Alloy 850, with its tin content, is mainly a bearing alloy. I wouldn't use it for pistons. It does not have good strength at elevated temperatures (5 rating on a 1-5 scale with 1 being best).Fluidity is rated a 4, so it is not as easily castable as other alloys.Pressure tightness is rated 5, so you would be more likely to encounter porosity.The trend these days for cast pistons is the hypereutic aluminun-silicon alloys, such as 390.
