The cylinder block is the most basic part of an engine. All other engine parts will be fastened to either the inside or the outside of the cylinder block. Older cylinder blocks will likely be composed of thick and heavy cast iron; many modern blocks are made instead of cast aluminum or a similar lightweight metal. Whichever metal is employed, it will be cast in a mold to produce the unique shape of the cylinder block.


The crankcase is located at the bottom of the cylinder block. It can be molded as a part of the block, or bolted on separately.

The primary purpose of the crankcase is to enclose and support the crankshaft. The crankcase will have parallel holes bored into its two ends. The ends of the crankshaft will be supported by these holes, either directly, or by bearings that have first been fitted snugly into the crankcase holes; the ends of the crankshaft will then rest against the inner races of these bearings(BEARINGS).

Another important function of a crankcase is to hold engine oil. Oil is dispensed through a fill tube into the crankcase, and as the engine runs, is used to lubricate internal parts like the connecting rod bearings, piston, and camshaft assembly. Oil seals are installed in the crankcase where the crankshaft ends protrude through to prevent oil from leaking out during operation. Because the crankcase is used as an oil reservoir, it will sometimes be called a sump(OIL FILL).

Frequently, especially in four-stroke engines, a crankshaft will be supported on one end by the crankcase, and on the other by a crankcase cover. The crankcase cover is bolted to the bottom of the crankcase. It will possess a hole in its center to support one crankshaft end which will almost always contain a bearing. The crankcase cover is removable to allow for internal engine maintenance.

For the crankshaft to turn properly during engine operation, the holes in the end of the crankcase and crankcase cover must be absolutely parallel in their alignment. To accomplish this, small steel pins called alignment dowels are fitted between the crankcase and the crankcase cover to hold these sister parts perfectly still as bolts are inserted and tightened to proper torque(TORQUE).


The crankshaft is the main rotating part of the engine. It converts the up and down motion of the piston into rotary motion which can be used to power blades, wheels, etc. The crankshaft is also used to drive the flywheel, the camshaft, and other parts of an engine that are timed to perform in symphony with specific movements of the piston.

A typical crankshaft in a single cylinder engine will be made in one solid piece, and will therefore require a two piece connecting rod. The ends of a crankshaft are joined together by a crankpin (also called a connecting rod journal), which when the crankshaft is properly positioned in the crankcase will be right beneath the cylinder bore(CYLINDER BORE). The two pieces of the connecting rod will be clamped around this crankpin. The opposite end of the connecting rod will support the piston. You may sometimes encounter the word, throw. Throw denotes the combined unit of crankpin and two piece connecting rod (not including the piston or other portions of the crankshaft).

Heavy counterweights are used on the crankshaft to balance the weight of the connecting rod. The counterweight will be positioned opposite the connecting rod and piston assembly. This creates balance as the crankshaft rotates. The crankpin is also offset from the parallel ends of the crankshaft. It is this off center positioning of the crankpin which causes the crankshaft to rotate with the up and down motions of the piston in the cylinder.

Some crankshafts employ just a one piece connecting rod. You may hear one of this type referred to as a built up crankshaft. The built up crankshaft is composed of multiple pieces which separate to allow for removal of the connecting rod or connecting rod bearing. In a built up unit, the crankpin threads like a needle through a hole in one half of the crankshaft, the roller bearing of the connecting rod, through a hole in the other half of the crankshaft, and is tightened down to proper torque to keep the entire assembly together(TORQUE).

Some older small engines may contain a one piece connecting rod and a single counterweight which are attached by a bolt. This bolt will pass through a spacer which serves as the crankpin, supporting the one piece connecting rod and its bearing. A thrust washer is then used to keep the connecting rod from slipping from its bearing during engine operation.

A small engine crankshaft is always positioned perpendicular to the cylinder bore(at a very precise 90 degrees). Without this positioning, the piston will not travel up and down in the bore properly. The crankshaft and bore will always be perpendicular to one another. However they may or may not be perpendicular to the wheels or drivetrain of a machine. There are three distinct operating positions for a crankshaft: horizontal, vertical, and multi positional.

In a horizontally configured engine, the crankshaft is positioned horizontally to the machine drivetrain(as the machine rests on its wheels on the ground). This means that the cylinder and piston are vertical to the ground. Lawn tractors use this type of crankshaft to operate their transmission shafts(TRANSMISSIONS).

In a vertically configured engine, the crankshaft rests vertically to the wheels or drivetrain(as machine sits on the ground). The piston and cylinder, meanwhile, are horizontal or parallel to the wheels and ground. Rotary lawn mowers use a vertical crankshaft because it can be attached directly to the blade the way a handle attaches to an umbrella top, making it easy to spin.

The multi positional engine is designed to operate in a variety of positions. They are found in equipment such as chain saws, which may be held and operated at myriad disparate angles, sometimes even upside down.

The important thing to keep in mind is that horizontal and vertical engines are designed to be operated in only one position. Their lubrication systems will function properly only when they are assuming this one position. In other positions, lubrication will be inadequate; in extreme alternative positions, lubrication may even be nonexistent. Multi positional engines, on the other hand, possess lubrication systems which can provide adequate and effective lubrication regardless of engine position.


During a rebuild, all areas of the crankshaft should be inspected. This inspection will entail not only visual scrutiny of the crankshaft assembly, but measurement of its main bearing journal and crankpin, and of crankshaft end play.


You should visually inspect the crankshaft for obvious signs of damage. Here are three things to keep in mind when performing the inspection:

1)The crankshaft must be straight; always replace it if bent or out of round
2)Replace the crankshaft if the keyway has been damaged
3)Inspect the journal surfaces (the surface around which a bearing rotates), gear teeth, and threads for damage; if damage is found, crankshaft should be replaced

Check first for overall integrity: this will include signs of cracking or bending, and other physical damage which compromises structural integrity. Bent crankshafts are quite common, especially on rotary (push style) lawn mowers which have their cutting blades directly attached to the engine crankshaft. If the mower blade strikes something solid during operation, the impact can easily bend the engine crankshaft. Generally speaking, any time a crankshaft is bent or bears signs of physical damage, it should be replaced. For safety reasons, a bent crankshaft should never be straightened and replaced in an engine Once it is bent, its structural integrity will never be the same.

Inspect the keyways: you should visually inspect the crankshaft keyway slots and flywheel keys (which secure flywheel to crankshaft) for signs of damage. Most crankshafts will have a keyway at each end, one where the flywheel attaches, and another where the blade or other output device is attached.

Keyways can be damaged under the same conditions in which a crankshaft is bent. Some manufacturers design their keys to break easily, a feature that centers damage in the key rather than the harder to replace crankshaft. Check the crankshaft keyway for wear. Notice if it appears to be spread open. If metal burrs or rough spots are present on the keyway edges, remove them with a small file so that the key will fit easily into it.

Check the journal surfaces(surface around which bearing rotates): of both the main bearings(located at either end of crankshaft, where it exit crankcase), and the crankpin. Journal surfaces should be free of metal particles and deep scratches (known as score marks). Small scratches can usually be treated by polishing the journal surface with fine emery paper or a crocus cloth soaked in oil. You should polish any area of the crankshaft, especially the crankpin journal, to remove only minor scratches. Removing too much metal from any journal will cause its diameter to be too small.

If journal surfaces do contain score marks or other damage, or if the journal diameter is smaller than specifications call for, the solution to the problem will depend on engine type. If an engine lacks replaceable bearings, the entire crankshaft will normally have to be replaced. If an engine possesses replaceable bearings, the journal in question can be reground to a smaller size, and then a thicker bearing used. These thicker replacement bearings will be referred to as undersize bearings because their inner diameters are substantially smaller than their overall thickness or side width. In engines which use the rod end itself as a crankpin bearing, connecting rods with undersize ends are often available.

To grind smaller the exterior of a crankpin or other journal, you must use a machine specially designed for this purpose. Since these machines are costly, few at-home shops or even small-engine repair businesses like ours will possess one. You will need to take your crankshaft to a local automotive machine shop to have its crankpin or other journal ground.

If a journal diameter is reduced in this way, be sure to visually check the area which has been ground before engine reassembly. It should be smooth, and free of score marks. Pay close attention to the ends of crankpin journal where they transition to the main part of the crankshaft. These transition areas should be smooth and round. The crankpin ends should not stop abruptly, or harbor squared corners, where they meet the main crankshaft. Instead, the ends of the crankpin should gradually merge with the crankshaft metal. The rounded ends of a crankpin journal are known as fillets. Fillets are positioned at the ends of the crankpin journal to reduce the stress concentrated in that transition area; they make the crankpin stronger, and less likely to shear during machine operation.

Several conditions can damage journal surfaces. The most likely of these are 1)improper lubrication, and 2)dirt inside an engine. If bearings and journals are receiving inadequate lubrication, it may be a result of several things: a low oil level, the incorrect type of oil, oil not changed at proper intervals, or even a defective a oil pump in the engine. If bearing surfaces are not properly lubricated, the bearing will overheat and damage the journal around which it is rotating. Where the crankpin is concerned, damage can be severe enough to cause its bearing material to melt onto the crankpin, a phenomenon which will eventually cause an engine to seize.

Dirt usually accumulates inside an engine because there has been a lack of maintenance, often due to improper oil change intervals. If dirt has entered an engine, it can work its way into small spaces between a journal and its bearing, and leave substantial score marks on both surfaces.


Once you have visually inspected the journal surfaces, they must be measured to ensure that dimensions are correct. A journal that is oversized can bind inside a bearing and prevent the crankshaft from rotating freely. If any journal diameter is too small, there will be too much clearance between it and the bearing, allowing the crankshaft to vibrate inside the bearing and causing damage to both surfaces.

Check the size of the crankpin and other journals: all journals should be measured according to recommendations of an engine manufacturer. A micrometer is normally used to obtain these measurements(MICROMETER). In most cases, measurements are taken at three locations on a journal surface: at either end, and again in the middle. Note that the spots where these three measurements occur can be estimated; they do not need to be fastidiously selected.

Two distinct measurements must be taken at each of these three locations. Imagine that the crankshaft is sitting inside of an engine, with the crankpin directly below the cylinder bore. One journal measurement will be made parallel to the cylinder bore, the other perpendicular to it. Since there are three locations on each journal surface to measure, each of which demands two measurements, a total of six measurements will need to be taken for each main bearing and crankpin journal an engine possesses.

A typical crankshaft will possess a journal at each end for use with a main bearing, and one crankpin journal for each cylinder. A one cylinder engine normally has three journals, each of which should be measured in six places.

Once your journal surface measurements are obtained, compare them with the specifications contained in the machine service manual. If any measurements fall outside of these recommendations, the crankshaft will need to be reground or replaced.

Check the roundness of the journal: size is not the only thing for which a crankshaft main or crankpin journal must be measured. Remember that two measurements are taken at each of three locations along a journal surface. Because two measurements were taken, you can compare them by subtracting the smaller from the larger, to determine if a journal is out of round. Out of round means that a journal surface is no longer perfectly round, that it has become oval from wear or age. A certain amount of ovalness is permissible. If a journal surface diameter proves to be outside of specifications because it is too much larger in one direction than the other, it is considered to be out of round, and must be reground or the crankshaft replaced.

Check journal taper: you do this by comparing the measurements taken at each of the three locations (left end, right end, and middle) along a journal surface. If the diameter at one end of a journal is smaller than the diameter at the other, the journal is tapered. As with out of roundness, some taper is permissible. The engine manufacturer will specify maximum taper allowed for each main bearing and crankpin journal a crankshaft possesses.

Record the measurements taken: draw a crude inspection form on a piece of paper which contains spaces to note measurements from journal surface areas. This will you’re your measurements better organized, and keep you from needing to repeat them. If measurements are written down as they are taken, you can judge out of roundness and taper by measuring just once.


1)When it comes to measuring a crankshaft, your service manual will usually specify the main bearing surface O.D.(outside diameter). This is another term for each main bearing journal surface of a crankshaft. The specification will provide a permissible size range for the main bearing journal diameter. The diameter you measure should fall within this specified range.

2)Another specification you will see is the main bearing(journal) surface maximum wear limit. This specification indicates the smallest diameter allowed for a usable main bearing journal.

3)You will find a specification for crankpin O.D.(outside diameter). This is the acceptable diameter range for a crankpin journal.

4)There will be a specification for the maximum acceptable out of roundness for a crankpin journal. You should use this specification to determine whether any crankpin on a crankshaft has become excessively out of round. To determine this, you must compare the two diameter measurements taken at each location(of three) along the crankpin. If the difference between any pair of measurements is greater than the specification, the crankpin journal must be reground or the crankshaft replaced.

5)Another specification listed will be maximum allowable taper for a crankpin journal. Taper is found by comparing the maximum measurement taken at each(of the three) positions along the crankpin to the minimum measurement taken at the other two positions. If the difference between these measurements is greater than the specification, the crankpin journal will need to be reground, or the crankshaft replaced.

6)Another measurement not touched on above is crankshaft end play, or the amount that a crankshaft can be shifted in and out after it is installed in the crankcase. Installation will obviously need to be performed first.

You should always begin crankshaft appraisal with a through visual inspection, then proceed to comparing measurements to service manual specifications. If your crankshaft passes all these tests, it can be reinstalled in the engine.