How to tell if I have an Acme or M8 threaded rod?

Yet another silly question. I bought my x-carve used and I’m not sure if the Z axis rod is the Acme or the M8 variety. I’d rather not take it apart to test it. Does anyone know a simple method for figuring what I have? Perhaps the threads per inch of the Acme?

In this picture the top one is the M8 and the bottom is the ACME
http://x-carve-instructions.inventables.com/step08/

Malcolm,
As Zach references, it is as simple as looking at the rod as the differences are readily visible, if you understand what to look for. No measurements are required. The M8 rod uses “standard” triangular threads as on almost any bolt. The ACME threaded rod uses a trapezoidal (almost “square”) cut thread with flats on the bottom of the thread channel.

If it isn’t clear between Zach’s photos or this description, I recommend searching for photos of acme threads. Wikipedia also has a good description.

Good luck getting it running! I suspect you will have lots of fun running it over the holidays!
–Rick

Thank you so much guys! It’s quite clear now that i have the Acme rod. Woot!