9" stretch ruined my engine, how to prevent??

[B-rad]

So, i have a 2011 taotao vip gy6 150. The scooter had 10k on it when i got it and was bone stock. Ive added a few things, most recently i did a 9" stretch. I did not use a kit, i did some research and decided to build it myself. I measured twice cut once etc etc, made extra sure alignment was correct and no binding/ rubbing. Got about 100 miles with no unusual tire wear, handling issues except for more vibration. Well, my mount bushings went bad, and it was like it happened all at once it felt like i was riding a fish. I went ahead and tore it apart to do the new bushings, and then... I saw a nightmare no scooter owner never wants to ever see. The rear of the case, just in front of the clutch/transmission unit SNAPPED OFF!! I was planning on doing the swingarm delete with shorter axle later, but thank God i didnt because that steel swingarm was the only thing holding it together!! So now, i am shoppi g for an engine, but how can i prevent this? As far as i know i have kept every measurement, mount height, geometry etc the same, but something must have caused this . what step did i miss? I have searched for similar stories but found none, so if any1 knos better advice would be golden about now!!