www.brownells.com/aspx/ns/store/ProductDetail.aspx?p=19914&title=TRITIUM+SHOTGUN+SIGHT

Mossberg 500..  What thread size do I need here?

Mossberg is 5-40.

Thanx!

When installing this, how do I time it to line up correctly?  And should I use blue Loctite?

The threads should already be regulated to position the sight correctly, but Mossberg is notorious for having some pretty "abstract" tollerences. Be prepared to coerce it with pressure (but not too much lest you crack the tritium vial) or conversely, be prepared to use blue loctite should the hole be too broad.

Assuming the threads are right, i'd use red loctite. There should be enough grip on the threads themselves to keep it on snug, red loctite would be an extra measure while still being a tad more forgiving should you ever have to replace it.

Thanx!

Too much pressure might also strip or shear the threaded post. Proceed with caution! A drop of blue loc tite is a good idea.