Thanks for your reply. We are at 3 x binned pixel size. It's a close call. I'll try to increase the threads number again and see if that helps. I was trying 400-1,000 classes. To make it work, I was thinking I could use the 'rescale' option at the pre-processing extraction step, clean up my data and then do the final 3d refinement on the original data. Is that possible? L