I received my Centaur bow yesterday. As I was tuning the bow with my FMJ I notice the nock wasn't tight on to the string. It was fairly loose. If the string was not deep into the grove the arrow sometime comes off the string while drawing back. The bow string it came with the bow is fairly thin. As I kept shooting I was able to keep it in place. It is so different then my Sage recurve. Is it alright for the nock to be loose or should I wrap some serving to keep it snug?

A loose nock can lead to a dry fire which can hurt you and the bow. A nock should fit just tight enough to keep it on the string during the draw. When held point down the arrow should not fall off without tapping the string. To tight of a nock can also be a huge problem.

Take a pot of water to a slow boil and insert the nock into the water for about 5-10 seconds.  When it is removed, you can then clamp slightly with your fingers for a few seconds to tighten the nocks onto the string.  As was mentioned, don't over-tighten.  Hope this helps.

The arrow falls off the string if it is pointing down without me tapping the bow string.

D.Key, I will give it try tonight.

Have your string reserved.

Get yourself a serving tool and two or three different sizes of serving thread.  Reserving a bowstring is simple and easy to do.  For me at least, it's a lot less hassle to reserve one string than custom fitting dozens of nocks by boiling or filing.

Have heard several people mention serving just the area the arrow grips with "flat" style dental floss.  Sounds easy, easily adjusted, and easily undone if it does not work for you...