a Subaru rally car block for Darton thick wall cylinder sleeves. We’re using our 5-axis CNC vertical machining center, which has precision digital probing, so we can not only locate bore center accurately, but also tilt the block in left/right (B-axis) and/or font/rear (A-axis) to machine the bores perfectly perpendicular and aligned with the crank bearing bore… what we call “square to the world” and aka “blue printing”.The first operation is to bore out the original sleeves, as well as some of the upper block deck, to leave a counterbored step. The Darton sleeves will fit precisely when pressed into place, using a small shop-built hydraulic. The precision of the fit is important for not only locating and holding the sleeves in place, but also to ensure proper heat transfer while the engine is running. If its too tight, broaching may occur while pressing a sleeve in; this can not only shift the sleeve off center, but also create a “hot spot”, where the sleeve cannot transfer heat evenly and may result in a scuffed piston. If the sleeve fit is too loose, it will not transfer heat efficiently, and eventually wallow out the block. Loss of power, blown head gaskets, etc are signs of this. So, this is why we use a very rigid CNC machine to precisely bore the blocks and sleeve, rather than a small “Bridgeport” style mill… we do it right the first time, every time.After the sleeves are installed, we put the block back on the CNC machine, so it is “square to the world” again, and deck the gasket surface so it too is perfectly align with the bores, flat, and has the proper Ra/Rz surface finish.Not shown is finish boring and diamond honing the sleeves to mate perfectly with the pistons. For more info on block boring and other performance engine work, including 5-axis CNC head porting and flow bench testing, please call Automotive Machine & Supply, Inc at 1-800-210-8675.
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