I’m trial fitting a neck to my tb2 pot and the neck currently sits in the original resonator such that the fretboard is proud of the head. Is it acceptable to cut around 7mm off the heel depth so that the fingerboard will be flat and in line with the head?
On a flat sided rim there would be no problem in trimming the heel, or deepening the cutout in the resonator wall. But if your neck heel is fitted to a Gibson style flange, you would be throwing off the fit by altering the position of the neck. ( I can't really tell from your photos exactly how your heel butts against the rim/flange)
Generally speaking, having your fretboard higher than the tension hoop is not a big deal for playability. It's mainly an esthetic thing, and would actually be preferable for clawhammer playing as it would give more clearance between the strings and the head.
Another solution that wouldn't affect the neck position would be to extend the fretboard over the head--by either installing a new bound fretboard, or adding an extension to the existing fretboard, hiding the splice under one of the frets.
... if your neck heel is fitted to a Gibson style flange, you would be throwing off the fit by altering the position of the neck. ( I can't really tell from your photos exactly how your heel butts against the rim/flange)
It looks to me that he has a neck with a heel cut for 1-piece flange and a rim that's flush sided because it has shoes and a flange plate.
Tim: While you could take a little off the bottom of the heel, you can't take too much since the lower lag is in there and you don't want to be weakening that area any more than it already is, seeing as the heel is cut for 1-piece flange.
There's nothing inherently wrong with the plane of the fretboard being above the plane of the head. I have a 1970s RB-250 on which the fingerboard surface is higher than the head by about its full 1/8-inch thickness.
So I'd be inclined to do as Dan suggests and lower the bottom of the notch in the resonator to see if that brings the neck down to a level you can live with.
Seems to me that the real issue with fitting this neck is that the rim you're using presents the same outer diameter to the upper and lower lag areas of the heel, while a heel cut for 1-piece flange mates to a rim with a smaller outside diameter at the lower lag -- meaning the lower lag area is longer. I guess you've cut the lower lag area short to meet up with the heel properly.
I actually prefer the neck to be mounted with the fingerboard above the head plane for more string clearance. It helps keep picks from hitting the head when playing near the neck. Here you have another issue of the head bottoming out in the notch, leaving no more space for tightening, so lowering the neck would be appropriate unless you plan to cut it out more. As mentioned above, location of the lower lag screw is the only concern in having enough wood below to retain strength.
There were several previous threads about height of fretboard vs height of head/tonering, including adjustments made by stewmac and Sullivan/1st quality, I linked a few at bottom of this thread, "Search Forums" box if you need more
Thanks for the great advice and replies! To update the information it is a straight sides 1926 Gibson fat rim with a shoe bracket assembly and diamond flange. However . The neck I happen to have has a OPF heel cut so I will be taking the lower heel mating surface back to the same depth as the upper one and probably then adding a piece of maple to fill the OPF cut out. The heel to fingerboard top depth will be just over 68mm as per a prewar five string neck. My slight concern is whether the small and thin lag bolts from the 26 tenor neck are up to the task of the modern maple five string neck with truss rod and modern tuners ie a fair bit heavier! But I really don’t want to change from original parts unless absolutely necessary.
Clearly Gibson learned the need for heavier lags. I see no need to relearn that lesson. Take advantage of their experience and use modern size if you are building this to play and hold up to regular handling.
My slight concern is whether the small and thin lag bolts from the 26 tenor neck are up to the task of the modern maple five string neck with truss rod and modern tuners ie a fair bit heavier! But I really don’t want to change from original parts unless absolutely necessary.
Well, I assume that maple 5-string necks made in 1926 used the same small and thin lag bolts. Prucha still makes the smaller bolts and matching co-rods for those who want or need that level of pre-war accuracy. So you could get new bolts of the right size rather than risk breaking the old bolts in removing them from one neck and installing them in the other.
The neck I happen to have has a OPF heel cut so I will be taking the lower heel mating surface back to the same depth as the upper one and probably then adding a piece of maple to fill the OPF cut out.
I'd love to see a photo of how this turns out. I eventually want to deal with the same issue, but for a different reason. I have a 1970s RB-250 that a previous owner converted from 2-piece flange to 1-piece flange, but in the process vastly over-sized flange cut-out. And did an ugly job of it, too. My guess is he cut the flange notch before he added wood to lengthen the lower section of heel. I assume that's when he discovered his cut-out went much further into the heel than needed.