remove the whole thing) and this time sanded down the two rings to make the fit a bit less snug. Applied the Elmer's and slid on the tube and...

It froze about 2mm before reaching its destination. I give myself credit for not simply throwing the whole thing in the garbage at this point; I was pretty steamed but taking deep breaths and trying to say calm. There was absolutely nothing to be done about this, once it's frozen it is irreversible. Also important to remember that I'm working with BT20 and BT50, both of which are pretty flimsy, so I had to be very careful not to manhandle it trying to force it into position.
Step 4: Damage Control
After resigning myself to the fact that that spot was just not going to look great, I decided that I needed to at least *try* to make it better. After several rounds of very carefully applied and shaped CWF, I actually got a decent result:

I applied some thin CA to harden it up, and considered my job done. The tube hangs over the end of the rocket a bit but that's not a problem.
Oh, and then I cut myself a new piece of BT50 and filled and primed it to replace the one I destroyed.
So there you are, a long 9-picture post on constructing an 18mm motor mount. You're welcome.