Whenever machine work is performed on a crankshaft’s journals, these surfaces must be adequately finished before use in an engine. Besides maintaining a sufficient microfinish, it is also important that any burrs are removed. A critical area where burrs are often found are just inside of the journal’s oil passages. The way to remove these burrs is by using a process known as chamfering.

Burrs develop on oil passages from a variety of machining operations. Just grinding the crankshaft .010” can leave a small burr that would restrict the proper flow of oil while the engine is running. This poses a serious concern since engine bearings need to be adequately lubricated at all times. Chamfering is the industry standard in deburring crankshaft oil passages, which is explained in more detail below.

If you look to the image to your right, you will see what the oil passages look like when they have been lightly chamfered. This deburring process was completed with a die grinder, small grinding stone and an automotive machinist’s steady hands. The die grinder rotates the stone at a high RPM and the machinist gently follows the outside of the existing passageway with the stone to make a light cut, while also being careful to deburr the entire circumference around the oil passage outlet.

Generally speaking, chamfering is performed as part of the crankshaft polishing process and must be performed after most significant crankshaft repairs have been made. Just before the crankshaft is polished, it is chamfered in the grinding machine. Because chamfering can also create slight burrs, a polishing belt is all that is needed to remove any burrs left behind from the surface where the stone has met the journal’s outside diameter.

When a crankshaft is welded, it almost always needs to be chamfered. Instead of simply cleaning up the outside of the oil passage after a welded journal has been roughed in, the automotive machinist will often use a long and narrow stone to cleanup the inside of the passageway as well. Because welding penetrates existing steel, a small amount of weld may enter the passageway that is easy cleaned out with the right stone and chamfering process.

When a journal has been welded, some automotive machine shops prefer to instead drill out the oil passage using a special carbide chamfering drill bit. Although this type of chamfering may be needed when an excessive amount of weld has blocked the interior oil passageway, the finish it leaves behind is often less than desirable. Many automotive machinists that must use a chamfering drill bit to clear out an oil passageway will then smooth out the passage with a die grinder and fine stone.

There are some automotive machinist that will not only chamfer the circumference of the outer oil passage hole, but they will add a directional cut with the die grinder’s stone that leads into the journal. The theory behind this is that such an elongated chamfer will aid in the distribution of oil, especially on high performance engines. While this may be true while the engine is running, on initial startup of the engine it will take longer for the oil to completely surround the void between the engine bearings and journals. For this reason, most automotive machinists opt to perform a light chamfering process that does not elongate or extend the size of the oil passages.

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First, many novice engine builders believe if their crankshaft is gouged that all it needs is to be polished. In most cases this line of thinking is wrong. Gouges, caused by material that may have been embedded in engine bearings, can damage a journal. Polishing can’t take these defects out of journals; at least with any degree of accuracy. If there are a lot of deep gouges, lines or other marks in a journal, chances are grinding the crankshaft first is the best course of action.

Generally speaking, crankshaft polishing is the last step in the crankshaft repair process. Normally the crankshaft has been ground first, and the polishing process is used to take off the microscopic peaks that the grinding wheel leaves behind on the finished journals. Many automotive machinists also choose to chamfer the oil holes just before polishing the crankshaft, which aids in removing burrs and helps the engine oil leave the oil passages when the engine is running. Regardless, polishing removes the peaks on journals and seal surface areas so that a smooth microfinish is created. This helps to reduce bearing and seal wear so that rebuilt engines will last for many miles under normal operating conditions.

Although there are dedicated machines to polish crankshafts, like the one pictured to the left, most crankshaft polishing operations occur right in the grinding machine after the last journal has been ground. The reason for this is simple; with the crankshaft already in the grinding machine, there is no setup time required to use a different piece of equipment. To polish the crankshaft in a grinding machine, the automotive machinist will simply need a portable polisher that has been designed specifically for use on crankshafts.

A portable crankshaft polisher, like the one pictured to the right, operates on a standard 120 volt outlet. When not in use, the polisher can be easily unplugged and stored in a safe location that is out of the operators work area. This polisher uses a special type of belt that is designed to come in direct contact with the crankshaft journals and seal surface areas as the crankshaft is slowly rotating within the grinding machine.

Crankshaft polishing belts, like the one pictured to the left, are available in a variety of lengths, widths and grits. Depending on the type of portable polisher used, which is adjustable, the length of the polishing belt can vary from 60” to 72” in length. The width of the belt being used to polish crankshafts is determined by the crankshaft journal or seal surface width. For V8 crankshafts, a one inch belt is normally used while V6 crankshafts normally require the use of a ¾” belt that fits between the narrow counterweights.

To polish crankshaft journals, a 400 grit belt is almost always used. When a new belt is put into production, it is often first used on a piece of steel to take the sharp edges off of the belt before it is used on any journals or seal surfaces. The concept of polishing is to finish a crankshaft with a microfinish that is appropriate for engine bearings and seals. Using a coarse belt will not only leave an undesirable finish, but may also take off too much material and throw the journals out of tolerance. By using the appropriate belt and operation, polishing should remove no more than .0002” of material.

It is important to note that some less experienced engine builders attempt to cleanup their crankshaft’s journals with a 200 grit belt. Although these belts can take out minor gouges and lines in journals, the end result almost always consists of journals that dip in the center. This dip, and out of tolerance condition, exists because polishing belts are not ridged on their ends. The absolute center of a polishing belt has the highest capacity to remove material. Polishing should never be used to repair visual defects on crankshafts. Crankshaft grinding is the only process that can correct visually damaged journals.

Polishing Belt Materials and Design

Most quality crankshaft polishing belts are designed with an aluminum oxide abrasive that is bonded with either glue or resin. Although the most common types of belts have straight edges, others have scalloped edging that is designed to polish into the radii. Though paper based belts are widely used for most crankshaft polishing operations, abrasive cork belts are also available for super fine finishes. Regardless of what type of belts an automotive machine shop uses, the belts lose their effectiveness over time as the bonding agent deteriorates and as the belts become loaded with material.

Since there is no way to dress a belt, proper belt maintenance includes blowing of the belt with compressed air. This process helps to dislodge some of the material that is embedded between the abrasives. Should the embedded material on the belt become too great, it should be discarded and a new one placed into production.

Polishing belts are normally sold in quantities of ten per box. Depending on which type of belt is purchased, the average cost can range from $2.50 to $4.00 per belt. Ordinarily, a polishing belt that has been maintained properly can be used to polish fifty crankshafts or more.

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