I just installed a 150 cc BBR Big Bore Kit on my son's TTR 125 and I thought I would share some of my findings. First let me say I have never done anything like this before and I was amazed at how simple it was. I used the Yamaha manual and was able to do this in one afternoon. It was very straightforward and I am a novice "mechanic" (using the term loosely so as not to offend a true mechanic). I left the engine attached and it with extremely minor wiggling the head bolts were manipulated out. The biggest trick for me was the cam gear. When I took it out - I realized it was going to be a problem getting it back in. It took me about 30 mins to figure out the right combo of moves before I could wiggle it back in. TIP for those to undertake this: I lowered the gear as deep into the head as possible affording me the greatest amount of slack in the chain. Trial and error will get the gear in and low enough to accomplish this feat of mystery and magic. I had my son by my side the whole time. I had him help with nearly everything. He was a huge help in getting the piston pin in place because that was a real 4-handed job. We also installed a 26mm Kehin carb. No major issues. It took me a few tries to get the slider down all the way. I realized it was not bottoming out because when I started it (started second kick!) it rev'd way up and I had to kill it. Biggest bummer for me was i was expecting mass power. The piston is dome shaped, the cam is noticeably different profile and for the money i was expecting bang for my bucks$$$$$. I rode it it and was unimpressed. My son said he could tell the difference but I think he was being nice to me. That's my story. We went on a ride the very next day and he enjoyed the bike but he always has. He learned to do a "no footer". I unexpectedly did a "flatliner" on my YZ250F coming off a step-up jump and created an impact crater on the landing (but I lived to ride again).