This is a common question we receive, and the best answer is that it is formed in the cylinder during the casting process.
Unfortunately, there are a lot of misconceptions about porosity. Some customers are worried that that a ring will catch on it or that it will cause poor ring seal. This is typically not the case. Kept in proper ranges rings will not be negatively affected by porosity. The holes fill up with oil creating a better ring seal and lowering the coefficient of friction. Some bore coating techniques actually rely on deliberately introducing holes in the surface of the bore to achieve this.
To understand porosity you have to understand what it is. Simply it is trapped gas bubbles from when the aluminium was poured. Most foundries have technologies that reduce this porosity but as a rule the deeper you go in a casting the more porosity you will find. That is why cylinders that are bored oversize are more susceptible to porosity than stock replates.
Welding can also introduce porosity. As a rule, everything is done to eliminate it but, depending on the material or its age, it may not be possible.
Another question is why does it have porosity on it now when it did not before? There are two reasons for this. Our plating process chemically removes material to allow the plating process to work. That etching sometimes exposes new porosity just under the machined surface of the bore. Our plating fills in most of the porosity but some of the bigger pits may be leftover.
The most important factor about porosity is that generally it is nothing to worry about. Every OEM has a specification on allowable porosity and we won’t normally send a cylinder out that would not pass their typical specification. After all, the piston and ring/s in a two-stroke engine are perfectly happy to pass the ports hundreds of times every second, so why would there be a problem with holes in the surface that are far smaller?
In very rare cases where it might be necessary to ship a cylinder that is perhaps marginal we would call our customer before shipping to discuss the issues.