It fits ok without any messing. You just push it down towards the wiring and it just clips under the edge of the 510 housing. As to whether it is worth it or not that's debatable given that it is not a true 1300mAh i only did it because the original was goosed and it was a straightforward replacement. [video]https://www.youtube.com/watch?v=W4aWC8tHPEg[/video]