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Line Boring Equipment: A Boring Subject?

As the old saying goes, when something works, you stick with it. Of course sticking with the same old routine hasn't exactly been the route to success in the engine building industry, so perhaps "change is the only constant," might seem to better describe this business.

By Larry Carley

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But as the British philosopher Bertrand Russell noted, "Change is one thing; progress is another." While some might say the basic techniques for line boring main and cam bearing bores in engine blocks hasn't changed much in 30 years others say that isn't such a bad thing.

A horizontal boring bar with cutters mounted on it is inserted into the block and then centered in the main bearing or cam bearing bores with support bushings or fixturing. The bar is then turned and advanced to shave metal off the inside of the bores so the inside diameter (ID) of the bores can be resized to the desired dimensions (back to standard size or to oversize).

An alternate method for machining the bores is to use a line hone. A hone uses abrasive stones rather than cutters to remove metal. Line honing typically removes less stock and leaves a smoother finish than line boring, making it well suited for applications where only minimal stock removal is necessary or where a smoother bore finish is desired or required (as in overhead cam cylinder heads where the cam journals have no bearing shells or inserts).

Why Bore?
There are three basic reasons for line boring the main bearing and cam bearing bores in engine blocks. One is to restore worn, out-of-round or damaged bores. If an engine overheats or loses oil pressure, one or more bearings on the crankshaft or camshaft may seize and spin. The resulting damage to the bearing bore must then be repaired by either machining the hole to accept a standard sized bearing or an oversized bearing.

With main bearings, a worn, out-of-round or damaged bore can be restored back to standard ID by grinding or milling the mounting surface of the main caps, bolting the caps back on the block, and then cutting the holes back to their original dimensions.

In the case of worn, out-of-round or damaged cam bearings in an engine block, there are no removable caps. The only option is to enlarge the bores so new oversize cam bearings with a larger outside diameter (OD) can be installed.

Reason number two for line boring a block is to restore proper bore alignment - a process which is often called "align" boring (or honing if a line hone is used instead of a boring bar). As rigid as an engine block might seem, there is actually quite a bit of residual stress in most castings. As a new "green" block ages and undergoes repeated thermal cycles, the residual stresses left over from the original casting process tend to distort and warp the engine. This affects the alignment of the crankshaft and camshaft bores as well as cylinders. Eventually things settle down and the block becomes more or less stable (a "seasoned" block). The bearings as well as the crankshaft and camshaft journals gradually develop wear patterns that compensate for the distortion that has taken place.

Additional warpage can occur if the engine is subjected to extreme stress (like racing) or overheats. If the original crankshaft or camshaft is then replaced without align boring the block, it may bind or cause rapid bearing wear. Likewise, if you're building a high performance engine with close tolerances, you don't want any misalignment in the main bores or cam bores.

The third reason for line boring or honing a block is to correct or change bore centers or bore alignment (as when "blueprinting" a high performance engine). The camshaft and crankshaft should be parallel in the block. If they are not, line boring can correct the misalignment to restore the proper geometry. With performance engines, there may also be a reason to change the centerline of the crankshaft or camshaft slightly to alter the piston or valvetrain geometry.

Line boring will also be required if the original main bearing caps are replaced with stronger aftermarket caps, or the block is being converted from two bolt main caps to four bolt main caps. For best results, four-bolt main caps should be machined in three-steps. First, bore the housings to within .030" of the desired size. Then bore again to within .005" of final size, and finish to size by align honing. Harder honing stones work best on cast iron while softer stones (such as J45 silicon carbide #150 grit) do better on bimetal applications where the block is aluminum and the main caps are cast iron, steel or powdered metal.

Even though the end goal of line boring has remained constant, some innovations in the recent past have made line boring anything but a boring subject.

One of the disadvantages of using a traditional horizontal boring bar is that it tends to sag. This has to be countered by using adequate support so all the bore holes are cut straight and true with no misalignment between holes and no variations in bore size.

One way to eliminate the effects of gravity on the boring bar is to use a vertical boring machine. Rotating the block and bar 90° so the block and bar are straight up and down provides a truer, straighter cut says one manufacturer of this type of equipment. It also saves floor space because the machine has a smaller footprint.

Another way to circumvent the issue of bar sag is to use a 90° right angle cutter attachment on a milling machine. Instead of using a long steel bar to pass single or multiple cutters through the main bores, the 90° cutter is lowered into the space between each main bore, then moved sideways to machine the bore ID. It's sort of like working around a corner. With computer numeric controls (CNC), each hole can be precisely machined to exact dimensions and the centerline of each hole perfectly located and aligned with all the rest. This technique works especially well on large, heavy blocks that may be too long for most boring bars.