COMMENT ON MACHINE SHOPS AND REBUILT ENGINES
Automotive machine shops are different from conventional machine shops.
The machinery found in a conventional machine shop can take a piece of metal and make a part from it. Feature location, dimensional accuracy and surface finish are all important.
The machinery found in automotive machine shops is very specialized and used to create new wearing surfaces on worn parts. In general terms, automotive machinery cannot make the original part. Dimensional accuracy and surface finish are important, however feature location is not as important or controlled. Automotive machine shops also provide a variety of repair services such as cleaning, detecting and repair of cracks, installing thread inserts and welding.
In a conventional machine shop, the accuracy of the part being machined is directly related to the accuracy of the machinery doing the cutting, and the quality of the part can be determined by measuring it and comparing it to the drawing from which it was made.
In an automotive machine shop, previously machined surfaces are usually used for alignment. Accuracy is dependent on both the machinery and the location of previously machined surfaces, and can diminish with each succeeding rebuild. An automotive machinist is forced to make assumptions that previous rebuilds and machined surfaces are accurate. Unfortunately that is not the case for many engines.
A new Model A engine had very closely controlled feature locations, dimensions and surface finish. Over the years, and after many rebuilds (especially with modified and "improved" engines), the features are not necessarily in their correct locations.
During the Great Depression when Model A’s were just old used cars, many engine rebuilds were done by "shade tree" mechanics with substandard equipment. Ford factory rebuilds were held to high standards and dealer rebuilds using K. R. Wilson equipment were second best.
Take cylinder bore as an example. With a fresh casting, the manufacturer (Ford, or a conventional machine shop) will first machine the bottom of the cylinder block. Next, this bottom surface is clamped to a machine table and the cylinder is machined. The cylinder is perpendicular to the bottom of the cylinder block, which is clamped to the machine tool table. Please note that the bottom of the cylinder block also happens to be the crankshaft axis centerline, which means the cylinder will be perpendicular to the crankshaft. Accuracy is dependant on the machine tool.
After years of use and when a worn cylinder bore needs to be rebored, an automotive machine shop is called upon. To rebore a worn cylinder, the typical automotive machine shop will clamp a boring bar (the machine that rebores the worn cylinder) to the upper surface of the cylinder block, and then determine the position of the new cylinder bore by locating the new oversize cylinder bore from the unworn bottom portion of the worn cylinder bore. Next, the cylinder is bored and is perpendicular to the upper surface. The cylinder diameter is exact and surface finish looks great.
However, two important dimensions are not controlled. These are perpendicularity to crankshaft axis and cylinder location. What if in 1934, Uncle Clyde fixed a crack in the top of the cylinder block, resurfaced the upper surface of the cylinder block with a belt sander, and then rebored the cylinder perpendicular to this new surface that he created? How would a present day automotive machinist know this, and how could he compensate? Even if the upper deck is resurfaced, is the cylinder in the right place? Accuracy is dependant on a number of unknown factors.
Similar stories can be told regarding crankshaft axis miss-location in the crankcase, miss-location of connecting rod journals relative to the woodruff key at the front of crankshaft that determines valve timing, and flywheel flange runout relative to rear main journal diameter.
Balancing is making things equal. New engines have all of their moving parts individually balanced and there is very little crankshaft flywheel mounting flange runout so that any order of assembly will result in a balanced assembly. Rebuilt engines with crankshaft flywheel mounting flange runout must have the crankshaft and flywheel balanced as an assembly and match marked for proper reassembly. Switching flywheels from one rebuilt engine to another will result in an unbalanced assembly.
Since the accuracy and exactly what was done during previous rebuilds is unknown, it should be no surprise that two different rebuilt engines from the same automotive machine shop held to the same dimensional tolerances and using the same machinery can have two entirely different personalities. There is some truth to Jonathan Swift's adage, "You can't make a silk purse out of a sow's ear"
Automotive machine shops are different from conventional machine shops.
The machinery found in a conventional machine shop can take a piece of metal and make a part from it. Feature location, dimensional accuracy and surface finish are all important.
The machinery found in automotive machine shops is very specialized and used to create new wearing surfaces on worn parts. In general terms, automotive machinery cannot make the original part. Dimensional accuracy and surface finish are important, however feature location is not as important or controlled. Automotive machine shops also provide a variety of repair services such as cleaning, detecting and repair of cracks, installing thread inserts and welding.
In a conventional machine shop, the accuracy of the part being machined is directly related to the accuracy of the machinery doing the cutting, and the quality of the part can be determined by measuring it and comparing it to the drawing from which it was made.
In an automotive machine shop, previously machined surfaces are usually used for alignment. Accuracy is dependent on both the machinery and the location of previously machined surfaces, and can diminish with each succeeding rebuild. An automotive machinist is forced to make assumptions that previous rebuilds and machined surfaces are accurate. Unfortunately that is not the case for many engines.
A new Model A engine had very closely controlled feature locations, dimensions and surface finish. Over the years, and after many rebuilds (especially with modified and "improved" engines), the features are not necessarily in their correct locations.
During the Great Depression when Model A’s were just old used cars, many engine rebuilds were done by "shade tree" mechanics with substandard equipment. Ford factory rebuilds were held to high standards and dealer rebuilds using K. R. Wilson equipment were second best.
Take cylinder bore as an example. With a fresh casting, the manufacturer (Ford, or a conventional machine shop) will first machine the bottom of the cylinder block. Next, this bottom surface is clamped to a machine table and the cylinder is machined. The cylinder is perpendicular to the bottom of the cylinder block, which is clamped to the machine tool table. Please note that the bottom of the cylinder block also happens to be the crankshaft axis centerline, which means the cylinder will be perpendicular to the crankshaft. Accuracy is dependant on the machine tool.
After years of use and when a worn cylinder bore needs to be rebored, an automotive machine shop is called upon. To rebore a worn cylinder, the typical automotive machine shop will clamp a boring bar (the machine that rebores the worn cylinder) to the upper surface of the cylinder block, and then determine the position of the new cylinder bore by locating the new oversize cylinder bore from the unworn bottom portion of the worn cylinder bore. Next, the cylinder is bored and is perpendicular to the upper surface. The cylinder diameter is exact and surface finish looks great.
However, two important dimensions are not controlled. These are perpendicularity to crankshaft axis and cylinder location. What if in 1934, Uncle Clyde fixed a crack in the top of the cylinder block, resurfaced the upper surface of the cylinder block with a belt sander, and then rebored the cylinder perpendicular to this new surface that he created? How would a present day automotive machinist know this, and how could he compensate? Even if the upper deck is resurfaced, is the cylinder in the right place? Accuracy is dependant on a number of unknown factors.
Similar stories can be told regarding crankshaft axis miss-location in the crankcase, miss-location of connecting rod journals relative to the woodruff key at the front of crankshaft that determines valve timing, and flywheel flange runout relative to rear main journal diameter.
Balancing is making things equal. New engines have all of their moving parts individually balanced and there is very little crankshaft flywheel mounting flange runout so that any order of assembly will result in a balanced assembly. Rebuilt engines with crankshaft flywheel mounting flange runout must have the crankshaft and flywheel balanced as an assembly and match marked for proper reassembly. Switching flywheels from one rebuilt engine to another will result in an unbalanced assembly.
Since the accuracy and exactly what was done during previous rebuilds is unknown, it should be no surprise that two different rebuilt engines from the same automotive machine shop held to the same dimensional tolerances and using the same machinery can have two entirely different personalities. There is some truth to Jonathan Swift's adage, "You can't make a silk purse out of a sow's ear"
