Engine Terms

Definition of Terms Used In Engine Repair, Rebuilding and Maintenance

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This glossary is courtesy of Chilton's Motor Age magazine.

Align Bore: A procedure used to re-align main bearing or camshaft bearing bores to bring them back into alignment after warping of the block or alignment shifts due to long-term mechanical stresses. By securing the block and then passing a single boring tool through all the bores without changing the alignment of the tool, the bores are restored to proper position. Properly aligned bores ensure that each bearing will carry equal load.
Alkaline Base (Cleaner): A form of chemical cleaner used in automotive parts cleaning; includes most soaps and detergents, as well as resin-, silicate- and phosphate-based cleaners. Most work best when heated to 180�F and above.

Backlash: (usually said of gears) Total play measured in both directions. For example, timing gear backlash measures the total rotation of the timing gear when forced as far as possible in each direction with the crankshaft gear held fast.
Bearing: Also sometimes described as a "journal bearing." A bearing is a special, replaceable insert composed of several layers of such metals as lead, tin and antimony. These metals are capable of withstanding high loads and wearing for a long time, provided they remain coated with a thin layer of oil under pressure. The main and connecting rod bearings of a gasoline engine are each formed of two half-circle sections which are assembled together to form a complete, circular bearing.
Bearing Crush: When journal bearings are assembled, each insert installed either in a semi-circular bore in the block (or rod), or a similar area of a bearing cap. When assembled into the cap and the bore section of the block, the ends of the inserts stick out slightly. When the cap is put in position, the ends of the bearing inserts butt up against one another before the cap actually seats against the block. In this position, the inside diameter of the bearing will be slightly greater when measured between the centers of the inserts than when measured between the two ends of either insert. The difference between these two diameters is the bearing crush. The crush is removed when the main cap bolts or connecting rod cap bolts are torqued.
Bearing Oil Clearance: The difference between the outside diameter of the crankshaft bearing journal or crankpin and the inside diameter of the fully assembled bearing. This is the space that becomes filled with oil when the engine is started and remains filled with oil as it runs. Bearing oil clearance is usually measured with a crushable Plastigage insert. This clearance is critical because too large a clearance results in low oil pressure, while too small a clearance usually destroys the bearing because of lack of lubrication and cooling.
Bearing Spread: The difference between the diameter of a bearing insert when measured at the ends prior to assembly, and after assembly into the bearing bore. The bearing is snapped into position because of the spread.
Boring: The act of removing large amounts of metal from a cylinder or bearing bore in order to remove damaged metal, correct alignment problems, and increase diameter.
Bushing: (as contrasted with "bearing") Mechanically, almost identical to a bearing. The difference is that a bushing forms a continuous circle. Therefore, bushings are drawn in and out of continuous bores in a block, rather than being retained by a cap.

Chamfer: Creating a cupped section leading into the threads cut into a casting, such as a cylinder block. The top of the cylinder bore is also chamfered. The chamfer tends to prevent the metal from cracking when it is subjected to the stress imposed when the bolt is later torqued.
Concave Wear: Wear pattern typical on a high-mileage valve lifter. The convex or bulged character of a new lifter is replaced by a dished shape, in which more material has been removed from the center area of the lifter bottom than from the edges.
Connecting Rod Side Clearance: The clearance between the side surfaces of the connecting rod and cap adjacent to the edge of the bearing and the nearby sections of the crankshaft. This clearance must be controlled to keep the rod from shifting from side to side in the engine while it is running, and to prevent excessive oil flow through the connecting rod bearing. Excessive clearance may be corrected by replacement of worn parts or, if worthwhile, by welding material to the crankshaft and machining it down to provide the correct clearance.
Convex: Said of a new camshaft lifter. The lifter bottom is rounded with the lowest point occurring near the center, so it will contact the camshaft lobe somewhere near its center, rather than wearing the edges.
Compression: An engine's ability to trap air and reduce its volume to create pressure. This ability is lost when engine parts such as piston rings and cylinders wear, or when valves lose their ability to seal due to burning or mechanical damage. Compression makes an engine fire smoothly (helps good combustion) and operate efficiently. An engine compression test is one very effective way of measuring wear and checking for mechanical problems.
Concentricity: A situation where a smaller circle is exactly in the center of a larger circle when measured in every direction. Valves will operate efficiently and give long service life only if the valve guide is concentric with the valve seat.
Crankshaft Journal: The sections of the crankshaft which ride on a thin layer of oil coating the main bearing surfaces. These surfaces must be of the right diameter and properly polished to ensure long service life.
Cross-Hatch: A cross-hatch is honed into the cylinder in the last step of finishing the cylinder bore. The cross-hatch pattern retains oil for proper lubrication. If the bore was perfectly smooth, lack of lubrication would damage the cylinder bore, pistons and rings.
Cupping: A form of valve damage in which excessive heat causes the head of the valve to become cupped because it cannot withstand the normal forces created when it seats.

Deck Height: The height of the cylinder block surface on which the cylinder head rests. Deck height is measured from the crankshaft centerline to the deck surface. It should be uniform from front to rear and may be corrected by machining, if necessary.

Edge Loading: Said of a new camshaft on which an old lifter has been used. The removal of the convex characteristic from the bottom of the old lifter due to wear will cause the outer edges of the lifter to contact, and wear, the outer edges of the cam lobes.
End Gap: The distance between the open ends of a piston ring, as measured with the ring installed in the cylinder bore. End gap is required to ensure that the ends of the ring do not touch when the ring expands as the engine heats up. If they do touch, the ring will typically score or break, or the engine may seize. Excessive end gap produces excessive oil consumption and loss of compression. End gap must be measured with the ring installed squarely in the cylinder. Standard procedure is to position the ring at the lower end of the area of ring travel. However, the manufacturer of the engine or parts being used may specify another position.
End-Play: The distance the crankshaft can be forced back and forth when installed. End-play is controlled by vertical sections of one of the main bearings and the clearance between those sections, and an adjacent surface on the crankshaft.

Free-Standing Height: The height of a valve spring with no pressure tending to shorten it. If valve spring height does not meet specification, it should be discarded. If re-used, it may allow for valve bounce or improper sealing.

Grinding: A method of refinishing a crankshaft's bearing journals and crankpins. Grinding removes significant amounts of damaged metal and produces journals and crankpins suitable for use with undersize bearings, or bearings with a smaller asssembled inside diameter than the standard one.
Guttering Valve: Guttering occurs when leakage causes small grooves to form in the valve face due to the passage of hot gases. A common cause of guttering is poor seating. The valve face near the channel eventually burns away.

Honing: A method of removing metal from a cylinder bore. Honing is less aggressive (removes metal more gradually) than boring. The final step in finishing a cylinder is finish honing, which removes very little metal, and is used primarily to create the proper surface.
Hoop Stressing: A form of valve failure that occurs due to overheating. Cracks develop on the rim of the valve face because the center cools much more slowly than the outer edge. Hoop stressing may be a symptom of combustion or cooling problems, or it may result because the vehicle is working against a load greater than that for which it was designed.

Impact Failure: A form of valve damage that occurs when the valve seats too hard or hits the piston. It can occur due to a broken timing chain, faulty keeper installation, or operating the engine at excessive rpm.
Insert: It may be necessary to install a valve seat insert or insert guide if a valve seat or guide surface in a cylinder head is worn excessively. In other words, there is not enough undamaged metal remaining to create a guide or seat of suitable dimensions. The head is bored out to the proper diameter, then the new part is pressed in and must be finished. Some aluminum cylinder heads use insert guides and/or valve seat inserts because the head material is not hard enough to ensure good wear characteristics.
Installed Height: (said of a valve spring) The vertical distance from the valve spring pad (where the bottom of the valve spring rests on the cylinder head), to the lower edge of the valve spring retainer. Incorrect installed height can cause improper sealing or valve bounce if the dimension is too great. If installed height is not great enough, there may be damage to the valvetrain due to having the spring coils contact each other and put excessive strain on the valvetrain.

Knurling: A process which raises or displaces metal inside a worn valve guide to restore it. The inside diameter is reduced and then the guide is bored out to the proper dimension. The end result is spiral grooves that provide excellent lubrication and allow operation at a very small stem-to-guide clearance.

Magnetic Particle Detection: A process used to find cracks in cast iron heads, especially the combustion chambers. A powerful magnet is positioned directly above the area to be inspected. Then, an iron powder is sprinkled over the area. If there is a crack, it will interrupt the magnetic field and produce a broken field pattern, rather than the single pattern that is normally visible. The check should be made twice, at 90 angles in case a crack would align with the field lines in such a way as to prevent detection.
Milling: A process of quickly removing with carbide cutters a thin layer of metal from the surface of a head or block deck. This is done to restore the flatness of the surface.

Necking: A thinning of the valve stem where it passes out of the head at the bottom of the guide. Abrasives or high temperatures may wear the stem surface, thinning it and possibly causing the valve head to break right off.

Open/Closed Pressure: Two of the tests done to determine the ability of a used valve spring to perform satisfactorily. Spring force is precisely measured at two different dimensions, one representing the normal valve-closed position, and the other representing the wide-open position. If the valve spring is square, stands at the right height and passes these two tests, it may be re-used. out-of-round - the difference between the cylinder bore measurement parallel to the crankshaft and perpendicular to the crankshaft.

Positive Crankcase Ventilation (PCV): A system that draws fresh, filtered air through the crankcase to remove blowby gases. The gases are then pulled into the intake manifold to be reburned. The PCV valve controls the airflow so it will be stable, regardless of intake manifold vacuum. A malfunctioning PCV system can cause engine oil leaks and high crankcase pressure, mimicking more severe engine problems.
Polishing: A process used to remove grinding "fuzz" or roughness remaining on the journals and crankpins after a crankshaft has been ground to size. The shaft will typically be micro-polished with a 320 grit belt to 0.0001 in. by moving the crankshaft in the normal direction of rotation.
Pyrolitic Oven Cleaning: A cleaning process that uses heat. Parts are baked at 400-800F, causing deposits to harden and flake off or to burn off.

Reaming: A process used to remove metal from the inside of a valve guide and increase the guide hole size. It may be done to allow the use of oversize valve stems, restore the diameter after knurling, or finish a new insert guide to standard inside diameter.
Ridge Reaming: A process used to remove the ridge worn into the top of the cylinder at the top of ring travel. The ridge need not be removed if the cylinder is to be rebored and oversize pistons installed.
Runout: A measurement taken to check concentricity of a valve seat or squareness of a camshaft drive gear. To check a valve seat, a special instrument which rides in the valve guide is installed and zeroed with its stem resting against the valve seat. It is rotated 360 and the highest measurement recorded to determine seat runout. To check a camshaft drive gear, a dial indicator is mounted along the axis of the camshaft with its stem resting against the outer edge of the gear. The indicator is then zeroed, the camshaft rotated 360 and the highest measurement recorded to determine runout.

Shim: A thin metal ring used to adjust (shorten) valve spring installed height. The shim is installed between the bottom of the valve spring and the spring seat on the cylinder head. Multiple shims of different thickness may be used to get just the right height. In this case, the thickest shim should be installed next to the spring.
Side Clearance: The clearance between the side of a piston ring and the side of the ring groove. Side clearance is checked with a feeler gauge and the ring to be used in the assembled engine. If clearance meets specifications, the piston may be re-used. If clearance is excessive (the groove is too wide due to wear), the piston will have to be replaced.
Spring Squareness: A check made to ensure that a used valve spring will produce even pressure all around the valve spring retainer. A spring that is not square will cause wear of the valve stem and guide. Spring squareness is checked with a T-square.
Stem-To-Guide Clearance: A precise measurement of the clearance between the valve stem and the guide. To take the measurement, the valve is inserted into the guide and a dial indicator is used to measure total motion of the valve tip as the valve is rocked back and forth in the guide. The clearance also may be determined by measuring stem outside diameter and guide inside diameter, and subtracting the smaller measurement from the larger one. This clearance must be great enough to ensure adequate lubrication, but small enough to prevent excessive oil consumption.

Taper: The difference between the diameter of the cylinder bore at the bottom and at the top, just below the ridge.
Torque-To-Yield: A type of head bolt. Torquing it to the correct figure actually causes the bolt to permanently stretch a small amount, but maximizes sealing force. This type of bolt must never be re-used.

Unseating: Said of a valve that does not remain tightly seated, but actually bounces after it closes. This commonly occurs because the valve spring is too weak. It may be accompanied by hydraulic pumping-up of the lifter, which creates poorer and poorer seating on successive closing cycles. It also may occur when a camshaft with a high lift and a rapid opening-and-closing profile has been substituted for a standard cam without modifications to the valve springs.

Valve Contact Line: The ring on the valve head and seat that represents the actual location where metal parts touch one another when the valve is lightly seated. Proper machining of seat and valve face width and angles will position the contact line 1/32 in. above the valve margin.
Valve Lapping: A process where the valve face is coated with a grinding compound and the valve rotated against the seat. This causes the surfaces to more perfectly mate with one another. Lapping is done after the valve face and seat are machined, or if the engine shows very little valve and seat wear.
Valve Lash: The clearance between the tip of the valve stem and rocker. Valve lash is always measured with the lifter positioned off the lobe of the cam. This measurement is taken while adjusting a solid lifter valvetrain, or to make sure hydraulic lash adjusters can adjust properly. Hydraulic lash adjusters must have their pistons bottomed in the lifter bores in order for this adjustment to be checked. Improper valve lash can cause valvetrain noise or improper seating.
Valve Margin: The distance between the valve's lower surface and the bottom of the face (the straight vertical section at the bottom of the valve head). When re-machining the valve face, it tends to get wider, and the machined area may get too close to the bottom of the valve, weakening its structure and leaving a sharp edge that is vulnerable to heat. There must be 3/64 in. of margin on older valves, and 1/32 in. on valves made of the latest materials.
Valve Seat Recession: A condition that occurs when the seating area of the head naturally recedes into the head due to pounding that occurs as the valve seats. Rapid valve seat recession becomes a problem when unleaded fuel is used in an engine without induction hardened seats. It also may be caused by valve spring tension that is too great. Seat recession is typically repaired with an insert seat.

This page last updated April 28, 2001. Send us your feedback, and come join the Imperial Mailing List - Online Car Club