Wel the story goes like this. You start with molten Aluminum and disolve all the silicon dioxide in the melt as can happen. This is about 16-17% by weight I presume.

The block is cast using hypo eutectic cooling. This is done by slow cooling to a temp where just the Aluminum with the highest concention of silicon solidifies first. This causes Aluminum with about 35% SiO2 to form. The queching is done on the cylinder walls first. In the end the block has 35% SiO2 in the surface of the cylinder walls and probably less than ten percent through the rest.

The holes are bored with a Sunnen CK10 cylinder hone to approximately the right bore and then to the right bore with increasingly fine grit with the end stage almost polishing grit. There are no visable hone marks.

Now for that last stage called the siliconizing procedure, It is the exact opposite of what people think. Silicon is not added. it is already there. The siliconizing process is the leather hone soaked in an acid etch. The process is done for a certain time during which the aluminum is etched from the surface leaving the SiO2 matrix.

The casting process is done to leave a depth of high silicon large enough for one 0.5mm resizing. (Boring)

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician