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The Long (Over)haul

Part II: CNC starts to shine as the metal chips fly.

Dave Verna - January 17, 2013 10:00 AM

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You can clearly see how much rusty material came out as we cleaned the threads. We used a thread chaser from ARP, or cleaner, and not a tap. A tap creates threads and a chaser just cleans them up.

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The caps have to be fitted to the block and need .002-inch clearance under the splayed portion in order to maintain pre-load to the block. A cap grinding fixture is used to sweep the cap across a grinding wheel to obtain the correct fit to the block.

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Once the caps are set, it is time to put the block on the CNC and get the extra holes drilled and tapped. The benefits of CNC show as we drill, countersink, and tap on the correct angle each time.

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The caps come smaller than needed to fit them specifically to each block. A few thousandths are left to hone them afterwards. The uniformity of these steps assures accurate oil clearances when we start to assemble the engine.

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The Rottler F67A takes the information it gained from the Reinshaw Probe and calculates where it needs to bore to center each cylinder correctly. The block will be rotated on the fixture to get to the passenger side.

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After the first pass, you can clearly see the amount the deck was out of square to the crank centerline. The darker side has yet to be cut. Several passes will be made to keep things uniform and correct. We elected not to cut the top of the block so we would have tighter fitting intake manifold rails.

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We are using a Cometic MLS gasket for this build. The multi-layered steel gasket loads the head and block differently than a typical fire ring composite gasket. The block should have the mains, and both heads torqued down to achieve the proper distortion the block will receive when it will be assembled and fired up.

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Since we can’t hone through the head we use a spacer called a torque plate. If you did not torque down the head, mains, or torque plate you would get perfectly straight bores, until you assembled the motor and torqued everything down.

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The Sunnen CV-616 is adjusted to achieve the proper rotational speed, stroke speed, and number of cycles per stone configured to obtain a uniform, equal hone on each cylinder. Honing fluid keeps the stones and block cool for uniformity.

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Several grit stones are used to create the correct pattern. We also elected to plateau hone as a last step. From left to right we have the plateau, 280 grit and 220 grit. We started off with the 220 grit stone and began the honing process. The 280 follows next.

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Last month we took you through some of the parts and detailed the start of the process that takes place when you drop your block off at the machine shop.

Usually, we just drop off a bunch of parts. Then we wait patiently for the call to pick the parts up when they are done. A lot takes place during that time and the process is the same no matter what make or year engine is being used.

We have to elaborate a bit on just dealing with the block this month – CNC machining is where it’s at! When the block we are using was worked on previously, CNC was in its infancy. In the last 20 or so years, CNC has really brought precision to a new level. These machines are accurate in reading and creating parts within the ten thousands of an inch (.0001-inch – that’s four decimal places!). The set-up that Fonse Performance has is much more accurate than what was previously done to the block. There was no reason to be skeptical, as the Reinshaw probe sets the record straight as to exactly where the block is, measurement wise.

The blueprinting of the block goes a long way to determine the overall performance of the engine as well as the assembly process. Once you know everything is square and true, you don’t have to worry as much about the differences in clearances from front to back.

Be sure to keep up as we will be getting into the specifics that you need to look for when electing to get your next engine rebuilt.

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