What is a good thead for tying the elk hair caddis fly. When I try to tighten thead it breaks. I am using Danville 6/0 Also what is good to t1e deer hair?

I've used gel spun thread like UTC's 100 or 200 denier with some success, but it is a little heavier in my opinion than regular 6/0. Just need to be careful with 6/0 and it should work. Has your thread been exposed to a lot of sunlight or anything that might have weakened it? The UTC is some TOUGH thread, good & strong. There's also a Kevlar type thread that I've used for big pike streamers...can't think of the name right now though.

ive used the kevlar thread for deer hair bass bugs. if you pulled too hard to flair the deer hair, the thread would actually cut through the hair. dont know how many sizes of kevlar are available since i havent tied for years, but if theres a smaller diameter kevlar available, i dont think itll break.

UTC, unithread, or almsot any of the GSP

its always been a funny balance with elk hair and deer belly to get them to flare correctly, but not be cut by the thread

I use 6/0 for mine. With practice you'll get a feel for when there's enough tension. I should only take a few flies. As said above, you won't break Kevlar, but you may cut the hair or even bend your hooks. Keep after it and it will become easy.