Automotive Lower End Theory and Service--part 3--CRANKSHAFT

CRANKSHAFT

Crankshafts (Fgr__39) are made of cast iron, forged cast steel, or nodular iron, and then machined.

At the centerline of the crankshaft are the main bearing journals. These journals are machined to a very close tolerance because the weight and movement of the crankshaft are supported at these points.

The number of main bearings varies with engine design. V-type engines have fewer main bearings than an inline engine with the same number of cylinders.

A V-type engine uses a shorter crankshaft.

Offset from the crankshaft's centerline are connecting rod journals. The degree of offset and number of journals depends on engine design (Fgr__40). An inline six-cylinder engine has six connecting rod journals. A V6 engine may have only three. Each journal has two rods attached to it, one from each side of the V. A connecting rod journal is also called a crank pin.

The position of the rod journals places the weight and pressure from the pistons away from the center of crankshaft. This creates an imbalanced condition.

To overcome this imbalance, counterweights are added to the crankshaft. These are positioned opposite the connecting rod journals.

The main and rod bearing journals must have a very smooth surface. A clearance between the journal and bearing is needed to allow a film of oil to form.

The crankshaft rotates on this film. If the crankshaft journals become out of round, tapered, or scored, the oil film won’t form properly and the journal will contact the bearing surface. This causes early bearing or crankshaft failure. The main and rod bearings are generally made of lead-coated copper or tin, or aluminum. These materials are softer than the crank shaft. This means that wear will appear first on the bearings.

It’s important that the journals receive an ample supply of clean oil. Each main bearing journal has a hole drilled into it with a connecting bore or bores leading to one or more rod bearing journals. Pressurized oil moves in, over, and out of the journals.

A crankshaft has two distinct ends. One is called the flywheel end and, as its name implies, this is where the flywheel is attached. The front end or belt drive end of the crankshaft has a snout for mounting the crankshaft timing gear and damper.

Crankshaft Torsional Dampers--Combustion causes an extreme amount of pressure in a cylinder (more than 2 tons each time a cylinder fires). This pressure is applied to the pistons and moves through the connecting rods to the crankshaft. This downward force causes the crankshaft to rotate. In an engine with more than one cylinder, this pres sure is exerted at different places on the crankshaft and at different times. As a result, the crankshaft tends to twist and deflect, causing torsional harmonic vibrations. These vibrations constantly change but there are specific engine speeds where these harmonics are amplified. This increase in torsional vibration can cause damage to the crankshaft, the engine, and/ or any accessories that are driven by the crankshaft.

These harmful vibrations are often limited by a torsional damper located at the front of the crank shaft. There are two common types of torsional dampers: harmonic balancers and fluid dampers.

Both use friction to reduce crankshaft vibrations.

Harmonic Balancer -- A harmonic balancer (Fgr__41), also called a vibration damper, is the most common. A harmonic balancer has a cast-iron hub and an outer cast-iron inertia ring that is connected to the hub with an elastomer (rubber) sleeve. The hub of the harmonic balancer is pressed onto the snout of the crankshaft. The inertia ring is heavy and is machined to serve as a counterweight for the crank shaft. As the crankshaft twists, the hub applies a force to the rubber. The rubber then applies this force to the inertia ring. The counterweight is snapped in the direction of crankshaft rotation to counterbalance the torsional vibrations from the pulsating crankshaft.

The connecting rod journals also snap as they receive the high pressure from combustion and the snaps cause the counterweight to snap.

To allow the outer ring to move independently of the hub, the rubber sleeve deflects slightly. The condition of this sleeve is critical to the effectiveness of the balancer. Check the condition of the rubber; look for any broken areas or tears. If it looks good, press on the rubber; it should spring back. If the balancer fails the checks, it should be replaced.

Fluid Damper -- This type of torsional damper is seldom used by the OEM. However, it’s commonly installed by the aftermarket. They are effective in a wide range of engine speeds, especially high speeds.

Fluid-filled dampers have a hub surrounded by an inertia ring. Rather than connecting the two with rubber, the outer ring encases the hub. A high-viscosity silicone fluid surrounds the hub. As the outer ring snaps in response to the snapping of the connecting rods and crankshaft, the outer ring rubs against the hub. This rubbing creates friction. The friction is absorbed by the fluid and turned into heat. Therefore, the vibrations are changed to heat and the heat is dissipated from the damper.

Flywheel -- The flywheel also helps the engine run smoother by applying a constant moving force to carry the crank shaft from one firing stroke to the next. Once the fly wheel starts to rotate, its weight tends to keep it rotating. The flywheel's inertia keeps the crankshaft rotating smoothly in spite of the pulses of power from the pistons.

Because of its large diameter, the flywheel also makes a convenient point to connect the starter to the engine. The large diameter supplies good gear reduction for the starter, making it easy for it to turn the engine against its compression. The surface of a flywheel may be used as part of the clutch. On a vehicle with an automatic transmission, a lighter flexplate is used. The torque converter provides the weight required to attain flywheel functions.

Flywheel Inspection -- Check the runout of the fly wheel and carefully inspect its surface. Replacement or resurfacing may be required. Excessive runout can cause vibrations, poor clutch action, and clutch slip page. With both manual shift and automatic trans missions, inspect the flywheel for a damaged or worn ring gear. Many ring gears can be removed and flipped over if they are damaged on one side.

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Fgr__41 A vibration damper or harmonic balancer. Timing seal surface -- Inner hub

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CRANKSHAFT INSPECTION AND REBUILDING

Check the crankshaft for the following:

¦ Are the vibration damper and flywheel mounting surfaces eroded or fretted?

¦ Are there indications of damage from previous engine failures?

¦ Do any of the journal diameters show signs of heat checking or discoloration from high-operating temperatures?

¦ Are any of the sealing surfaces deeply worn, sharply ridged, or scored?

¦ Are there any signs of surface cracks or hardness distress?

If any or all of these conditions are present, the parts need to be repaired or replaced.

To measure the diameter of the journals, use an outside micrometer (Fgr__42). Measure them for size, out-of-roundness, and taper (Fgr__43). Taper is measured from one side of the journals to the other. The maximum taper is 0.001 inch.

Compare these measurements to specifications to determine if the crankshaft needs to be reground or replaced. If the journals are within specifications, the journal area needs only to be cleaned.

USING SERVICE INFORMATION

Crankshaft specifications can be found in the engine specification section of a service manual.

Crankshaft Reconditioning

If the crankshaft is severely damaged, it should be replaced. A crankshaft with journal taper or grooves, burnt marks, or small nicks in the journal surfaces may be reusable after the journals are refinished. This process grinds away some of the metal on the journals to provide an even and mar-free surface.

Minor journal damage may be corrected by polishing the journals with a very fine sandpaper. A polishing tool rotates a long loop of sandpaper against the journals as the crankshaft is rotated by a stand.

The constant movement of the sandpaper and the rotation of the crankshaft prevent the creation of flat spots on the journals.

Checking Crankshaft Straightness:

To check the straightness of the crankshaft, ensure that it’s supported on V-blocks positioned on the end main bearing journals. Position a dial indicator at the 3 o'clock position on the center main bearing journal.

Set the indicator at 0 (zero) and turn the crankshaft through one complete rotation. The total deflection of the indicator, the amount greater than zero plus the amount less than zero, is the total indicator reading (TIR). Bow is 50% of the TIR. Compare the bow of the crankshaft to the acceptable alignment/ bow specifications.

A special machine is used to straighten crankshafts but will only be found in serious engine rebuilding shops. In most cases if the crankshaft is warped, it’s replaced.

Crankshaft Bearings:

Bearings are used to carry the loads created by crank shaft movement. They are a major wear item and require close inspection. Main bearings support the crankshaft journals. Connecting rod bearings connect the crankshaft to the connecting rods. Fgr__42 Measure the diameters of the crank journals with an outside micrometer.

A vs. B = Vertical taper C vs. D = Horizontal taper A vs. C = Out of round B vs. D = Out of round

Check for out-of-roundness at each end of journal.

Fgr__43 Checking crankshaft journals for out-of-roundness and taper.

Modern crankshaft bearings are known as insert bearings. There are two basic designs (Fgr__44). A full-round (one-piece) bearing is used in bores that allow the shaft's journals to be inserted into the bearing, such as a camshaft. A split (two halves) bearing is used where the bearing must be assembled around the journal. Crankshaft bearings are the split type.

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Fgr__46 Six bolts secure each main cap in this engine. Each bolt must be tightened in correct sequence and to the correct torque.

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Many crankshafts have a main bearing with flanged sides. This is called a thrust bearing and is used to control the horizontal movement or end play of the shaft. Most thrust main bearings are double flanged (Fgr__45). Some crankshafts use flat insert thrust bearings.

Some late-model engines don’t have separate main bearing caps; rather, they are fitted with a lower engine block assembly. This assembly works like a bridge and contains the lower half of the main bearing bores. The assembly is torqued to the block.

On some engines, the main bearing caps and lower block assemblies are given additional strength Fgr__44 Full-round and split insert bearings.

Flat; Oil groove

Fgr__45 A thrust bearing with grooves cut into its flange to provide for better lubrication, through the use of additional bolts. The main caps are secured by four bolts, two on each side of the cap.

Other designs may have additional side bolts to fasten the side of the cap to the engine block. Regardless of the number and position of the bolts, proper tightening sequences (Fgr__46) must be followed.

Bearing Materials

Bearings can be made of aluminum, aluminum alloys, copper and lead alloys, and steel backings coated with babbitt. Each has advantages in terms of resistance to corrosion, rate of wear, and fatigue strength. Aluminum alloy bearings are the most commonly used.

These bearings contain silicon, which helps to reduce wear. Some bearings use a combination of metals, such as a layer of copper-lead alloy on a steel backing, followed by a thin coating of babbitt (Fgr__47). This design takes advantage of the excellent proper ties of each metal.

Bearing Spread:

Most main and connecting rod bearings have "spread." This means the distance across the outside parting edges of the bearing insert is slightly greater than the diameter of the housing bore. To position a bearing half with spread, it must be snapped into place (Fgr__48). This provides a good fit inside the bore and helps keep the bearings in place during assembly.

Bearing Crush:

Each half of a split bearing is made slightly larger than an exact half. This can be seen quite easily when a half is snapped into place. The parting faces extend a little beyond the seat (Fgr__49). This extension is called crush. When the two halves are assembled and the cap tightened, the crush forces the bearing halves into the bore. Bearing crush increases the contact area between the bearing and bore, allowing for better heat transfer, and compensates for slight bore distortions.

Bearing Locating Devices:

Engine bearings must not be able to rotate or shift sideways in their bores. Many different methods are used to keep the bearings in place. The most common way is the use of a locating lug. As shown in Fgr__50, this is a protrusion at the parting face of the bearing. The lug fits into a slot in the bearing's bore.

Oil Grooves:

To ensure an adequate oil supply to the bearing's surface, an oil groove is added to the bearing. Most OEM bearings have a full groove around the entire circumference of the bearing, and others have a groove only in the upper bearing half.

Oil Holes:

Oil holes in the bearings allow oil to flow through the block and into the bearing's oil clearance. These holes control the amount of oil sent to the connecting rod bearings and other parts of the engine. For example, oil squirt holes in connecting rods are used to spray oil onto the cylinder walls. The oil hole normally lines up with the groove in a bearing. When installing bearings, make sure the oil holes in the block line up with holes in the bearings.

Bearing Failure and Inspection

As shown, bearings can fail for many reasons. Dirt and oil starvation are the major reasons for bearing failure. Other engine problems, such as bent or twisted crankshafts or connecting rods or out-of-shape journals, can also cause bearings to wear irregularly.

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Fgr__48 Spread requires a bearing to be lightly snapped into place.

Fgr__49 Crush ensures good contact between the bearing and the housing.

Fgr__50 The locating lug fits into the slot in the housing.

Fgr__47 The basic construction of a bearing composed of three metals. By Dana Corporation

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Fgr__51 Common forms of bearing distress. By Dana Corporation Normal wear Overlay fatigue Scoring Corrosion Dirt embedment Cap shift Distorted crankcase Oil starvation Accelerated wear Hot short Dirt on bearing back Wiped Fretting Fatigue

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Mark 1, 2, 3, 4 or 5 Mark 1, 2, or 3 Mark

0, 1 or 2 No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Fgr__52 Size markings on a crankshaft, connecting rod, and rod bearing. By Toyota Motor Sales, U.S.A., Inc.

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INSTALLING MAIN BEARINGS AND CRANKSHAFT

Before assembling the short block, make sure the engine is thoroughly cleaned. Hot water and deter gent are used to clean blocks, crankshafts, and cam shafts. After the parts are cleaned, blow them dry and immediately coat them with oil to prevent rusting.

Also make sure all oil passages are free of dirt and foreign particles.

A gap or clearance between the outside diameter of the crankshaft journals and the inside diameter of its bearings is necessary. This clearance allows for the building and maintenance of an oil film.

Proper lubrication and cooling of the bearing depend on correct crankshaft oil clearances. Scored bearings, worn crankshaft, excessive cylinder wear, stuck piston rings, and worn pistons can result from too small an oil clearance. If the oil clearance is too great, the crankshaft might pound up and down, overheat, and weld itself to the insert bearings.

During an engine rebuild, if there is little or no wear on the journals, the proper oil clearance can be restored with the installation of standard-size bearings. However, if the crankshaft is worn to the point where standard-size bearings will leave an excessive oil clearance, a bearing with a thicker wall must be used. Although these bearings are thicker, they are known as undersize because the journals and crank pins are smaller in diameter. In other words, they are under the standard size.

Undersize bearings are available in 0.001 inch (0.0254 mm) or 0.002 inch (0.0508 mm) sizes for shafts that are uniformly worn by that amount. Undersize bearings are also available in thicker sizes, such as 0.010 inch (0.2540 mm), 0.020 inch (0.5080 mm), and 0.030 inch (0.7620 mm), for crankshafts that have been refinished (or reground) to a standard under size. The difference in bearing thickness is normally stamped onto the backside of the bearing. Bearings may also be color-coded to indicate their size. Often engines are manufactured with other than standard journal sizes. The crankshaft is marked to show the size of bearing used (Fgr__52).

If the housing bores have been machined larger by align boring or align honing, oversized bearings are used to take up this space.

Make sure the new bearings match the crankshaft journal diameters and main bearing bores. Before the bearings are installed, make sure the bore is clean and dry. Use a clean, lint-free cloth to wipe the bearing back and bore surface.

SHOP TALK -- The critical bolts in most engines must be tightened with a torque-angle gauge after a specific torque has been reached. Make sure to do this.

Also check the length and condition of all bolts before reusing them.

Put the new main bearings into each main bearing cap and the bearing bores in the cylinder block.

Make sure all holes are aligned. The backs of the bearing inserts should never be oiled or greased.

Place the crankshaft onto the bearings. The oil clearance is now measured with Plastigage. Make sure the journals are clean and free of oil. The presence of oil will give inaccurate clearance measurements.

Plastigage is fine, plastic string that flattens out as the caps are tightened on the crankshaft. The procedure for using Plastigage is shown in Photo Sequence 7.

One side of the Plastigage's package is used for inch measurements, the other for metric measurements. The string can be purchased to measure different clearance ranges. Usually, the smallest clearance range is required for engine work.

If the oil clearance is not within specifications, the crankshaft needs to be machined or replaced, or undersized bearings should be installed.

SHOP TALK -- If the journals measure within specifications but they are pitted or gouged, polish the worst journal to determine whether or not grinding is necessary. If polishing achieves smoothness, then grinding is probably not necessary.

Crankshaft End Play:

Crankshaft end play can be measured with a feeler gauge by prying the crankshaft rearward and measuring the clearance between the thrust bearing flange and a machined surface on the crankshaft. Insert the feeler gauge at several locations around the rear thrust bearing face (Fgr__53). You may also position a dial indicator so that the fore and aft movement of the crankshaft can be measured.

If the end play is less than or greater than the specified limits, the main bearing with the thrust surface must be exchanged for one with a thicker or thinner thrust surface. If the engine has thrust washers or shims, thicker or thinner washers or shims must be used.

Most engines require the installation of main bearing seals during the final installation of the crankshaft.

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HOW-TO

Checking Main Bearing Clearance with Plastigage P7-1 Checking main bearing clearance begins with mounting the engine block upside down on an engine stand.

+++2 Install main bearings into bores, being careful to properly seat them. Wipe the bearings with a clean lint-free rag.

+++3 Wipe the crankshaft journals with a clean rag.

+++4 Carefully install the crankshaft into the bearings. Try to keep the crankshaft from moving on the bearing surfaces.

P7-5 Place a piece of Plastigage on the journal. The piece should fit between the radius of the journal.

+++6 Install the main caps in their proper locations and directions. Wipe the threads of the cap bolts with a clean rag.

+++7 Install the cap bolts and tighten them according to the manufacturer's recommendations.

P7-8 Remove the main caps and observe the spread of the Plastigage. If the gage did not spread, try again with a larger gage.

+++9 Compare the spread of the gage with the scale given on the Plastigage container. Compare the clearance with the specifications.

+++10 Carefully scrape the Plastigage off the journal surface.

+++11 Wipe the journal clean with a rag.

+++12 If the clearance was within the specifications, remove the crankshaft and apply a good coat of fresh engine oil to the bearings.

+++13 Reinstall the crankshaft and apply a coat of oil to the journal surfaces.

+++14 Reinstall the main caps and tighten according to specifications.

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Connecting Rod

The connecting rod is used to transmit the pres sure applied on the piston to the crankshaft (Fgr__54). Connecting rods are able to swivel at the piston and the crankshaft. This allows them to freely move the pistons up and down while they rotate around the crankshaft. A connecting rod faces great stress. The force applied during the power stroke is applied to the connecting rod as it moves through a variety of angles. The rod has great force applied to it from the top and has great resistance to movement at the bottom. The center section of a rod is basically an "I-beam." This pro vides maximum strength with minimum weight.

Connecting rods are kept as light as possible.

They are generally forged from high-strength steel or made of nodular steel or cast iron. Cast iron is rarely used in automotive engines. Aluminum and titanium connecting rods are also used. Aluminum rods are light and have the ability to absorb high pressure shocks, but they are not as durable as steel rods. Titanium rods are very strong and light but are rather expensive. Some late-model engines, such as the Ford 4.6-liter and the Chrysler 2.0-liter engines, have powdered (sintered) metal connecting rods.

Fgr__53 Crankshaft end play can be checked with a feeler gauge. By Federal-Mogul Corp.

These rods are light and strong and are easily identified by their smoothness.

The small end or piston pin end is made to accept the piston pin, which connects the piston to the rod.

The piston pin can be pressed-fit in the piston and free fit in the rod. When this is the case, the small end of the rod will be fitted with a bushing. The pin can also be a free fit in the piston and pressed-fit in the rod. In this case no bushings are used. The pin simply moves in the piston using the piston hole as a bearing surface.

The "big" end of the rod is used to attach it to the crankshaft. This end is made in two pieces. The upper half is part of the rod. The lower half is called the rod cap and is bolted to the rod. The connecting rod and its cap are manufactured as a unit and must always be kept together. During production, the rod caps are either machined off the rod or are scribed and bro ken off. Since the cap of a powdered metal rod is bro ken away from the rod during manufacturing, there is an uneven mating surface due to the break and the grain of the powdered metal. When the cap is assembled on the rod, these imperfections ensure that the cap is positioned exactly where it was before it was broken off. With other rods, there is the possibility that the rod and cap will be slightly misaligned when they are assembled.

The big end is fitted with bearing inserts made of the same material as the main bearings. Many connecting rods have a hole drilled through the big end to the bearing area. The bearing insert may have a hole that is aligned with the hole in the rod's bore. This hole supplies oil for lubricating. Some rods have an oil squirt hole. This hole sprays oil onto the cylinder wall to lubricate and cool the piston skirt. When the rod is properly installed, this squirt hole is pointed to the major thrust area of the cylinder wall.

Inspection--Closely examine all piston skirts and bearings for unusual wear patterns that may indicate a twisted rod (Fgr__55). Rods suspected of being bent or distorted can be checked with a rod alignment checker. Normally a damaged rod is replaced, although equipment is available to straighten them and to re-bore the small and big ends. Many manufacturers recommend a check of the rod bolts before reusing them. The typical procedure involves measuring the diameter of the bolt at its tension portion.

If the diameter is less than the minimum, it should be replaced.

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Next: Pistons, etc.

Prev.: Camshafts