Nearly everyone is familiar with the automotive term "balancing," but most associate it with the familiar wheel-balancing that's done when tires are changed or rotated. Engine balancing, though less familiar, is essentially the "flip-side" of engine blueprinting or prefitting. It is a process that is an essential part of building a high-performance engine. The process takes place at a machine shop where skilled technicians used specialized equipment to weigh engine components and adjust their weights as necessary. The objective behind balancing the engine is to achieve equality in weight among various moving components so that those parts don't "fight against each other" when the engine is working at operating speed. Balance among these parts (the pistons, in particular) reduces engine vibration, maximizes performance and increases reliability. A successful balancing ultimately results in a high-revving engine that is capable of running very smoothly. Balancing the Pistons Once the parts have arrived at the machine shop, each piston is weighed on a specialized scale (figure A) to determine which is lightest. This is a combined weight that includes the piston, wrist-pin, ring package and the retainers that secure the wrist-pin to the retainer. The weight of the lightest piston assembly will be used as a benchmark for adjusting the weight of the others.
The scale measures in grams giving precise measurements. The one used in our demonstration is an analog scale, but there are digital scales available as well. The machinist records the weight of each piston.
Once each piston is weighed, and the lightest of the eight is identified, small portions of metal may be removed from the other seven to approximate the weight of the lightest one. The metal is removed by carefully drilling out small portions of the piston on a drill press (figure B).
The drill press is used to make small indentations on the inner surface of the pistons (figure C) in order to match the weights of the pistons as closely as possible.
Balancing the Connecting Rods
The same process is used to balance the connecting rods, but with a few variations. The rods are weighed on the same scale that was used for the pistons, but a specialized mounting fixture is used to support one end of the rod while the other rests on the scale.
The rods being used in this engine were the original rods, not new ones. Each was reconditioned on the larger (bearing) end, outfitted with new bushings on the wrist-pin end and upgraded with high-performance fasteners. With all of these modifications, it is especially important to balance the rods in the same manner as the pistons. The amount of metal that was removed from the larger end of the rods during reconditioning can render them out of balance with one another.
The large end of the rod is weighed first (figure D), using the mounting unit to hold the smaller end suspended.
Once all of the rods are weighed and the lightest one identified, the technician grinds away small amounts of metal from the remaining rods (figure E) to approximate the weight of the lightest one. All rods are then reweighed, and their exact new weights are recorded.
After balancing the large ends of all eight rods, the mounting fixture is adjusted so that the large end can be supported while the small end of the rod is weighed (figure F). The same process of balancing is performed on the small ends of the rods.
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.
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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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