yep red loctite, but also check that the brass bushings on each side are not tight, as i looks like maybe one side is grabbing when hot and causing it to break the shaft.

T1

Red Locktite as it can handle the heat...not sure if blue loctite is up to that job.

Dynoharley is correct. If the screws are too tight it will encourage the shaft to crack. I would use a little longer screws, a bit softer metal, then do then up as D H suggested and stake the end. I do this on V-Rod TB's and Rocket threes etc after fitting larger butterflies and have never had an issue. If one uses an allen head screw the allen key bit can be used as a mandrel to hold the screws when staking the end.