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How do you it? Normally with a dovetail I just shove a bag of birdshot between the TB and the block and let fly with the hammer, but in this case I'm worried about knocking the tenon off the FB extension from hammer vibrations because of the gap between the tenon and the plywood slab in which it sits. What do you folks do?

Open up the fret slots on the extension with a dremel bit, then press and glue the frets in.

I do the same as Barry only I use a back saw to make a wider cut. Then just glue them in place.

I may be misunderstanding the problem, but... Why not remove the neck, make a support caul that fits the tennon, (drilled out, if necessary, where the T nuts protrude) clamp the caul in your vice & hammer away.
The main joy, for me, with a double M&T neck is that you can install the neck, secure the fingerboard extension, final shape the fretboard to address any minor hump or dip at the body joint, and then remove the neck for fretting.

Double M&T as in totally bolt-on neck?
Level your fretboard with the neck mounted on the guitar, do your fretting with the neck apart from the guitar. You can put together a holder held in a vise for the fretboard extension in 5 minutes.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Ha! Well yeah I guess that's pretty obvious now that you point it out! Thanks!

Oh yes if it's totally bolt on. My bolt on neck joints have the tongue glued down and I have had hump problems when fretting the board prior to gluing in place.

To get around the hump problem with a bolt-on, with a glued down tongue, try putting a couple of countersunk screws through the fingerboard & the top... ( make sure you miss the UT brace) Put a nut on the inside with a nice big washer under it & screw the extension down nice & tight. Then do your final fingerboard shaping. Once that is done, remove the screws & bolts & do your fretting on the bench.
You should have most of your alignment issues worked out first, but this trick works well to perfect the extension drop-off & eliminate a minor 12th (13th 14th) fret hump.
If you put the screws in where fret markers go, it's easy to plug the holes & your shell markers will cover the evidence.
I did this on a few guitars, before I started using the "double M&T joint". It worked great.
Just make sure you glue in your fret markers & sand them flush, before you install the frets.

Someone else like me!

I love how obvious things are once they make perfect sense.

Of course that can take a long time for me.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

For a total bolt-on which I've done on all my guitars the last several years I've evolved to this,
*Attach neck and level with a little tension on the russ rod
*Sand some drop off into the end
*Fret to around 12-13 with neck attached.
*Support the peghead and put about 6lbs on each shoulder and check the drop off at the end of the fretboard.
*Sand more in if needed and make sure the fret slots are deep enough.
*Take the neck off and fret the extension. I compress the tangs on the frets with a fret tang pliers and re-radius so they go in easier and use glue. I have a caul I made to support the extension while doing this but it's hard to get total contact for support if your tenon does not go completely to the edge of the fretboard as in the technique shown by Mayes and Sylvan Wells. I do it very carefully. It's not foolproof

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It's not what you don't know that hurts you, it's what you do know that's wrong.

For those using a second tenon that only glues to the fretboard, take a look at the technique that Colin S uses. I've used a similar version on 5 guitars and have had great results. From a post a couple of months ago...

Here's a link to a tutorial that Colin S did. http://luthiersforum.com/forum/viewtopi ... 954&hilit=

I add a 3/4" thick paddle to the top of a standard neck block. The paddle is as wide as the neck block (2.75" from memory) and long enough to butt against the main brace on the top of the guitar. I then continue as if it was a standard single mortise build. Cut the mortise and tenon, install the hardware and set the neck. With the neck mounted to the body I attach rectangular template which I use to route the second mortise. Be sure that the mortise is narrower than the fretboard (I leave ~1/4" overhang on each side). As far as length, I mount the template so that the pocket stops ~ 1/4 - 3/8" from the end of the paddle and extends back to ~ the 12th fret on a 14 fret neck. I make the mortise ~1/2" deep to keep the truss rod slot buried (that's why I switched to a 3/4" paddle). Glue the tenon to the neck and plane it flush. You'll either have to cut the entire truss rod slot now or just recut the tenon portion. Hardware location depends on what size paddle you use. Give yourself ~3/16" clearance between the paddle and the o.d. of the bolt head and you should be fine. On my first attempts I installed mounting hardware into the second tenon as well. It doesn't seem to need it and no longer do. But if you want to, it's easier to drill pilot holes through the neck and paddle before you add a fretboard.

Bob