Extreme Service Mainspring Housings are one of the few parts in modern day 1911s that will readily "drop-in" many frames. However, due to the over 20 manufacturers of 1911s worldwide having different standards for dimensional tolerance cotrol, sometimes you find a combination of frame and mainspring housing that need some fitting to fit together smoothly. It is also possible to find combinations that have slight mis-alignments of the pin hole between the frame and MSH. Both are easy fixes well within the skill of the DIYer.