all castings (not spun) have some flaws in them.. I work in a machine shop full time and we only use 6061 or 7075 aluminum for that reason. (and that castings are expensive for parts you wont mass produce..) That small pocket is no big deal, it's outside the sleeve that will distribute pressure . If you're really worried about it, get a putty knife and fill it with some liquid aluminum like the stuff you use if you accidentally over-port a head or manifold, it might prevent stress cracks. I wouldnt lose sleep over it. I'd be willing to bet if you took that block down to nothing .005 at a time, you'd find hundreds of them.

 

Internal voids like this are present in all castings. These are not "air bubbles" created by a frothy mix going into the mold. These internal voids result when the molten metal cools down and contracts into a solid. Because the liquid takes up more volume than the solid, there always areas in a casting that get cut off to the flow of new liquid metal. Casting companies do a great deal of research in how to control the solidification process so that an area doesn't fully solidify and cut off downstream passages. When that happens you have a huge chunk that's just not there.

Like was mentioned before, if you machined it down 0.005" at a time, you'd likely find them everywhere. There's actually a pretty good chance there's another void like that just under the surface, but until you machine them off (or X-ray), nodoby knows the difference. Basically, if you think of any casting on a microscopic level, it has these voids everywhere. Its just the nature of any casting process.

 

Thanks alot guys very good input I will not be worrying about it anymore time to hit it with the 200 shot!