Hi everyone.

I am looking at the notion of converting a Fly into a "12-string Concert” of sorts; more info on that soon. At this stage, however, I have a question about string tension. I’ve modified the bridge and saddles of a Fly to physically accommodate 12 strings (6 two-string courses), and intend to “fix” the bridge by replacing the tension spring with a wood block.

My question is:

*what are the limits on overall tension that a Fly can handle? *

I’d like to use D'Addario EJ38 12-String Phosphor Bronze Acoustic Guitar Strings, (10-47), which have the following gauges:

Plain Steel .010/.10, .014/.014, Phosphor Bronze Wound .023/.008, .030/.012, .039/.018, .047/.027

I’ve never used 12s on my Fly, but, in principle, a Fly with a 12 spring could handle the below string tension of about 150 lbs. of tension (with D’Addario EJ21 12s) as follows:

**String Note Gauge Tension**

1 E 4 0.0120 in. 23.36 lbs

2 B 3 0.0160 in. 23.31 lbs

3 G 3 0.0240 in. 28.02 lbs

4 D 3 0.0320 in. 27.63 lbs

5 A 2 0.0420 in. 25.60 lbs

6 E 2 0.0520 in. 21.15 lbs

Total *149.07 lbs.* of tension

For the 12-string idea I’m proposing, with a wood block used instead of a spring, the guitar (truss rod, bridge, etc.) would need to support about 250 lbs. of tension in total resulting from a set of 10-gauge 12-strings. D’Addario lists the tension for 12-String 10’s (EJ38 set) as follows:

**String Note Gauge Tension**

1 E 4 0.0100 in. 16.22 lbs

2 E 4 0.0100 in. 16.22 lbs

3 B 3 0.0140 in. 17.85 lbs

4 B 3 0.0140 in. 17.85 lbs

5 G 3 0.0230 in. 27.90 lbs

6 G 4 0.0080 in. 14.68 lbs

7 D 3 0.0300 in. 26.71 lbs

8 D 4 0.0120 in. 18.54 lbs

9 A 2 0.0390 in. 24.66 lbs

10 A 3 0.0180 in. 23.41 lbs

11 E 2 0.0470 in. 19.91 lbs

12 E 3 0.0270 in. 26.91 lbs

Total *250.86 lbs.* of tension

I know that the carbon fiber exoskeleton is strong, but I don’t want to push it beyond its limits. I also don’t want the bridge to pull up from where it’s secured. If anyone is knowledgeable about long-terms effects of having this added tension, please let me know. At this stage, I haven’t done any modifications to the guitar itself that are irreversible. Ideally, if successful, I’d like to continue this project and document how this 12-strong mod was executed.

Thanks for the help!