When the term “align bore” is applied to the internal combustion engine, it typically means a boring of the journals where the crankshaft resides. It actually could refer to align boring the camshaft journals or balance shaft journals, etc, but the common use of the term refers to making the crankshaft journals properly sized and in line with each other within a specific tolerance.
Machinists use the term boring when a sharply pointed cutting tool is used to remove metal from a previously created hole. The hole could have been drilled or, in the case of an engine casting, it may have been cast in place. The boring process typically uses a boring bar which is a rigid steel or carbide bar containing one or more cutting tools. Typically the engine is clamped in place and the rotating bar is fed through all of the journals at a constant feed rate. In other words, it travels along its axis at 0.001, 0.003 or 0.005 inches per revolution of the bar. Due to the rigidity of the set-up, this procedure can remove more metal from the left side of the hole than the right side or more metal from the top than bottom, etc. So, if the various journals are out of line prior to boring, they can made to be in line after boring.
This procedure of align boring is performed on every engine when it is manufactured. So why would anyone need to do this again? The answer is: STRESS!! After inspecting hundreds of Honda B-Series engine journals, we know that main bearing alignment is out of spec on most stock used engines before any machine work is performed to install sleeves in them. This happens due to the natural stress relieving process. When an engine block is cast, the aluminum starts out in a molten state. As it cools, the thin areas cool more quickly than the thick areas. Hence, they become rigid earlier in the
cooling process than the thick section. Therefore, when it is cooled, there are internal stresses in the block. Basically, some areas pushing on other areas while other areas are pulling on yet other areas. Even though stress relieving is done in the manufacturing process, some stresses remain once it is fully assembled and in the car.
There are two common techniques for stress relieving metals: The first is to heat the metal and then cool slowly. The second is to vibrate the metal at various frequencies. As the car is used, we find the engine is repeatedly heated and cooled and constantly vibrated while running. Over time these effects result in misalignment of the Main Bearing Journals*.
So, in summary, when the engine was built, it was align bored and thus the bores were in alignment and on size, but as a result of use, stresses relaxed and misalignment occurred. Typically, the bore diameters are still correct, the only problem is misalignment. Now when a race engine builder performs an align boring operation he must remove as much material as the alignment is out. But the bores were already the proper size so, after boring, we end up with properly aligned but over sized journals. This is not a problem if “over sized back” main bearings are available. It is a serious problem if such bearings are not available.
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