I'll guess you're talking about a sleeve. Basically you get a steel sleeve with the inside diameter you need, bore a hole in the aluminum of a slightly small diameter then the outside of the sleeve and then press the sleeve into the whole. They make collapsable sleeves that are made for just this operation in mind. They use them for cylinder head alignment on cars a lot. You just gently tap them into the cylinder head bolt hole with a hammer. The bolt then fits down the middle of the sleeve and the sleeve lines the head up on the cylinder block.