Engine Boring
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Joined: Feb 2014
Posts: 2,629
From: Gillespie County, Republic of Texas
Over time the bores get out of round from piston wear. Boring makes each cylinder perfectly round so the the rings can seal effectively.
Also you will find a ridge near the top of each cylinder bore where the rings stop their travel. You don't want new rings running into that shelf.
As F-85 4-4-2 said, when you rebuild you want everything back to the way it was intended
Also you will find a ridge near the top of each cylinder bore where the rings stop their travel. You don't want new rings running into that shelf.
As F-85 4-4-2 said, when you rebuild you want everything back to the way it was intended
Old(s) Fart
Joined: Mar 2007
Posts: 50,802
From: Northern VA
I'll expand on these responses a little.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
Thread Starter
Registered User
Joined: Jul 2012
Posts: 221
From: Oregon
I'll expand on these responses a little.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
Thank you all for your response. Joe, so if a engine is bored do not use the original piston always replace it with new ones?
Old(s) Fart
Joined: Mar 2007
Posts: 50,802
From: Northern VA
Each engine has a specific piston-to-wall clearance. Boring the cylinder requires a new piston with an equally larger diameter to maintain this clearance.
Registered User
Joined: Mar 2007
Posts: 3,446
From: eastern MA
I'll expand on these responses a little.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
The seal between the piston rings and the cylinder walls is an important part of maintaining pressure in the engine's combustion chamber. If combustion gasses leak past the rings, performance suffers and the caustic gasses also contaminate the engine's oil. As an engine ages, the rings riding on the cylinder walls cause wear, which causes this seal to deteriorate. The most practical way to repair this wear during a rebuild is to machine the cylinder walls back to the proper shape and finish - this is called boring. Doing so enlarges the diameter of the cylinder, which requires new, oversized pistons. Standard oversize pistons are available (typically 0.030" and 0.060" over original diameter for Olds motors), but custom sizes can be made at a price.
A byproduct of boring an engine is an increase in displacement. The engine displacement is a function of the swept volume that the piston covers during a stroke - the volume of the cylinder defined by the bore and stroke of the engine. Increasing the bore increases this volume and thus the displacement. For standard oversize pistons, this increase is modest. As an example, boring an Olds 455 to 0.030" oversize yields a 462 cu in motor; going 0.060" over yields a 468. The original thickness of the cylinder walls limits the amount of overbore that a given block can handle, and some blocks are able to handle large overbores and thus more significant displacement increases. The DX block from the Olds diesel motors are one such example. The cylinder walls are extremely thick for the diesel combustion pressures, but when used to build a gasoline engine, this added thickness allows large overbores and thus large displacement gains. Some earlier Olds blocks, like the 330 and 400 motors are also capable of larger than normal overbores.
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