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LS3-Based 418ci Engine Build - Big Power, Small Budget

Texas Speed Builds An Affordable 418CI LS Engine Part I: Assembly
April, 2009
By Randall Allen
Photography by Randall Allen
418Ci Engine Assembly 418Ci Engine
418Ci Engine Assembly Aluminum Block
A 6.2L (376ci) aluminum L92/LS3... 
   
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418Ci Engine Assembly Aluminum Block
A 6.2L (376ci) aluminum L92/LS3 block is the foundation for both the 418 and 427ci configurations. In stock form, the block carries a 4.065 bore and 3.622 stroke. To create a 418ci LS3, the bore will be opened up to 4.080 and the stroke elongated to 4.0 inches.
418Ci Engine Assembly Gm Ls3 Main Caps
GM LS3 main caps are torqued... 
   
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418Ci Engine Assembly Gm Ls3 Main Caps
GM LS3 main caps are torqued to spec in order to maintain load conditions on the block before machining operations begin. The block is then set up on a Rottler F-67A three-axis CNC machine by Heber Ramirez of Sunset Racecraft to both deck and bore it. The deck height is measured to determine how much machining is necessary to ensure a flat deck. According to Ramirez, "A production LS3 block has a stated deck height of 9.240 inches, and traditionally we see 0.003-0.005 of variability across the deck surface." In this case, the variability was quite low, so a total of three passes were taken at 0.001 per pass, and the deck height was set to 9.238 inches.
418Ci Engine Assembly Measure Bores
After the bores are measured... 
   
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418Ci Engine Assembly Measure Bores
After the bores are measured for size to begin the boring process, the critical next step is to determine the center of each bore, as shown. If the top of the bore isn't what GM states, then each cylinder is carefully measured with a dial-bore indicator to determine the actual bore from top to bottom.
418Ci Engine Assembly Boring Head
The boring head is attached,... 
   
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418Ci Engine Assembly Boring Head
The boring head is attached, and the CNC machine is programmed with the bore center, deck height, boring stroke, and the amount of material to achieve the desired bore. Since the bore specified was 4.075 inches, a single pass on each cylinder brings them to the final spec. Dial-bore gauge measurements are taken to ensure the desired bores are obtained.
418Ci Engine Assembly Carbide Bit
In order for the longer connecting... 
   
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418Ci Engine Assembly Carbide Bit
In order for the longer connecting rods (6.125 inches versus stock length of 6.098 inches) to clear the bottom of the bore with the planned crank/rod combination, a carbide bit is used to remove 0.006 inch of aluminum at the bottom edge straight across each bore, after the block is mounted to a traditional engine stand. Once the aluminum is removed, a sanding roll is employed to smooth and radius each cut to ensure that a piston won't nick the area on the downstroke.
418Ci Engine Assembly Cylinder King Honing Machine
At the Sunnen CV616 Cylinder... 
   
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418Ci Engine Assembly Cylinder King Honing Machine
At the Sunnen CV616 Cylinder King honing machine, machinist Rodney Pope takes over. A metal bar (fixture) is bolted through the crank journal, and a bank of cylinders is set up in preparation for machining.
418Ci Engine Assembly Clean Block
After cleaning the cylinders,... 
   
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418Ci Engine Assembly Clean Block
After cleaning the cylinders, a cone-shaped sanding disc is used to add a slight bevel at the top of each one, and the block is cleaned again.
418Ci Engine Assembly Piston Bore
The final bore size that the... 
   
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418Ci Engine Assembly Piston Bore
The final bore size that the honing process achieves is a combination of the piston bore (4.075 inches) and the piston-to-wall clearance (0.0050 inch). A BJH 4.080-sized torque plate is utilized with a Cometic MLS gasket, and the 10 15mm bolts are torqued in sequence to 60 ft-lb.
418Ci Engine Assembly Rougher
To achieve a hone of 0.005... 
   
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418Ci Engine Assembly Rougher
To achieve a hone of 0.005 inch, a rougher set of stones is affixed and 0.003 inch is taken off each cylinder. A second pass takes off an additional 0.001 inch, before the stones are changed out to the final finish hone, and the bore is verified to be 4.080. According to Rodney, "Honing stones range in grit from 70 to 280, and the final finish is dependant upon the type of rings that will be employed."

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