Line Boring Main Housings

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What is Line Boring?

Mitsubishi Evo Block mounted For line boring on our Berco BT6

Mitsubishi Evo Block mounted For line boring on our Berco BT6

The most important thing to remember when dealing with any bearing housing which features one half of its diameter in the main casting and one half in a separate cap or carrier is that they will be manufactured as a matched pair from the factory. On no account should they be mixed up, interchanged or turned around from their original position within the engine. When dismantling any engine it is good practice to always mark bearing caps for both position and location in the cylinder block. The same applies when working with connecting rods and their associated caps.

When referring to cylinder blocks, the term line boring is usually applied to the main bearing housings although it can equally be applied when referring to camshaft housings where the camshaft runs in the cylinder block itself. The line boring operation essentially restores both the sizing and alignment of these housings to ensure that the rotating components (camshaft or crankshaft), turn freely.


When is it necessary?

With cylinder blocks the most frequent reason to need a line bore is following a crankshaft failure. A badly worn crankshaft can “whip” within the main bearing housings which causes distortion within them. A seized crankshaft (through lack of oil or pressure) can cause the shell bearings to lock onto the crankshaft resulting in them being forcibly rotated within their housings causing both distortion and wear.

Equally, if a seizure occurs within the camshaft housing the cam bushes may be damaged or in extreme cases might rotate within the block itself. This can result in damage to the block which would require the housings to be bored oversize, removing the damage and correcting the alignment.   Corresponding oversize bushes will be needed to fully repair the damage.

In high performance engines strength in the crankshaft area can be improved by fitting steel main bearing caps. These will be supplied in a semi-finished condition and line boring will be necessary to ensure perfect final alignment.

All or any of the above could mean that your casting needs to be line bored.

How is it done?

Essentially, the principals are the same whether working on a cylinder head or block and there are several ways that the end result can be achieved.

Probably the easiest method both in terms of machining and understanding is simply to enlarge the diameter of the housings in question. This will correct any misalignment but will require the use of oversize bearings or bushes. The downfall with this method is that quite often neither are readily available. Oversize bushes can quite easily be made for machined camshaft housings however oversize bearing shells to accommodate crankshafts can be quite difficult to obtain. In this instance, a slightly different approach will be required.

The only way to restore the damaged housings to their original size involves the method termed “cut and shut”. This means that the parting faces of either the cap or parent casting (or both) are machined. When both halves are assembled together again this results in an ovality in the housing.  This creates a “tight spot” (which is greatest at 90 degrees to the split line) from which material can be removed.  When this is rounded out, the original diameter is restored. The most material is removed from this point and virtually zero at the parting line itself.

Mitsubishi Evo Block - Main Bearing Ladder Machining using a Berco STC361

Machining Main Bearing Housings Prior to Line Boring using our Berco STC361

In all cases the machining processes performed using our BERCO BT6 machine ensures that the housings concerned are not only finished to the correct sizes but that they are bored in alignment with each other along the full length of the job.

Call us for expert advice and a quotation for your engine machining requirements.