It was around 1985 that Martin started building their great guitars with truss rods, a very welcome thing.
In around 2006 Martin seeing the advantages that I speak of till I'm blue in the face here on the OLF...
Martin started building with an excellent two way rod. I never built with this rod I used the Blanchard rod and now 18 years later the rods continuer to perform flawlessly. But the Martin 2 way rod is something that I would have built with had I continued to build it works great and gets great reviews from the people who I listen to.And now.... drum roll please Martin has been doing some pretty innovative things and one if them is yet another new truss rod a two way with a 4 mm adjustor instead of the 5 mm adjuster that they have used since 1985.
We are also seeing a paddle/neck block arrangement too on some of the models such as the excellent D-28 Modern Deluxe that I worked on yesterday which, by the way has EVO... RIP wire.
So if you are going to be working on or purchasing a new Martin you may need a new truss rod wrench.
Any ordinary, quality 4 mm Allen wrench will work but I do all my adjustments unless I am doing fret work under full string tension and tuned to pitch. An ordinary Allen key does not have the leverage on the short side to make this effortless and accurate in my experience and you have to feel around in there to get proper engagement too. So a longer handle Allen wrench such as the one available on Martin's site in 4 mm is a good way to go.
You can make your own too but even the Martin wrench is only around $15, mouse nuts and likely well worth it. Amazon has no frills ones too with a longer short side but no nice handle like the Martin 4 mm wrench.
