The Reader's Digest version is that the iron GM sleeves are cut out of the block and replaced with stronger sleeves capable of being bored out to sizes that are impossible with the factory offerings. The three ingredients for this are a donor block, a set of Darton MID sleeves, and someone with the skill to meld them together. Two-thirds of our recipe was handled leaving us needing to source a block. The good news is that since it was going to be heavily machined, we didn't need a new one. In fact, used is better since it's cheaper, and that helps offset the cost of the sleeves and labor. We found an old LS2 block with scored cylinders, but according to Steve, various LS blocks would be suitable candidates. As he explained, "I like the LS1 block because it has solid main webs with no cast-in breather holes. I will install the large bore, 4.200-inch sleeves, in the LS1 blocks but not the LS6. The LS6 blocks have the cast-in breather holes, making them too weak in my opinion, to bore out for the larger bore sleeves. Now, the Gen IV blocks also have breather holes, but on these blocks the floor of the coolant section has been raised leaving more material for the sleeves to sit on with much less chance of cracks developing than on the LS6 block. My preference in order on the Gen IV blocks is LS2, LS7, and then the L92/LS3/LSA/LS9 blocks, which are all the same in terms of sleeving. The LS7 sleeves are not cast into the block but pressed in at the factory, so they machine out, leaving sufficient parent aluminum to machine for the liners. The remaining blocks have cast-in liners with a larger outside diameter than on the LS2 blocks, which in itself isn't a problem. The problem is the lousy placement of the sleeves at the factory when the blocks are cast. Sleeves are not at 4.400-inch centers, or are shifted slightly to the front or rear or side to side, in the block, which gives me headaches during machining. I usually end up juggling the bores slightly away from the crank centerline axis to get the old sleeve completely machined out to the parent aluminum of the casting. This is especially important with MID where I need a nice smooth surface for the O-rings to seal against, or else coolant will leak into the crank case."
The process of installing sleeves is an exercise in uber-tight tolerances. Bore centers must be held to within +/- .0005 inch. The bores themselves, for sleeve fitment, must be held to +/- .00025 inch, or a quarter of a thousandth of an inch! This is why having the right tools and skills are imperative. The CNC mill needs to be regularly qualified to make sure the backlash tolerances are within specification. Steve also opined on the importance of using a machine with flood coolant since the heat associated with the machining can screw up a block beyond repair. According to Steve, "Our shop is air conditioned, which I use in the summer to keep the temperature constant when machining. Otherwise, that twenty or so degree rise in temp from morning to afternoon will cause the block to expand in length and height. I've had blocks in here from shops doing the installs dry that were six-thousandths out of spec. In these cases the block can't be saved and the only option is to pull the sleeves and install them in a correctly machined block. I have done this for close to a dozen folks over the past few years. These shops did a pretty good job of screwing up the MID name when we first came out with the design for the LS blocks. I gave up my time fixing these screw ups for free to show people that when the work is done correctly there are very few issues with the MID blocks."
If all of this sounds labor intensive, that's because it is. The basic charge to machine, stress relieve, install the sleeves, and deck the block is $1,175. Add in $100 to bore the block to within honing range and another $75 if you want the notches cut for rod clearance. The sleeves retail right around $1,300, so if you add it all up, you're at $2,650 for parts and labor. Well, plus a block. But good used donors can be found for $400 give or take. That means the total for a big-bore aluminum small-block would be in the three-grand neighborhood. Not cheap but very competitive to the aftermarket LS blocks currently on the market and less than two-grand more than a comparatively small 4.065-inch bore stock LS3 block from GM.