Tech: Block Preparation And CNC Machining With CNC-Motorsports

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Building an engine is possibly the most difficult task many enthusiasts may ever attempt; from first-timers to those who do it regularly, the challenge of taking pieces of metal and turning them into a harmonious, well-oiled machine (pun intended) is incredibly rewarding. But if one doesn’t start with a solid plan in-hand, and an understanding of not only the parts and pieces required but the machining processes and why they are required, it can be an exercise in abject failure, and in fact ruin someone’s pocketbook for years in the process.

That’s where Clint Anderson and the guys at CNC-Motorsports can help. Opened in 2000, CNC-Motorsports offers everything from block machining to parts sales and the experience to help you make it all come together to build the project you desire.

The company has invested in the equipment and employs highly-trained manpower to take your engine block from a non-machined hunk of metal to a ready-to-assemble bedrock for horsepower construction.

“People expect to buy a new block and have it be perfect out of the box, but there’s a lot of machine work that’s required to get it ready for use,” says Anderson.

CNC-Motorsports' Rottler F68A CNC mill needs to have accurate locations in order to machine the block's surfaces properly for the builder. This probe maps out the deck surface and the bore locations from the factory blueprints.

The block in these photos is one of Dart‘s Big M Chevrolet-style big-block castings, but these operations can be (and should be) performed on any engine block that’s going to see severe duty, be those race days, high-performance street use, or any other usage where maximizing performance is important.

Anderson explains that starting with a solid base that has been machined properly makes the rest of the engine build an easier task. In other words, block preparation is key.

Checking the deck height with the CNC Machine probe before machining

The Foundation Of Performance

Blueprinting an engine is an age-old process, where each critical dimension is measured, recorded, and most importantly, matched; the point of doing so is to ensure that the parts you’re using and their tolerances are designed to fit together “just so” to provide maximized performance as the engine is used. It’s incredibly expensive and time-consuming to properly blueprint an engine — from matching piston weights and sizes down to the smallest dimension to matching ring gaps to the ten-thousandth of an inch, the process is such that it’s not for the faint-of-heart.

Boring the block to the proper finished dimension.

Boring the block to the proper finished dimension.

Blueprinting an engine block is the beginning step, and shops like CNC-Motorsports use the factory’s dimensions and recommended specifications to machine the engine block and properly reflect those elements.

How does this help the enthusiast? Regardless of who eventually assembles the engine block in question, the vital dimensions of the block will be exactly to the specifications recommended by the manufacturer to achieve improved performance.

By starting with the specifications provided by engine block manufacturers like Dart, Brodix, and others, the machine is able to finish the engine block as close to perfect as is possible. These companies do not ship their engine blocks in a fully-machined state — the process is left up to the machinist as various types of parts require fitting on the spot, like pistons, lifters, and engine bearings, to achieve those recommended specifications.

When machining, the fasteners that will be used in the build should be installed, and torqued properly. CNC-Motorsports prefers to use this extreme pressure lube.

“Blueprinting not only puts dimensions such as bore center-line and deck angle where the OEM intended, but also assures specifications such as deck height and bore diameter correspond with the specific aftermarket components used,” says Anderson.

Things To Fix

Valvetrain Geometry

It’s also critical to get the lifter bores corrected to the blueprint. CNC-Motorsports not only checks the bores to ensure they’re laid out properly, they also correct them if necessary, then hone the bores to the lifters that will specifically be used in the engine. The lifter bores can also be bushed if necessary; this is often done on high-level race builds. The tops of the bores are also chamfered to remove any burrs from the machining process, and ease lifter insertion.

When blueprinting a block, there are several operations that take place to ensure the block is ready for assembly when it’s removed from the CNC machinery.

Basically, the entire point of the blueprinting process is to ensure that the block is equally machined everywhere possible, from the front to the back to the deck surfaces, mains, and even the lifter bores.

One of the most critical measurements are the main bore dimensions from front to rear — are they correct, straight, and true? CNC-Motorsports checks and corrects the alignment of the main housing bores where required, in order to provide a clean-running surface to ensure the crankshaft can spin freely from 0 rpm all the way up through redline without fear of binding in the bores.

Another important measurement that’s corrected during the CNC machining process is the square-up of the bellhousing surface to the crankshaft centerline. Ensuring that this dimension is correct will prevent potential issues with clutch release in a manual transmission vehicle, or torque converter problems with an automatic transmission in place.

This particular block had stroker clearance engineered in right from the outset; cleaning up the bottoms of the cylinders for crankshaft and connecting rod clearance, as well as the pan rails, simplifies the process for the builder, as this type of machining takes forever when done with a hand grinder. The CNC machine makes quick work of the operation. Tuning up the deck surface to the required dimensions is also part of the block blueprinting process; not only will they set the deck height to the user’s exact parameters, they also square up the deck surfaces to the oil pan rails and one another.

Machining in clearance for a stroker crankshaft. In the case of this Big M block, the bottoms of the cylinders require notching for the connecting rod on both major and minor thrust sides.

Optimizing The Block For Assembly

All bolt holes are chamfered to allow for easy fastener insertion.

All bolt holes are chamfered to allow for easy fastener insertion, as well as many other locations on the block’s surface.

Chamfering various surfaces on the block is important; by setting up what could be hard edges with a softer, smooth transition, it eases the process for the builder.

Chamfering the top of each bore ensures that the piston rings won’t be damaged during installation by catching on a sharp edge, while chamfering the bottom of the bore ensures that there’s no possibility of the piston skirts acquiring any nicks or gouges as they rise and fall in the bores.

The edges of the areas where the block is clearanced for the stroker assembly are also chamfered to prevent hard edges in those spots where the mill has made a pass.

Freeze plug holes and the rear camshaft hole are also chamfered to ease the process of installing the plugs for those locations, while the distributor hole not only sees the chamfering bit but also a ball hone to eliminate any high spots in that location, easing distributor installation and preventing any issues down the line.

People expect to buy a new block and have it be perfect out of the box, but there’s a lot of machine work that’s required to get it ready for use – Clint Anderson, CNC-Motorsports

“Compared to the previous machining operations chamfering may seem a bit minute in comparison, but just like all other aspects of blueprinting it is of paramount importance in assuring the customer smooth assembly, maximum performance and longevity,” Anderson explains.

Chamfering operations continue on all bolt holes and oil galley holes; in the former’s case to ease fastener installation and prevent stripping out the threads, and in the latter’s case to clean up any potential issues that can affect oil flow and pressure.

The oil galley holes, along with the head and main bolt holes, all receive taps to clean out the threads from any potential machining debris, as well as to ensure the threads are in tip-top shape and will maintain torque readings as the engine is built.

Special attention is paid to the cylinder bores and their finish. Anderson consults with the piston and piston ring manufacturer to determine the required bore dimension for each particular engine, then mounts a set of torque plates to the decks and hones the block to the correct final bore size. The process is then verified with a profilometer, for a number of reasons.

The profilometer ensures that the bore has been machined to provide the correct finish, allowing the rings to seat quickly and completely so that the engine doesn’t burn any oil. More specifically, the profilometer measures the bore finish RA, or roughness average, which is measured in microinches, or millionths of an inch (one microinch is one millionth of an inch, or .000001-inch.)

Measuring and correcting the main bearing saddle dimensions is another step in the process; this operation ensures that the crankshaft will spin straight and true.

“We have general guidelines regarding profilometer values from the ring manufacturer, but we also acknowledge the need to adjust these values to suite the demands of more critical applications, such as with forced induction or nitrous use,” Anderson explains.

The profilometer has a diamond-tipped stylus that measures the peaks and valleys created by the honing stones. Some of the other measurements taken by the profilometer include RPK (peak height), RVK (depth of the valleys), and RK (average core roughness based on the previous two measurements). The measurements are compared with those provided by the ring and block manufacturer’s specifications to determine whether the block has the correct cross-hatch depth, the load-bearing area to support the piston rings, and whether it’s smooth enough to minimize ring wear as the ring seats in.

The profilometer is used to measure the machined finishes on all surfaces; this is key to ensure gasket retention and proper ring seal, among other things.

The profilometer is used to measure the machined finishes on all surfaces; this is key to ensure gasket retention and proper ring seal, among other things.

Wrapping Up

It’s not as simple as purchasing a new block, cleaning out the shipping goo, and starting the assembly process. Regardless of whether a project includes a brand-new block like the one shown here, or a seasoned OE casting, checking and correcting the surfaces ensures that the finished product will perform as intended. Reducing the potential for gasket failures and optimizing ring seal are paramount for performance; adding clearance for stroker crankshafts, installing lifter bushings and ensuring that the main saddles are of the proper dimension are also critical operations that must be performed.

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About the author

Jason Reiss

Jason draws on over 15 years of experience in the automotive publishing industry, and collaborates with many of the industry's movers and shakers to create compelling technical articles and high-quality race coverage.
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