I was right about the splines, but you were actually right about the problem, as the OP had machined the splines and had a install problem, not a machined problem. But the whole conversations has been productive. I added this thread to the X2 sticky as it has a lot of great information.
And I get it, but you will find those on facebook too. In the last few days I have been getting mocked by a number of people on FaceBook for suggesting that modified power wheels should have seat belts, roll bars and/or helmets. Some of these parents are bumping these cars to 24-36v, putting on larger tires with gearboxes from smaller cars, pushing to 15mph with toddlers with no safety considerations at all. I'm a "karen" with my "panties" in a wad for suggesting safety needs more consideration. Hate to go to a party with you, sucks to be you as a kid, glad you're not my dad they've said. Totally uncalled for responses. The ridiculous is everywhere.