GY6 150 upgrade

hey guys I have a few questions for everyone here I have a 157QMJ MOTOR I did a 171cc big boar kit on it and it fit perfect due to A hold my beer moment the case had cracked now I’ve rebuilt it again with a different kit as the damage was to much but this time around my piston does not reach a flush point with the jug at TDC I measured the gap still remaining it’s 1.02mm is how much it still needs to go up for it to be flush only this kit did it both were 61mm pistons any ideas on what might be happening