I have a Martin DC-16RGTE which has a mortise & Tenon with bolt too. Its neck is also loose. So if I wanted to reglue the neck, do you remove that bolt, then free up the fretboard extension, and inject steam into the joint as in typical dovetail joints?
No steam. Pop the cover on the head block and tighten the bolt.
To answer your question, you are correct in that steam will loosen the joint. First, loosen the fingerboard extension. If the fingerboard is black Richlite be very careful to not overheat it as it will bubble the top layer and there is no good fix for it. It's not the end of the world if it happens, but you can avoid it by judicious use of heat. I use a heat lamp. It takes about 4-5 minutes to get enough heat into it for removal. A flat, wide blade does the trick. As for steaming, the steam hole should be drilled about 3/8" off center which should clear the truss rod. Angle the drill slightly toward the center. Leave the bolt in to concentrate the steam in the joint. Remove it after a short time and work the neck out. I've come across many of these with poorly fitting joints. I typically glue thin shims to the sides of the tenon, then trim and sand until the fit is snug. Everything else is fairly straightforward. Have fun!
that wood plate is held on with thick double-sided foam tape. drip some naphtha down behind it to soften the adhesive, then pry it off with an offset screwdriver or something, not a big deal at all.
tighten the bolt, or if it's missing you can add one! some of these had the same neck joint but didn't use the bolt, instead just gluing the heel in. grab a 1/4-20 machine screw maybe 3/4" long and crank it in there, that heel should pull right in.
afterwards you just clean up the adhesive residue and re-apply some double-sided tape before slapping the coverplate back on.
Just now doing a neck reset on an older Martin B1 acoustic bass, with a mortise/tenon neck joint and the same 1/4"-20 bolt, so the timing was perfect to learn about the double-stick tape on the serial number plate, thanks!
Good advice, Steve, about installing a lock washer on that bolt. I'm sorta' surprised Martin never put one on in the first place but better late than never.
BTW, when steaming the neck out, I don't think it took over a minute's-worth of steam before the neck popped loose.... a piece of cake for a change!