I was afraid that thick 12 strings would appear too heavy to tune and make some harm to my 25yo korean jackson/charvel 550 dreadnought 10 or 11 (I don't remember). There were nothing to worry about. The strings were tuned perfectly. The guitar just needs some neck bend adjustment (slight clockwise turn of the anker) afterwards. After I re-stringed the Martin's red 12s the guitar new sound became awesome. Long sustain, bright and deep sound, no problem for fingers to give strings enough pressure in hard to press chords or in solo. No too much creak when move fingers along 3-6th. I like the overall result. Martin says it will last long due to their treatment. I hope so.