An OEM (original equipment manufacturer) mass-produced, cast block typically requires more corrective measures in a blueprinting approach, simply due to the nature of wide-tolerance mass-production practices. By contrast, today’s high-quality aftermarket performance blocks, such as those produced by Dart, World Products, RHS, Brodix, and others have CNC-machined bore locations and angles from the very start. In essence, they are already “trued,” but still need to be checked, typically requiring only deck height and bore diameter machining to the required sizes. OEM blocks, on the other hand, generally require more extensive machining in order to achieve the design geometry.
Aftermarket blocks from Dart, World Products, Brodix, and others are CNC machined to much tighter tolerances and quality standards than cast OEM blocks. (Photo Courtesy Dart Machinery)
Generally speaking, leading-brand aftermarket high-performance blocks are made with superior materials and are made in lower numbers than OEM blocks, and therefore benefit from a higher level of attention to detail. An aftermarket block costs more, but you’ll likely be faced with less corrective machining labor.
The manufacturing process for OEM blocks have time and technology constraints. As a result, factory blocks have a range of tolerances that can and usually do deviate from the initial design. In general, the older the block, the wider the OEM tolerance ranges are. Late-model blocks generally have tighter machining tolerances thanks to advanced CNC technology. However, OEM blocks are produced in mass quantities so, considering machine calibration and tooling wear factors, you’d still be hard-pressed to find an OEM block that’s precisely accurate to the design specifications. That’s not to say that they’re unusable, but they can always be corrected according to the design specifications and performance and durability can be improved.
This drawing of an LS block clearly shows how the main bore centerline is used as the primary reference point. (Illustration Courtesy General Motors)
To accurize the block, make sure that the main bore is not only straight and round, but also exactly in plane front to rear, and corrects (where needed) the main bore height location. When accurizing the block, you are correcting all critical areas to eliminate the tolerance range that is otherwise deemed acceptable in mass-production runs.
When the average enthusiast claims that his or her engine has been “balanced and blueprinted,” in reality it usually means that the crank has been balanced. True blueprinting involves much more attention to detail (and subsequently higher machining/prep costs) than merely honing the cylinders, performing a fresh cut of the decks, and balancing the rotating assembly. When someone claims that his engine was blueprinted, and, when asked how much he spent, he or she mentions something in the area of, say, $1,000 to $3,000, it’s obvious that a blueprint has not been performed. Blueprinting requires skill and expertise, and is very time and labor intensive. That doesn’t come cheap.
Before any machining takes place, flaw check the block to make sure that no cracks are present. Any cracks or damage must be addressed by repairing the damage or finding another block. Also soncially test each cylinder wall to verify wall thickness and determine whether it can be overbored. This needs to be done before any machining and crack repair is done.
Minimum acceptable wall thickness may vary depending on the block, but in general, you should have at least .200-inch thickness after boring and honing. If the walls are too thin, they can distort enough to result in ring blow-by or even eventual cracking. Keep in mind that a too-thin cylinder wall may be corrected by overboring and installing a quality cylinder sleeve (again, this can vary depending on the type of block).
Once the block has been deemed serviceable, the first order of business involves checking and correcting the main bore if needed. In addition to making the main bore straight and round, it needs to be centered per its design. Specialty guide fixtures are available, but today, with the increasingly common use of CNC machining technology, the appropriate software can handle this duty. Once the main bore is verified as correct, all remaining machining basically uses the main bore centerline as the reference point.
When the block has been deemed serviceable, the first order of business involves checking and correcting the main bore if needed. Or, the block can be machined to correct any anomalies on a CNC-milling machine (with generic programs for specific blocks, bore sizes, and deck heights, or by custom-programming by the CNC operator). Accurizing fixtures are essentially precision templates, or guides, that establish proper geometric centerline locations and angles. If a shop isn’t equipped with a CNC machine, these fixtures allow precision machining using traditional shop equipment boring machines, multi-axis milling machines, etc.
The sensor probe is slowly run along the cylinder wall (in a number of clock positions and from top to bottom) while monitoring the gauge readout.
Before measuring and/or align honing the mains, all caps must be fully installed (at spec’d torque value). If the block has additional side bolts (as in the GM LS block, or vintage-Ford FE side-oiler block, for instance), these must be installed as well. Fully tighten the primary main cap fasteners before tightening the cap side bolts. This is critical: The side bolts must be fully tightened to spec before measuring or honing, since they do affect main bore geometry.
If you plan to use main studs (instead of bolts), install the studs finger tight, then add a small bit of preload. Be sure to follow the stud maker’s specs! Do not overtighten the studs into the block. There’s no need to (since nut tightening achieves clamping load), and by overtightening studs, you can easily create a splayed stud problem. For instance, do not double-nut the studs and try to tighten to the main cap torque spec.
Here are the steps to perform a flaw detection to check for cracks:.
Use a sonic tester to measure and record all cylinder wall thicknesses. A minimum of three height locations and at four clock positions should be used. Record the data and use it for proper overboring.
Inspect