Originally Posted by berfinroy

Mea culpa. I didn't realize that the first single carburetored version of the 300 series engine was designated M186, followed by M187 (twin carburetors) and 188 (triple carburetors), while the later fuel injected version was designated M189. The carburetored versions did have the spark plug in the block, not the cylinder head, whereas the M189 placed the fuel injectors where the spark plugs had been located and moved the plugs to the cylinder head.

I don't believe the cylinder block of the M189 engine was made of aluminum until 1960-61 and thereafter for all applications until the engine was discontinued in 1967.