Preassembling the block is the smart way to build a performance engine. Any experienced engine builder will likely tell you that the build of an engine actually begins with the prefit (or preassembly) stage, a process also known as "blueprinting" the engine. Following this procedure allows the engine builder to preinstall and test many of the engine components -- including the rods, rings, pistons and crankshaft -- to ensure proper fit and operation. To achieve maximum engine performance, an engine-balancing is also performed. Although the steps in prefitting a performance engine are more critical than on an ordinary engine, the preassembly process is an important one for any engine rebuild. As mentioned in an earlier segment, it's important to realize that building a kit car is not simply a linear process of assembling pieces. In a number of stages, disassembly and reassembly are required -- and in some cases that process may be repeated more than once. This principle is most pronounced in the preparation and restoration of the engine prior to its installation on the chassis. The process of dismantling and rebuilding parts -- particularly some of the moving parts within the engine -- make possible the kind of optimization that can ultimately produce an engine capable of performing at peak operating specs. The 427 engine we selected for our CobraB. replica is a high-performance racing machine that can deliver a tremendous amount of thrust when it's operating at peak efficiency. In this segment we present the preassembly steps that the engine-rebuilder performs to help fine-tune the engine and get the maximum power that it can safely offer. The added benefit of this process is that by the time it's done, the person who has performed or overseen the engine work (ideally the builder of the kit car) will be thoroughly familiar with all aspects of this engine's operation. Though we used a 1966 block, a majority of the parts we used in the rebuild are new parts that, at manufacturer's tolerances, may not work optimally in this application. Those potential problems need to be eliminated early. Expert engine-builder Butch Capps, who has built half a dozen Cobra replicas, offers some insights on what should go into a thorough, meticulous prefit. Important: Preassembling and balancing a performance engine is a complex procedure with literally hundreds of steps, many of which require highly specialized equipment and tools. This is a job best left to professionals, preferably ones with experience at rebuilding these particular type of high-performance engines. There is a sizable difference between the precision work that goes into rebuilding a racing engine and ordinary mechanical maintenance and repair that's done on a normal automobile engine.
Pistons, Rings and Tolerances
The prefit process begins with preinstalling and test-fitting the rings in the cylinders. Individual rings are installed, moved around from one cylinder to another, and carefully checked along the way until the suitable matches and desired tolerances are achieved. During this "shell game" process of moving rings around, the tested rings are numbered and laid out in a grid so that they can be accurately referenced for reinstallation into the proper cylinder during final assembly (figure A). To preinstall the rings and achieve proper tolerances, each ring is gauged inside the cylinder to check the gap. First, the ring is placed into the cylinder (figure B), followed by the corresponding piston. Making certain that the piston is inserted squarely into the cylinder bore (figure C), it is then pushed down to its operating range inside the cylinder. The piston is then removed from, and a feeler gauge is used to check the end-gap in the ring while it is in position inside the cylinder (figure D). If the ring-gap and tolerance is not exactly what is desired, the ring can be removed, reinstalled on another piston and checked again. The tolerances must be precise, as disparity -- even in measurements as tiny as 1/1000"-- can adversely affect performance. There is no room in this particular process for a variance scale to be used. If done with the proper amount of care, this cylinder-by-cylinder, ring-by-ring prefit is a very time-consuming process. It is a process, however, that becomes easier as the engine-builder gains experience. When the exact tolerances for a cylinder are achieved, the rings to be used in that cylinder are carefully removed, marked to indicate what cylinder they will go back into (figure E) and carefully stored in individual containers.
Preparing Connecting -Rods and Bearings
To prepare and prefit connecting rods, each rod is disassembled and washed very thoroughly in a parts-washing solution (figure F) to remove any dirt, grease or residue from the machining process. Once cleaned, each rod and cap is given a designated number and marked with a permanent marker (figure G). This identifies each rod/cap combination and helps ensure that the parts won't accidentally get interchanged. The rod-assembly is then reassembled. New performance bearings are installed, along with the rod cap and bearing shell (figure H). At this point the connecting bolts on the rod are lubricated with a thread-lubricant to help ensure that each bolt can be torque-tightened to exactly the proper specifications during reassembly. The rod-assembly is then placed in a vise (with a shop rag being used to protect it from scratches), and the bolts are tightened to proper torque specifications using a torque wrench (figure I). It's important that the rod bolts be cycled at least three times before they are actually put into use, so this part of the process will be repeated more than once. Once assembled and torque-tightened to spec, an inside-micrometer is used to measure exactly the inside diameter of the bearing (figure J) so that it can be compared to the actual size of the corresponding crankshaft journal. The measurements are recorded in a logbook. The difference between those two measurements is known as the oil-clearance . On a performance engine such as the 427, an oil-clearance of around 2/1000 is considered optimum. (The measurement determined using the inside micrometer can be verified with an outside micrometer.) The engine's factory-recommendations for these specs are consulted (figure K), but keep in mind that because the bearing manufacturer may have recommendations that differ, it's advisable to refer to their literature as that may be the preferred guideline. A micrometer is then used to measure the main journals of the crank and determine exact diameters (figure L), and the measurements are recorded. This process detects any significant wear to the crankshaft and indicates whether it will need to be resurfaced or repolished. Bearing sleeves are mounted in the main bearings (figure M), and main-bearing caps are installed in the engine block. The main-bearing caps are first set in place and the bolts hand-tightened (figure N), then each bolt is individually torqued to spec using a torque wrench. In fact, in this exacting application, each bolt is step-torqued. This means that each bolt is first tightened (with the torque wrench set to a lower setting) to a particular interim spec, then each is tightened again (with the torque wrench on a higher setting) until the bolt reaches its full torque spec. Experienced mechanics utilize two separate torque wrenches so that each bolt can be step-torqued without the mechanic having to reset the torque wrench repeatedly. In this case, the first pass with the torque wrench will tighten each bolt to a 50-pound interim spec, and the second (final) pass will bring it to its full 90-pound specification. Once each cap is installed, the main-bearing diameter is measured using a micrometer (figure O). As before, the difference between the inner diameter of the main bearing and the size of the main-crank journals gives the oil clearance for the main journals. For this engine, ideal oil clearance on these positions is about 2/1000" to 3/1000". The measurement for each inside-micrometer reading may be verified using an outside micrometer (figure P). A moly-based lubricant is applied to the surfaces of the bearing shields (figure Q) to prepare for the step in which the crankshaft will be test-fitted in place. This step will allow the crank to be checked to make certain that it turns freely in the main bore. With the crank in place, the bearings will be torque-tightened once again. In the absence of engine oil, the lubricant ensures that neither the bearings nor the crank journals are damaged when the crankshaft is turned for testing.
RESOURCES :
cobracountry.com
Premiere online resource for all things related to CobraB. reproductions. Established by enthusiast Curt Scott, and maintained by Crown Communications of Santa Clarita, CA, the site contains news, articles archives, a directory of Cobra-replica specialists and manufacturers, aftermarket sources, announcements of upcoming shows and rallies, listings of Cobra clubs, links to other Cobra sites, and more.
Cobra Country
Website: www.cobracountry.com
Unique Motorcars
Unique Motorcars
(Unique Cobra Specialists)
Manufacturers of high-quality Cobra® replicas since 1977, Unique Motorcars also maintains an informational and well-illustrated web site providing information about their Cobra reproductions and related products. Assembly manuals for their kits (including the one used in DIY's Build A Kit Car workshop) are available on their site.
230 E. Broad Street
Gadsden, AL 35903
Phone: 256-546-3708
Web site: www.uniquecobrareplicas.com
Information also available at:
www.cobracountry.com/unique
The Cobra Story: A Man, His Dream and His Automobile
Author: Carroll Shelby and John Bentley
1965, Trident Press
Carroll Shelby website
Shelby Cobra: The Shelby American Original Color Archives 1962-1965
Model: 0879387572
Author: Dave Friedman
(1994)
To order this title from Amazon.com, click here.
Motorbooks International (MBI Publishing)
Essential AC Cobra : The Cars and Their Story 1962-67
Model: 1870979850
Author: Rinsley Mills
(1997)
To order this title from Amazon.com, click here.
Motorbooks International (MBI Publishing)
Shelby Cobra Gold Portfolio, 1962-69
Model: 1855200236
Author: R.M. Clarke
(1990)
To order this title from Amazon.com, click here.
Motorbooks International (MBI Publishing)
Carroll Shelby's Racing Cobra
Model: 0850454573
Author: Dave Friedman and John Christy
To order this title from Amazon.com, click here.
Motorbooks International (MBI Publishing)
Shelby's Wildlife: The Cobras and Mustangs
Model: 0879380454
Author: Wallace A. Wyss
To order this title from Amazon.com, click here.
Motorbooks International (MBI Publishing)
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