the Ace servo is a 23S spline. not a futaba 25 spline. none of the losi aluminium horns will fit corrctly.
your better off just running the stock horns because there arn't any aluminium horns that are a correct fit
Ace servos are a great servo but their stupid spline size really turns me off them
i used to run them in my 8ight buggy and truggy and used the stock horns. i never ever stripped one of the stock horns and i hit all kinds of solid objects with them