my output HIGH is 2.47V

I'm testing my new batch of HC74 shift-registers and after much frustration discovered that the output voltage on both my Uno and Mega2560 maxes out at 2.47 volts?
the VCC is 5V but setting any of the digital pins to HIGH only gives me 2.47Volts...
any idea why this is happening?

had neglected to define the pinModes ....

pinMode(pinData, OUTPUT);
pinMode(pinLatch, OUTPUT);
pinMode(pinClock, OUTPUT);

all is right with the world ever since ...

Yup, that would do it. There are generally three causes of it - what you said, or Vcc is much lower than you intend (usually because powering through Vin/RAW/input to regulator with intended Vcc, or via woefully underpowered supply), or that the pin is actually loaded very heavily (target device not powered, target device damaged and pins have fairly-low-impedance path to ground, or is directly driving a load too large to power from a pin). The last case is the most serious issue - so know what the first two are so you can very quickly rule them out - I say most serious because, if target device isn't powered when applying a voltage to it, that will damage the device you're trying to talk to (most frequent result is those pins then being near-shorted to ground, as mentioned as potential cause above), and in any event, if a pin set OUTPUT is so heavily loaded that it's only at 2.5v, the current exceeds the max current per I/O pin rating of the microcontroller, and can lead to those pins becoming damaged on the Arduino itself - though in my experience, the AVRs at least are fairly tolerant of this being done for short periods of time, even though it absolutely exceeds the absolute max specs.

Actually, there's one other potential cause - if the arduino pins themselves are damaged; the usual result of this is that they act like the output drivers are always on, either in HIGH, LOW, or both directions, which if either LOW was blown and you try to drive HIGH, or both are, you usually end up with ~half-Vcc as the output voltage. This can be the result of overloading the pins, or otherwise abusing them.