IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:
THE CYLINDER HEAD
THE CYLINDER BORE
VISUALLY INSPECTING THE CYLINDER BORE DURING A REBUILD
MEASURING THE CYLINDER BORE DURING A REBUILD
MEASUREMENTS A SERVICE MANUAL WILL PROVIDE
RECONDITIONING THE CYLINDER BORE DURING A REBUILD
Recollect that a cylinder block is the most basic part of an engine. All other engine parts are fastened to either the inside or the outside of the block.
In very simple terms, the cylinder of an engine is a large hole that has been bored into the metal cylinder block. The cylinder serves two main purposes in a small engine:
1)Supporting the cylinder head
2)Holding the piston
The proper name for the hole in the cylinder block is the bore. The inside surfaces of the bore are called the cylinder walls. The bore is machined to an exact diameter, and made very smooth. This inside smoothness allows the piston to slide up and down inside the cylinder with a modest amount of friction.
The outside of the cylinder block of an air cooled engine is deeply finned to increase the surface area exposed to the air. The metal cylinder block becomes burning hot as the engine operates, and this heat must be transferred away from the cylinder block to keep the engine from overheating and burning up. The greater amount of surface area exposed to the air, the more readily the heat will dissipate. In this way the fins are an important part of an engine cooling system. Many modern cylinder blocks use an aluminum alloy for the finned portion of the cylinder; not only is aluminum lightweight, it is a metal which effectively dissipates heat.
Most small engines contain a single cylinder and one piston. Garden tractors and other large equipment will frequently contain multi cylinder engines. These are classified by the positions of their cylinders: the straight or in line type; the V type; and the opposed type. In the straight or in line arrangement, engine cylinders are positioned in a row. In the V type arrangement, the cylinders are angled to create a V shape. In the opposed arrangement, a pair of cylinders is positioned directly opposite to one another.
In all multi cylinder arrangements, the pistons and connecting rods are attached to a single crankshaft. The crankshaft has offset counterweights and crankpins that stagger the firing order of the cylinders. Therefore when one cylinder is on its power stroke, another cylinder will be on its intake stroke, and so on. This staggering helps ensure that the crankshaft rotate smoothly.
THE CYLINDER HEAD
The cylinder head is a metal cover that covers the cylinder. It is bolted to the top of the cylinder block, and, in general, will be made of the same metal. Like the cylinder block, the head is finned to promote cooling.
The underside of a cylinder head is hollowed out to form the combustion chamber, a dome like a football stadium which, usually at its center, contains a threaded hole for the spark plug. A gasket(called the head gasket) is placed between the cylinder head and cylinder to seal the two together for an airtight combustion chamber. Gaskets are used throughout an engine to form seals between connected metal parts, but you will find the head gasket thicker than most others, and made of heat resistant materials like asbestos fiber and aluminum.
A four stroke engine needs an intake and an exhaust valve to operate. Openings for these valves will be drilled into the cylinder head. Depending on engine type, the valves are positioned at different locations in the cylinder head, each valve arrangement requiring a different shape to the cylinder head.
There are three cylinder head shapes: the L type, the T type, and the I type. An L type cylinder head has a combustion chamber which covers the bore, then protrudes to the side of the head, so that the head is larger on one side than the other. Intake and exhaust valves are positioned vertically in the cylinder block, and they open upward, toward the top of the head. This is known as the side valve engine configuration, and twenty years ago was used by almost all small four stroke engines.
In a T type cylinder head, the combustion chamber covers the cylinder bore, then extends to both sides of the head. The intake and exhaust valves are on opposite sides of the head, and again they open upward. You will rarely if ever see this configuration in a small engine.
In the I type cylinder head, the intake and exhaust valves are positioned vertically at the top of the head, and open downward, into the combustion chamber. This type of head is used in overhead valve(OHV) engines, once found only in automobile and light truck engines, but now common in small four stroke engines too. If an engine contains the overhead valve arrangement, the abbreviation OHV will usually be printed on a valve cover someplace above the cylinder, probably in the neighborhood of the spark plug.
THE CYLINDER BORE
The cylinder bore serves several purposes. It guides the piston within the cylinder, helps to seal the combustion chamber, and, in the case of air cooled engines, assists in cooling by releasing engine heat. An air cooled engine cylinder absorbs the heat of the combustion process, then releases it to the atmosphere through these fins on its exterior.
Like the crankshaft, the bore must be visually inspected, and then several measurements taken to ensure that it falls within service manual specifications. Before an engine is reassembled, the bore should be reconditioned to ensure that the piston rings properly seal the cylinder during the compression stroke.
VISUALLY INSPECTING THE CYLINDER BORE DURING A REBUILD
Note the composition of the bore: an aluminum bore may possess a chrome plated surface. Because aluminum is a soft metal, a bare aluminum bore will tend to wear when contacted by steel piston rings. Chrome plating is used to reduce this wear. A chrome bore will look very shiny. You may inspect and measure the chrome bore for size, however it cannot be reconditioned if it has become too worn. When a chrome plated bore sustains heavy damage, the entire cylinder block normally must be replaced.
A more common means of preventing wear in aluminum cylinder bores is the cast iron sleeve or liner. The bore will be manufactured to a larger size than necessary, then an iron sleeve either pressed or cast into the block. The inner diameter of this sleeve serves as the bore, allowing steel piston rings to contact it rather the aluminum block. This significantly reduces cylinder wear.
Check for overall integrity: the bore should be inspected for signs of damage. Its walls should appear smooth, and free of cracks and score marks. If the bore is scored, the rings will not effectively seal the combustion chamber, resulting in a reduction of engine horsepower.
Several conditions can cause a bore to become scored. Dirt or debris may have entered the cylinder as the engine was operating. This happens frequently if an engine is operated with no air filter. Without a filter, dirt can easily penetrate an engine through its carburetor, and from there work its way into the combustion chamber. It is possible for dirt to become lodged between the piston and bore. As the piston travels up and down, this dirt will scrape the bore, leaving score marks.
Another common cause of score marks is improper engine lubrication. The cylinder wall must be coated with a film of oil to allow the piston and rings to move freely. In many small four stroke engines, oil is flung from the crankcase onto the cylinder walls by an oil slinger or similar distribution device. In small two stroke engines, oil is normally mixed directly into the fuel. For whatever reason, if there is a problem with an oil distribution system, oil may not reach the cylinder walls. If the bore is dry, friction between it and the piston may score the cylinder wall, and possibly the piston skirt.
Score marks in a bore can also be caused by overheating. If an engine is run at extremely high temperatures, oil on the cylinder wall will not afford enough lubrication for the piston and rings. At excessively high temperatures, oil may also break down and lose its properties, leaving behind score marks.
Score marks found on the cylinder wall should always be removed before an engine is reassembled. If marks are small, they can be removed during routine reconditioning. If the marks are deep, removing them is more problematic; it may enlarge the bore too much for use with a standard sized piston. For this reason, larger pistons, referred to as oversize pistons, are available. With an oversize piston, the combustion chamber will seal properly even if its bore was enlarged to remove deep score marks. Before enlarging the cylinder, you must determine if oversize pistons are available for that engine, and what sizes they are offered in. Once the size of the larger replacement piston is known, the bore can be enlarged to match. There is a limit to how much a bore can be enlarged. In an engine service manual, the manufacturer will specify how much a specific cylinder can be enlarged.
Enlarging a cylinder is referred to as boring the cylinder. This procedure is performed with a piece of equipment known as a boring bar. The carbon steel or diamond cutting tip of the boring bar allow you to enlarge a cylinder accurately. The boring bar also ensures that a bore ends up perfectly round, not tapered. Boring bars are expensive, so few home shops will generally possess one. Look to a local machine shop to perform this work.
Check for a cylinder ridge: another phenomenon to watch for as you inspect cylinder is a small ridge or step along the walls of the bore. This raised area, called a cylinder ridge, can usually be seen; it can be felt by running a finger along the cylinder wall. Cylinder ridges are caused by wear in the cylinder. As the piston moves in the cylinder, its rings contact the cylinder wall. Over time, this contact will begin to wear the cylinder wall, making the bore slightly larger. Since the rings are not located at the very top of a piston, a small area at the top of the cylinder will remain untouched, where a ridge may eventually form.
You will only find ridges on engines that are very old and well used. A ridge can be removed with a tool known as either a ridge reamer or ridge cutter. The ridge reamer is designed to cut away this ridge so that the cylinder is once again uniform from top to bottom.
MEASURING THE CYLINDER BORE DURING A REBUILD
Once you have visually inspected the cylinder bore, you should take measurements to determine if it is properly sized. The measurement process involves noting the dimensions at various locations along the bore, and comparing your findings to service manual specifications.
Check the size of the bore: cylinder bore measurements are similar to crankshaft journal measurements(CRANKSHAFT). Exact measurement locations for an engine will be provided in the service manual. Generally speaking, measurements are taken at three different locations along the travel area of the piston rings in a cylinder; locations will be near the top, someplace in the middle, and at the bottom.
Check the roundness and taper: at each location(top, middle, and bottom), the bore diameter is measured in two directions. The first measurement should be parallel to the crankshaft as it sits in the crankcase, the other perpendicular to it. By comparing each set of measurements taken, subtracting the smaller measurement from the larger, you can determine if the bore is out of round at any of the three locations. Out of round means that the bore has become too much larger in one direction than the other. Acceptable parameters for out of roundness will be provided in an engine service manual. If your measurements fall outside these specifications, the bore will need to be reconditioned.
Taper of the bore is determined by comparing measurements taken at its top with those taken at its middle and bottom. If these measurements differ, it means that the bore is narrower in one place than in another. A certain amount of taper is acceptable. An engine service manual will specify how much. As with out of roundness, if your measurements fall outside of these specifications, the bore will need to be reconditioned before it can be used.
Record your measurements: to keep track of measurements, create a crude form on a piece of paper which provides space to note each one as it is taken. Using a form saves time, and reduces the chances of error when measurements are checked against specifications.
Compare the measurements: refer to the service manual for the engine on which you are working for official specifications.
MEASUREMENTS A SERVICE MANUAL WILL PROVIDE
1)You will usually find a specification for I.D.(inside diameter), new; this provides guidelines for the original engine cylinder bore.
2)Another specification you will see is the maximum wear limit. This is the largest acceptable cylinder bore diameter for use with a standard size piston. If the bore turns out to be beyond permissible limits, the cylinder itself must be replaced, either fitted with a new sleeve or replaced with an entirely new crankcase cylinder block, or, if a given engine permits, bored larger for use with an oversize piston.
3)There will be a specification for maximum out of roundness allowable for the cylinder bore of a given engine. This specification refers to the shape of the bore, the degree to which it is oval instead of round. The amount of cylinder out of roundness is found by calculating the difference between the two measurements made at each location within the bore(top, middle, and bottom; SEE ABOVE). If the difference between any pair of measurements exceeds the service manual specification, the cylinder has become excessively out of round and must be replaced or bored to a larger size. A cylinder which is just slightly out of round, within specifications, can likely be restored to new when the bore is reconditioned.
4)The final specification you will find is for maximum allowable taper within a cylinder. Amount of taper is determined by comparing the measurements taken at any of the three locations in a cylinder(top, middle, bottom) to those taken at the other locations(SEE ABOVE). If the difference between any two of these measurements falls outside of service manual specifications, the bore has become excessively tapered; it must be either replaced, or bored to a larger size for use with an oversize piston. A modest taper can usually be corrected during routine reconditioning of the bore.
RECONDITIONING THE CYLINDER BORE DURING A REBUILD
If the bore passes visual inspection and proves to be within service manual wear specifications, the cylinder can be reused. Before reuse, the bore should be honed. The honing process is designed to slightly scratch the cylinder walls, leaving small scratch marks behind. Honing helps ensure that the rings seal properly. It can also correct small amounts taper or out of roundness, and even remove effects of minor scoring from the cylinder wall. Since honing involves a slight enlargement of the bore, the bore should be remeasured after the process is completed to ensure that it remains within specifications. The cylinder head and bore are normally a single component in two stroke engines, which may entail extra care to avoid enlarging the bore too much during the honing process.
Honing a new cylinder: newly manufactured engines are honed to create scratch marks on their cylinder walls. These small scratches assist in lubrication by establishing crevices that retain engine oil. When rings are installed in a new engine, ring surfaces will not precisely match the cylinder bore. A piston ring which has not yet been worn in may contact the cylinder wall in only a few spots. It takes a short time for the surfaces of new rings to wear to the shape of a cylinder wall, and form a tight seal; this short period is referred to as the break in period of an engine. Manufacturers will recommend that you use new equipment under a light workload for the break in period of an engine so that it will occur correctly. The wearing in of piston rings is referred to as the seating of the rings.
Each ring contacts the cylinder wall at intermittent spots before it is seated. Force of contact at these spots is quite high, creating high friction. Rings will therefore require extra lubrication until they wear to the shape of the wall. Once rings are worn in, the force up and down the wall is distributed evenly. A cylinder wall is slightly scratched by honing to provide reservoirs for the increased oil lubrication that rings need during the break in period.
Scratch marks left by honing are referred to as cross hatching. The marks are usually placed at angles of 45 to 60 degrees along a cylinder wall(never vertically). Scratches must be placed at an angle to the motion of the piston to prevent compression gases from escaping past the rings. As an engine is broken in, honing scratches will quickly wear off, and the cylinder wall is left perfectly smooth.
Tools used for honing: a bore is typically honed to restore scratch patterns to the cylinder during a rebuild. A tool called a cylinder hone is used. The cylinder hone is a round, cylindrical tool that fits neatly into the bore, and then is rotated by an electric drill. Honing stones surround the outer edges of the tool. These stones do the cutting. Different types of stones are used depending on the material being cut, and the amount of metal which needs to be removed. If the cylinder must be heavily honed to remove deep score marks or repair other irregularities, coarse stones are used initially for a faster cut. After this initial cutting, they can be exchanged for finer stones that will leave the cylinder walls with a smoother finish.
Hones are adjustable to the diameter of a cylinder. They possess knobs which can be swiveled to extend the cutting surfaces outward until they reach the cylinder wall. As the stones do their cutting, the hone will need to be readjusted periodically, to extend its cutting surfaces enough so they continue engaging the enlarged bore.
The type of hone just described is called a rigid hone. Instead of allowing its cutting stones to conform automatically to the shape of a cylinder wall, a rigid hone will hold its stones in the alignment the operator sets until they are manually adjusted. Since the stones are always aligned, the cylinder is cut in a perfectly round shape. Because of its design, the rigid hone is best for cylinders that are slightly either out of round or tapered.
Another type of hone is called the flex hone. It is similar to a rigid hone in that it requires an electric drill to be rotated, and possesses similar cutting stones. Instead of held rigidly in place, the cutting stones of a flex hone are located on spring loaded arms. As the flex hone is operated, spring pressure insures that the cutting stones remain firmly against the cylinder wall, even as the bore is enlarged.
The advantage of a flex hone is that it works more quickly, and is easier to use; the operator does not need to stop work to adjust the cutting perimeter as would be required with a rigid hone. Another advantage is that flex hones come in a greater variety of sizes, small sizes in particular, and thus can be used on many two stroke engines where a rigid hone would prove too large. However flex hones can be used only on cylinders which are properly shaped; they will not remedy either out of roundness or excessive taper.
To hone a small engine cylinder, the engine block must be securely anchored to prevent it from moving. The hone is mounted on either a handheld drill or drill press. If one is available, a drill press is frequently better because it keeps the hone positioned more uniformly.
Deglazing a cylinder bore: a cylinder that does not require resizing, and which is shaped properly, can be lightly honed to remove the ceramic like petroleum glaze that builds up on all cylinder walls during routine engine operation. Light honing is often referred to as deglazing. Flex hones are usually used to deglaze cylinders. So little cylinder wall material is removed during a deglazing that the standard size piston and rings can be installed in the rebuilt engine. Any time a cylinder needs resizing, a rigid hone with coarse stones should be used first, then fine stones used to finish the procedure.
Honing to recondition an old cylinder: to correctly resize a cylinder, you must ensure that it is perfectly aligned with the hone. It is also exceedingly important to use the correct honing stones. The stones used to resize an aluminum cylinder differ from those used to resize a cast iron or steel cylinder. Remember also that the honing stones must be in good condition. If not, they should be redressed or replaced before you commence the project.
Once the engine block is securely anchored, and the cylinder in position, lower the hone into the bore until the bottom end of the hone protrudes ½ inch or 1.5 centimeters beyond the lowest point of the bore hole. Then adjust the hone until its cutting stones contact the narrowest part of the cylinder; this will normally be at the bottom of the bore. The hone should not be excessively tight against the walls of the cylinder. If you cannot turn the hone by hand, loosen the adjustment knob until you are able to.
The drill should be set to operate within the range of 300 to 700 rpm. When the drill is started, move the hone up and down inside the cylinder at a pace of 40 to 50 times per minute. If all you need to do is a deglazing, just a few strokes will be necessary.
When resizing a cylinder, you will need to work out its narrowest end first. As the cylinder acquires a more uniform diameter, the hone should move gradually further along the cylinder length. The ends of the honing stones should not move more than ½ inch beyond either the top or bottom of the cylinder as the hone is used.
Follow instructions of the hone manufacturer when honing is done. Lubricant such as cutting oil must be used with some hones. Refer to recommendations of the manufacturer to determine the correct lubricant. On the other hand, some hones will not work at all if there is lubricant on the cylinder walls.
As honing proceeds, be sure to check cylinder bore diameter regularly, particularly when resizing. Try to stop every 10 to 20 hone strokes to check not just cylinder size, but its cross hatching. A cylinder is normally honed only enough to produce a uniform pattern of cross hatching on its walls. If a cylinder is being resized for a larger piston, it should be honed until the desired diameter has been reached.
To check bore diameter, remove the hone from the cylinder, and take the measurements described above(INSIDE MEASUREMENT TOOLS). As you cease honing for a measurement, keep the hone traveling up and down in the bore until the cutting stones have stopped turning. As rotation speed decreases, reduce the speed at which you are moving the hone up and down the cylinder. This will maintain proper cross hatching patterns. Once the hone has stopped, loosen its adjusting nut so that you do not scratch the cylinder wall as the cutting stones are extracted from the bore.
If you are resizing a cylinder, it is important to check its size frequently to avoid enlarging the bore too much. When the cylinder is within 0.002 inches or 0.05 millimeters of your desired diameter, the coarse stones you were using should be replaced with fine stones to finish the project. The procedure for finish honing is exactly the same as for coarse honing.
Cleaning the reconditioned cylinder: once honing has been finished, the cylinder walls must be cleaned. It is critical to remove all grit and other particles left by the honing process. The engine block should first be washed with cleaning solvent. Solvent will remove any lubrication oil that was used from the cylinder, however it will not completely remove grit and other particles. To remove these remaining particles, the cylinder must be washed with soap and hot water, and a stiff brush. Be sure your soap solution is very sudsy. Suds will get into cross hatching and float out gritty particles. To complete the cleaning process, wash the cylinder with hot water and dry it with soft clean cloths, or better yet, compressed air. When the cylinder is dry, a fresh coat of new motor oil should be applied to its walls to prevent rusting.
After honing is completed, measure the bore again to be sure it is the proper size. Record your new measurements; you will need them later on to determine piston to cylinder bore clearance.