Converting rubber to Kevlar on old builds?

Here's the setup. I have an few old builds (18mm powered, BT-50 based designs) that I'm thinking about restoring and putting back into flight rotation. Problem is, they have old (+20 years) shock cords that are very questionable. They used the tri-fold/teabag mount, and space was tight then. A 2nd mount would be a real problem. I'm certain that with time and work, I may be able to remove most/all of paper, but I don't want to replace them again.

I'm thinking that if I can find a drill bit that is long enough to hit both centering rings, and fit between the two tubes, I could drill a hole through them, then thread a Kevlar shock cord through it, and attach my parachute from there.

Anybody tried this before? What were the results?

I know I could just clone them, but I don't want to if I don't have to.

On a couple of mine I took a thrust ring or centering ring ,if you so choice ,notched it and tied the kevlar to it and glued it in below the old tea bag.I had to get most of the bag off,but you can slide the ring down past it and then turn it sideways. I glued it by dripping CA into the tube on to the ring.
Anywhoo..worked for me..YRMV.
I'm sure the drill idea will work just fine if ya can get a small drill that has any lenght to it. All me small drills are short ones.

BT-50 models are tight, not a lot of space between the BT-20 engine mount tube and the main air frame BT-50.
You could drill a small hole through both rings and make a replaceable Kevlar line that loops around the rear centering ring.
When drilling you have to be careful not to go through or into either tube.

Going in from the back of the BT-20 engine mount tube,
I've taken a sharp awl and lifted a glued side of the engine block.
You need to use stiff Kevlar and feed it under the lifted side of the engine block.

After the Kevlar is fed under the lifted block, you can double knot the end and pull through up to but up against the engine block - or -
Feed the line out the back and tie a knotted loop that will tighten around the engine block.

You'll have to get some glue under the lifted side of the block, then press back down against the inside wall of the engine mount tube.

Another solution is to do a "Lariat Loop" around the back end of the engine.

Here is what I did to resurrect a few old birds that needed a new shock cord. If the root of the fin fillet is ahead of the top centering ring, I just made a small hole right thru the body tube at the leading edge of the fin. I then fed the Kevlar through from the outside and glued the tail down in the fin fillet. As long as your not using a big honking thick piece of Kevlar, it's not that noticeable.

I've Retro fitted a lot of older BT-50 and BT-20 models. Remove the old tea-bag bulge from the forward end of the tube and smooth out the glue with 80grit sandpaper wrapped around a dowel or spent motor casing. cut a standard JT-50 or JT-20 Coupling in half. use a needle file to create a groove on one side of the coupling deep enough to lay your 70 to 100lb Kevlar in. Tie the Kevlar around the coupling with the line laying in the groove. add some cotton ball to the end of a small diameter dowel long enough to reach the upper Motor mount ring. mix 20-30minute epoxy and apply to the motor mount and very bottom of the Body tube Above the MMT. using a dowel the fits your BT-50 body tube slide the new shockcord anchor coupling all the way down into the wet epoxy. Once cured it retro fit is complete ready for the recovery system.

PS it's possible to use a new CR-2050 centering ring but they are a little harder to control sliding in from the forward end. I use these also often

Don't know if this will work for you but it's an alternate. I built an Easter Egg rocket from a 16" tall plastic egg. I passed a 2.6" airframe through the egg with ttw fins. I got ahead of myself and glued the motor mount in before attaching the kevlar. So I drilled two holes in the airframe about 4" apart. I passed one end of the kevlar through the bottom hole and fed it out through the top of the BT, then tied a knot in it and then pulled it back to where the knot was held against the inside of the BT. Then I fed the other end of the Kevlar through the top hole and out the top of the BT. I glued the Kevlar to the outside of the BT. It was covered by the egg so it's not visible.

You might be able to work this technique to your advantage.

Pat

12" extension drill bits are readily available and not terribly expensive. Here they are at The Yard Store. Careful though, there are lots of fun things to buy here!