I should have noted - I have just upgraded my LMS 7x12 with a 7x16 Bed Extension and find the gnashing is actually Gear B just barely contacting behind Gear D on it's back edge. The gears have ~2/3 engagement, likely due to problems noted below.

My Left Lead Screw Bracket is a bit beyond flush with the left edge of the bed casting and my Change Gear Adjusting Stud might not be quite located properly. LMS instructions are "On your original bed way casting, measure the distance from the front edge to the center of the hole for the change gear adjusting stud. If it is over 21/32" or less than 19/32", use the dimension you measured instead of 5/8" in the step above." My stud was 0.5" set back on my 2013 model LMS 4200 7x12, so I went with 0.5" as indicated.

I'll get a picture when home again and post - sorry if I'm not describing it well. I'm hoping some tweaks might help fix the issues. At worst, I can grind back the read edge of the 80-tooth Gear B. :-(

Trying to precision drill holes in the bed casting proved quite difficult, even with a drill guide.

BRET