Engine Block Cylinder Bores

The cylinder bores of engine blocks tend to get quite a bit of attention for good reason. The pistons and piston rings travel inside the cylinder bores and problems inside of the cylinder bores can cause loss of compression, burning oil, crankcase pressure and a host of other problems. Because the cylinder bore is a critical close tolerance component of the engine block, just as the line bore is, it will be discussed in great detail on this page.

Most problems related to the cylinder bores can be diagnosed within the vehicle. For example, a compression test can determine if the rings are not seating properly. Additionally, excessive cylinder bore wear can often be diagnosed audibly when a cold vehicle starts and exhibits a slight knocking sound that dissipates after the engine’s heat causes the aluminum pistons to expand. This heat actually transfers down to the piston skirts and causes them to grow, which takes up some of the excessive clearance present while the engine is cold. Though most automotive technicians can diagnose problems with the cylinder walls in the vehicle, we are going to take a closer look at the engine block itself, how problems are diagnosed with precision measuring instruments and common machining processes that are used to repair problems with the cylinder bores.


Understanding the Desired Cylinder Wall Finish


Driver Side Cylinder Bores of 350 Chevy BlockIn the image to the right you will see an image of a 350 Chevy engine block. The fully machined and finished cylinder bores extend all the way from the deck to the main saddles, which is a distance that exceeds five inches in total length on this particular block. Though the cylinder bores extend this distance, the stroke of the crankshaft, length of the connecting rod and the compression height of the pistons determine how much travel the pistons and rings have within the bores. Because of this, many used engine blocks have a carbon ridge at the very top of each cylinder bore, which starts where the top piston rings end their upward travel. Providing the cylinder bore is within specification, a tool called a ridge reamer can be used to remove the ridges.

Cylinder Wall Cross Hatch CloseupIn the image to the left, you will see a close-up image of a bored and honed cylinder bore. As you can see, the walls of the bore have lines. These lines are known as the cross hatch and are needed to both aid in the retention of oil to lubricate the pistons and rings during travel and also to protect the piston rings and cylinder walls from scuffing. Though cylinder walls are honed with a stone, they are often finished with a brush. The brush is used to perform a machining operation known as plateau honing. By removing the plateaus left behind from the honing stones, less debris is created during the engine break-in process and the piston rings will seat much faster and with less wear. It is important to note that the lines within the finished cross hatch must be at a 45° angle to provide the most suitable surface finish for the pistons and rings.


Cylinder Bore Inspection and Machining Operations


To achieve the most suitable cylinder bore finish a variety of machining processes are needed to accommodate for the use of oversize pistons and rings. To first determine how bad the cylinder bores are on a block, a dial bore gage is used to check whether the cylinder bores are within their size specification. Additionally, a dial bore gage can be used to see if the cylinder bores are out of round. Other more intensive testing for cracks may include a magnaflux operation or pressure testing the block. Most automotive machine shops have the equipment, staff and skill to perform all of the inspection and machining processes needed on an engine block. To save money, it is smart to disassemble as much of the block as you can (except the freeze plugs) before taking it to a shop.

Boring

Boring is a machining operation that uses a cutter to remove material from the cylinder walls. Often the boring operation includes two cuts inside each cylinder bore, with the first cut being a rough cut and the final cut being a finish cut in preparation for honing. Cylinder bores that are being machined in excess of .030” may actually require two rough cuts, which is largely dependent on the equipment being used to perform the machining operation. For example, a boring bar is unable to take cuts that are as deep as a CNC machining center. Since the boring process is not used to actually finish the cylinder walls, the surface finish is not critical. However, this final bore size and finish should allow for a minimum of .003” to be honed in the next machining operation.

Honing

Honing is the final machining process used to provide the cylinder walls with the desired size and surface finish for the pistons and rings. Though drill driven honing stones and ball hones are available, they are not as accurate as modern day machinery designed solely for honing automotive and industrial cylinder walls. For example, a Sunnen hone can apply consistent pressure to the 220-280 grit honing stones and consistent vertical motion to achieve a cross hatch that finishes at the desired 45° angle that is suitable for cast, chrome and moly rings. Additionally, honing machines also have bottom and top stops that reduce the risk of honing stones colliding with the main journal webbing. For performance engines, torque plates may also be bolted onto the deck surface prior to honing to reproduce the distortion caused when cylinder heads are torqued onto the block during assembly.

Cylinder Bore Sleeves

Melling Cylinder Bore Repair SleeveWhen a cylinder bore on an engine block is too badly damaged to bore and hone to an available oversize piston, installing a cylinder sleeve (pictured right) is often a cost effective option to replacing the entire block. To install the sleeve, the automotive machinist must first bore the damaged cylinder bore to a size that will accept the sleeve’s outside diameter with some press fit. The normal press fit, which is also known as an interference fit, should be approximately .004” on cast iron engine blocks. Aluminum engine blocks, on the other hand, require special care as cylinder sleeves are used by manufacturers and can be removed and replaced if an available oversize piston is not available. To prevent a sleeve from dropping down in the block, automotive machinists typically leave a small step at the bottom of the cylinder being sleeved that is beyond the area where piston skirts travel. This step is where the new sleeve will bottom out and the cylinder head naturally will not allow the sleeve to move upwards. To install a press fit cylinder sleeve inside a cast iron block, many automotive machine shops first heat the block, freeze the sleeve or do both. By using thermal expansion to their benefit, installing a sleeve in a press fit cylinder bore is much easier. After the sleeve is successfully installed, the inside diameter of the sleeve may be bored to the desired size for honing.

When the cylinder bores have been fully machined, the bare metal is susceptible to corrosion and should be protected immediately with engine oil wiped that is wiped across the cylinder walls or with the application of an appropriate rust inhibitor. Even the moisture from humidity in the air can cause finished bores to rust. If the engine is not going to be immediately built, it is also a good idea to wrap the block with shrink wrap or even a garbage bag to prevent dust and other debris from contaminating the freshly machined cylinder wall surfaces.

 

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Cylinder Bore Information