The new unit is not a surface-mount type; it has a cast metal body that, when installed, is perpendicular to the guitar's body, and which is screwed to a steel plate on which 4 bridge saddles are mounted. That plate is attached to the guitar's body by 4 screws into the guitar's deck. No bridge is used.
In fact, this vibrato unit was intended for a cigar box guitar, which requires a wooden block to be added to the inside of the box to provide enough structural integrity for the vibrato. This necessitates a slot being cut through the lid of the box for the body of this unit to drop through, and that body rests against the end of the wooden block, with 3 springs stretched from the bottom of the unit towards the neck, and anchored about 3" away. (Please see examples below of cigar box guitars with this unit installed.)
So, the unit is positively attached to the deck with a row of 4 screws through the plate, but the part of the unit that is inserted in the body remains somewhat flexible. As I understand it, the unit's ability to return to the strings to being in tune after the vibrato arm is manipulated is due to the top plate of the unit being held securely to the deck, and the other end of the unit's body resting against the block inside of the box and being held under tension against it by the 3 springs, but still able to be slightly bent away from it to change string tension, by manipulating the arm.
The main reason I decided to go with this unit is because I am setting this guitar up as a tenor guitar, so the fact that the unit is intended for a 4-string guitar (as many cigar box guitars seem to be) makes it the best candidate I was able to find.
The unfortunate thing is that the body of this vibrato unit is 1.5" deep (more like 1.65" when one factors in the springs that attach to the bottom end), whereas the body of my guitar is 1.38" thick. Obviously, I don't want a third of an inch of vibrato unit sticking out the back of the guitar, so I will be cutting the lower 0.33" of the unit's body off. (See photos of body with cut line marked.)
Cutting it down will eliminate the holes for the 3 springs (which are only about .25" deep), but I will drill them .33" deeper before cutting off that piece, so the holes will remain, and in the correct positions. I will also be cutting a slot through the guitar's body for the unit to drop into. I'll make a flush cover-plate for the back of the guitar to cover the springs, etc.
Here are my questions:
1. Obviously, if the slot I cut for the vibrato unit's body is perpendicular to the deck, and only big enough for the unit to snugly fit, I won't be able to use the vibrato arm much, since the unit will be wedged immovably inside of the guitar's body. So when I cut the slot for the body of the vibrato unit, should the opening at the deck be close-fitting, but both the fore-and-aft sides of the slot be undercut at a slight angle (say, 5 degrees on either side), to allow the unit to move? (See my drawing of that configuration.)
I presume that the placement of the points to anchor the other ends of the springs would be somewhat critical for being able to hold the strings in correct tension. I don't know if this method would be likely to result in the strings reliably returning to being in-tune after the vibrato arm is used.
-or should I cut the side of the slot closest to the neck perpendicular to the deck, but undercut the other side of the slot, to allow the unit to flex only in that direction, replicating the effect of the wooden block added to the cigar box?
2. Since this unit does not use a bridge, I presume it will be critical for me to get the placement of the tailpiece very accurate for the scale-length. According to other "Vanguard II" owners, this guitar supposedly has a 25-15/16" scale length. However, measuring from the inside edge of the nut to the center of the bridge location on my guitar yields 25-11/16".
As I understand it, the bridge (or the saddles, in this case) must be exactly the same distance to the 12th fret as from the 12th fret to the nut, which in this case is 12-7/8" (ergo, 25-3/4" total).
If you didn't see my other post about the neck, this is not the original neck to the guitar, and it may be from a slightly different model or year (hence the discrepancy). But so as long as I install the tailpiece so the the saddles are at that magical 25-3/4" from the nut, the intonation should be good, right? And should I measure that distance from the saddles in their mid-point of adjustment (rather than tightened all the way) to allow for adjustment in or out for individual string intonation?
Thank you to anyone who has read this essay, and could help advise me!
My next question will be about pick-up placement, but one challenge at a time!
