On mine the only place they worked out without cutting and custom threading them is at the ailerons. That was with two different planes btw. With the servo output shaft towards the leading edge 4" works. At the elevators with the output shaft forward you would need something about 5-3/4" for the threads to have any bite. With the output shaft aft it would have to be about 5-1/4". You don't want to be cutting off threads to make them fit. You only have about 1/2" of thread on each end and you want maintain a minimum of 1/4" of threaded rod inside the ball link.

Just make the elevator linkage using 4-40 all thread and the composite tubes that came with the hardware kit. A lot cheaper to do the ailerons that way too. A side note about the elevator linkage. You want both sides to be exactly the same length if at all possible to equalize the servo geometry at the elevaor. If one side is shorter than the other you will run into deflection problems. This is with any plane that uses separate elevator halves. The servo arm at each elevator servo should also be positioned the same as the other side before you start making any other adjustments.