The locks that sit on tops would be difficult to measure diameter because they are not complete half circles the will seem small if you measure the edges.
You shouldn't have to tighten anything enough to break. You do not have to clamp any of the locks very hard at all just a bit snug. Nothing on here requires tight, everything snug. This is a first hearing those parts are too tight. All the other times were wrong parts printed or just hardware wrenched down really tight. It's plastic start light, you can always tighten it later if needed.