It is impossible to torque header bolts to spec. You cannot get a torque wrench on header bolts; you must do it by feel.

 

Threadlockers
Purple= light or up to a #12 fastener
blue=medium 1/4-3/8"
red= heavy 1/2" and up
Green=WTF grade. (theres actually 2 types of green one is wicking grade)

anyway a little heat and when there is a puff of smoke the loctite is soft again.
20 years ago loctite used to post specs on the slipperyness affecting the torques, I don't think they do that anymore. But yes when you put loctite on a bolt the torque changes about 10-20% I THINK (it's been a while). But that is part of why your bolt snapped.
And yes the more threads that are coated with loctite the more the hold LOL