There's not that much difference between the SG of Magnesium and...

There's not that much difference between the SG of Magnesium and Aluminium to warrant the use of the former over the latter in car engine cylinder heads, but Aluminium is much lighter than cast iron, so I believe Aluminium will be used to reduce the weight of car engines and not Magnesium. However weight is very important in aeroplane construction and would justify the use of magnesium alloys