Engine Block Line Bore

Line Bore of 350 ChevyOne of the most critical areas of an engine block is its line bore. The line bore, which is often called align bore, is the area of the block where the crankshaft’s main bearings are located. Since the crankshaft’s main journals must have adequate clearance between the bearings, maintaining the line bore tolerances is a must.

In the picture to the left you will see a 350 Chevy engine block with five 4-bolt main caps. The line bore consists of all five round main housing bores on this block, but many smaller engines have only four main housing bores. Half of the main bore housings are within the block while the other half is in the main caps. Each main housing bore runs parallel with one another and are collectively called the line bore.

Importance of Line Bore Tolerances

Each engine block’s line bore must maintain close tolerances to ensure that the main bearings have adequate crush when the main caps are torqued down. This crush occurs when the main cap bearing shell comes into contact with the bearing shell inside the block’s main housing bore and is responsible for holding the bearings in place. If the line bore is too large, the bearings may not crush/seat properly and could potentially spin. On the other hand, main housing bores that are too tight may cause excessive bearing crush that distorts the inside diameter of the bearings and could bind the crankshaft’s main journals. A precision measuring instrument called a dial bore gage is used to determine the size of main housing bores and also any out of round conditions that may be present.

Line Bore Damage and Machining Repairs

Damaged Main RegisterIn most cases, engine blocks that are going to be used for a stock engine build will not need any machining to the line bore because it is not a wear surface. This determination is made with the assistance of a dial bore gage, which measures the size of each housing bore after the main caps have been torqued to specification. However, if the crankshaft previously spun a main bearing the housing bore(s) may be damaged from the bearing spinning or from excessive heat. Detonation, on turbo boosted applications, can also cause the main caps to dance and often damages the surface where the main caps seat against the block (called the main register). Please view the picture to the left to view a damaged main register.

Another situation that arises, which may necessitate machining the line bore, is the addition of new main caps. More often than not new main caps are purchased for performance upgrades. New main caps are often made of billet steel for added strength, but they must also be machined to fit inside the main registers for adequate side clearance. A good automotive machine shop that installs billet main caps will first machine the block’s main registers and main caps. Those who fail to machine the block’s main registers often do not realize the full strength potential of billet main caps because of an uneven seating surface. Furthermore, new main caps that have not been properly fit for side clearance can also damage the block on assembly because of excessive press fit.

Though fitting new billet main caps involves additional steps to complete, the process to machine the line bore is generally the same for stock and performance blocks. Because adding material to each main housing bore is not an option, the housing bore size must be reduced so that it can be machined to its proper diameter. Milling the main registers will help to accomplish this, but most machine shops use a cap grinding machine to cut the main cap seating surfaces. After the mating surfaces have been machined and main cap studs or bolts have been torqued to specification, a line boring machine may be used to open up the housing bores when there is .005” or more material to be removed. A good automotive machinist can often avoid the line boring process and instead use a line honing machine to take out less than .005” of material. Which machining process is needed to open up the main housing bores is largely dependent on how much material was taken off of the main registers and/or main caps.

It is important to note that taking a large amount of material out of the main housing bores will also reduce the centerline distance between the crankshaft and camshaft. Because of this, many automotive part manufacturers produce timing chain sets that are .002”+ smaller than the OEM part. This consideration becomes more of a concern if a lot of material was taken out of the main registers.

Please visit our engine block information page to learn more about the critical areas of blocks, common damage seen and how a used engine block may be repaired with a variety of machining processes.


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Line Bore Information