Here's how you prevent that grinding/crunching noise (and prevent
chipping/breaking the teeth on reverse):
1) Always come to a COMPLETE stop before attempting to engage reverse.
2) Once you've stopped, make sure you have the the clutch ALL THE WAY to the
floor.
3) Engage a FORWARD gear; I use 2nd, but any gear will do. Keep the clutch
DOWN.
4) Engage reverse. Presto, no grind, no crunch, ever!
Since reverse gear is not synchronized, it is essential to make sure the front
half of the transmission stops spinning before you engage reverse. Even if
you're at a complete stop with the engine idling and you push the clutch all
the way down, the front half of the tranny still takes considerable time to
stop spinning. If you don't want to wait, step number 3 will makes uses one
of the forward gear synchronizers stop it for you.
If you continue to get grind/crunch noises when following this procedure, it
means you clutch is not disengaging all the way (dragging) and is spinning up
the front half of the tranny again...