I just bought a Martin Guitar and I have no idea what size Allen key I need to adjust the Truss Rod

[Chris]

I recently bought a Martin D-15, should be a 2008 series. I have no idea what size allen key I need to adjust the truss rod, plus it's way in there. I've never seen one so deep before. Probably because this is my first real nice acoustic. Any help would be much appreciated. THANKS!!

[Greg Mirken]

It's a 5 mm. The newer Martin 2-way rods are very deeply set; you need a long wrench to reach.