MIMF

I have enlarged the adjustment end with chisels and gouges in the past. I was about to buy a jig when the supplier ran out of them. How do you make an access hole for headstock adjustment? With a two way truss rod, it is really unnecessary to enlarge that end of the routed truss rod channel, so ...am I making more work? Can I just leave the channel square?

Yeah, don't cut away any more wood than necessary. This is a spot that you don't want to weaken anymore than necessary.

I route truss rod channels on a little router table before the head plate is glued on. After I've run the slot I bump the bit up a few thousands and bump the fence back a few thousands (precise, aren't I) and hit the end of the channel where the adjuster sits (I only use dual acting LMII rods but I would do the same if it was any other kind). The head plate gets drilled for access and I clean that up with a rasp.

I also make scarf joined head and thing they are a lot stronger than cut ones but I still make the access as small as I can.

Yeah, it's been four years since I've made a steel string guitar. Thanks. One of two I'm making now adjust from inside. They're Martin 000 and Gibson J-45 copies.