Go to www.cascadeeengine.com and follow the Yanmar/Manual/Marine links to get to the service manuals. Since the HM engine and the early GM engines were covered by the same manual I am sure that they are cast iron. I think all of the Yanmar designed and built engines (as opposed to their marinized auto/truck engiens) are all cast iron.a

It is only on the higher HP LY/LP/LH engines where aluminum components were used that antifreeze specs matter.

David