"Reconditioning Aluminum
Cylinder Bores
Most of the engine blocks you will work on will be either cast iron or aluminum with
cast-in or pressed-in iron sleeves or liners. However, some lightweight engine blocks use
no liners at all — the pistons bear against a specially prepared aluminum cylinder
wall. These are often referred to as "linerless" engines. Most linerless alloy
engine blocks use a special hypereutectic casting process that distributes small particles
of very hard, pure silicon throughout the alu-minum. During production machining, the
manufac-turers use an acid etching process to dissolve away the soft aluminum from the
surface layer of the cylinder bores. This leaves only the exposed silicon particles to
support the piston and to keep it from wearing the aluminum away, figure 11-56. The piston
skirts are generally plated with a thin layer of iron to protect them from the abrasive
silicon particles. "
"Machining limitations
Aluminum cylinder blocks cannot be machined in the same way that cast iron can.
Hypereutectic castings have to be reconditioned so that the silicon particles stand away
from the cylinder walls to prevent the alu-minum from contacting the piston skirts and
rings. After you open up the bore, you perform a lapping process to scrub the aluminum
away from between the silicon particles.
An alternative method is to fit the block with iron sleeves rather than trying to duplicate a factory bore. If you choose to recondition to stock specifications, be sure to check with both the engine and the machine equipment manufacturers for specific recommenda-tions. Although you can open up the bores using a boring bar, most manufacturers recommend performing the entire resizing job using an automatic honing machine. If a boring bar is used, it is essential that the cutting bit is extremely sharp and freshly lapped. If the bit is dull, it will gouge chunks of silicon from the bores instead of clipping them off cleanly. If this hap-pens, the cylinder wall is destroyed and the block must be sleeved or scrapped.
Following is the procedure Porsche recommends for reconditioning cylinders in their hypereutectic engine blocks. The entire operation is performed using a Sunnen CK-10 honing machine. Keep in mind that this is a brief summary of a detailed procedure. "
All description and picture courtesy of Chek-Chart Publications
