two things to consider here: a) string-thru and b) modding the bridge
since it's the easiest to discuss, I'll start with the bridge. some bridges
can be modded to accommodate a string-thru, it all depends on how much working room you have behind the saddle. in the case of a Hipshot Vintage bent plate, they can be ordered with or without the thru-body option while utilizing the same design. some plates are not possible to mod, though no specific model comes to mind at the moment
for the string-thru modification to the body ... it's a total pain to do afterwards, IMO. the biggest reason for this has to do with string ferrule alignment/spacing. if you're off by 1/64" (0.015" or 0.4mm) it really stands out as a glaring mistake to my eyes. when working this early in the body build workflow, the holes can be drilled against a backstop where you can set the inset distance against a reference center mark on the drill. once the body is sculpted it's an entirely different story, and you rely heavily on the 'guessing' the center position of the spotface since you have 3/16" thru holes from the top side ... and these can drift on occasion. the solution is to make a jig to hold the body centerline perpendicular to the drillpress fence. Since I don't get requests for this mod often, I've not taken the time to make a standard set of jigs for different body styles. typical street price for a professional install job is around $100 plus parts
but ... I don't really hear any significant sonic difference between top loaded and thru-body when everything is equal. the only time I have detected an A/B difference in a recorded blind taste test was when using taper core strings, and I attribute this difference to the top loaded configuration having the core resting on the saddle while the string-thru configuration had the windings on the saddle. there was a slight difference in string stiffness (often incorrectly referenced as 'tension') possibly due to the break angle over the bridge, but this wasn't anything that couldn't easily be overcome with slightly thicker strings
if someone was to tackle this as a homebrew job, it's imperative that you have a drill press (pillar drill) at your ready. you'll also need a 3/16" brad point bit to drill from the top side with minimal drift. add to the list a Forstner bit of a diameter that matches the dimension of the ferrule outer diameter, and a drill bit the is slightly under that ferrule shaft diameter. the drill press is going to allow you to drill perpendicular to the body and also allow you to set the drill depth for the backside drilling operations. unless you don't care how the install job looks, this is not something tackled with a hand drill.
the install workflow requires a minimum of three drilling operations for each hole after the body has been stripped of all hardware:
1) once the thru-body hole locations are identified on the top side, drill thru the body with the 3/16" brad point bit. drill all top side holes before moving to step 2
2) flip the body lay-out where the backside holes need to be recognizing that the 3/16" could easily have drifted. I like to reference the two outer holes and then average the spacing for the remainder of the ferrule locations. chuck up the Forstner bit and carefully bring the bit into contact with the body (centered on a hole so that the bit face and not the tip is in contact) adjust the drilling depth to match the thickness of the ferrule lip. lift the bit and center it where the ferrule will be placed. secure the body into position so that it cannot move, but be careful not to damage the body by over clamping. I also use a router pad on the drill press table to help avoid marking the body. drill the spotface
3) without moving the body, unchuck the Forstrner bit and add the bit for the ferrule shaft hole. set the drilling depth to exceed the ferrule shaft distance by about 1/8" and drill the hole. note that this shaft hole may be slightly off center from the hole drilled from the top side - this would be the case when averaging ferrule locations when the top side drilling effort drifted
unsecure the body and relocate it for the next ferrule location, repeating steps 2 and 3 until all of the ferrule holes are completed. since you unchuck the drille bit with each step, you'll need to reset the drill depths with each step. on a new body construction, all of the holes for a specific bit are drilled before changing bits - saving a TON of time.
usual install time for a new body construction is about 20 - 30 minutes. usual install time for a retrofit job can be as high as 90 minutes, not counting the time needed to disassemble and reassemble/set-up the instrument ... it could be as high as a 4 hour job when retrofitting thru-body on a 6-string bass
hopefully all of this makes sense. feel welcome to shoot a reply back if something needs further details
all the best,
R
