This a pretty specific and esoteric enquiry! I have settled on a 8 ring routine that includes the Al Baker threaded pair. I'm having trouble getting a thread that is just right. 2lb breaking strength cotton is OK but I would like a little stronger. Trouble is that stronger threads tend to be polyester which doesn't seem to knot so securely. Any bulletproof experience out there?

I also employ a threaded pair of two singles ala Al Baker. I use cheap commonly available thread , which typically is cotton covered polyester thread. first I tie a square knot and then another half hitch (overhand knot) to make it completely secure. I've broken the thread accidentally, but never has the knot come undone.

Around here, we specialize in esoterica!

I got a spool of grey "coat thread" from my grandmother about sixty-five years ago that has worked fine for both knot and controlled break.
I don't know how to test the strength or material. Probably pure cotton or linen.

I have silk thread that definitely will NOT work for knots. I have used soft wire separated from picture-hanging cable.
With my large brass rings I have used dental floss soaked in coffee.

Two great replies, Funsway is Coat thread the brand (which I see does exist), or a type?
Jonathan, cotton covered polyester is a new concept for me, but clearly a "thing", the go-to range of threads in Europe are Gutermann- but I will track some down (their range probably includes it) sounds the perfect combination of grip and strength.

I have also used the small spools of Gutermann.... I think k it is 100% polyester.

Coat thread is thicker than normal sewing thread. I think some are braided rather than twisted.
Methinks it is for sewing on buttons rather than joining the material.

Levent Cimkentli produced a definitive, excellent, 4 DVD set on the Linking Rings, he kindly just gave me an additional tip on tying the loops. Tie the first loop with a couple of square knots then take the loose ends around again to tie a another loop in the same manner. Repeat with the second loop, this means each loop has two rings, so if one breaks the other should hold.
This I must try....

It's also written up in the stellar Roy Benson by Starlight.

Try a bonded Nylon thread. It is pretty strong but can be broken by hand. I seem to recall the size as being L22. I will have to check.