Are you sure it's a Livery arrow? 3/8" is too small a diameter, and 8" fletchings are too big for Liverys. An 8" fletching is massive for any 3/8" shaft actually - even the huge 4oz military arrow only has a 7.5" fletching, with 3/8" diameter Standard arrows being around 6".
As for the thread, there are quite a few ways of preventing it ripping, but the way I do it is coat the binding in a good waterproof glue, then apply about 3-5 coats of clear varnish over the top. Superglue works well also, if you dab a bit along the bindings then use your finger to wipe it all over, making sure you coat all the threads. Then apply the coats of varnish over that. Hopefully that keeps it at bay for a bit, but eventually you will find they start to look a bit ragged.