Major Engine Failure Analysis

Unit Objective:

After completion of this unit, students will be able to associate causes with effects of major engine failure. This

knowledge will be demonstrated by completion of a quiz with 85 percent accuracy.

Specific Objectives and Competencies

After completion of this unit students should be able to:

- Identify engine parts that are obviously worn

- Associate worn parts with the cause of wear

- Understand constancy , change, and measurement (Standard 648.03)

Teaching Time:This unit should take 2 hours to complete

Steps for students to complete before the next lesson:Major engine failure analysis quiz

Work on removing paint

Steps for teacher to complete before next lesson:Check status of parts orders

Teaching Materials:Parts:

Examples of worn, scored, discolored, or broken parts

Resources:

“Major Engine Failure Analysis” video by Briggs & Stratton

Briggs & Stratton Repair Manual

Teaching Activities:Show “Major Engine Failure Analysis” video by Briggs & Stratton

Display examples of engine parts and explain the signs of failure related to each part.

Give written quiz included in this section

Prepare a quiz on engine failure analysis, use actual parts or pictures in this section

Knowing the causes of engine failures can save you time and allows discussion of the failure reasons with the

engine owner.

Four most probable causes of major engine failure:

Abrasive grit

Lack of lubrication

Overheating

Over speeding

Abrasive Grit: Abrasive grit can enter the engine in a number of ways, such as:

Through an improperly serviced or missing air cleaner

Through loose throttle or choke shaft

When the oil plug or dipstick is removed

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Insufficient cleaning during overhaul

Abrasive grit can enter an engine at almost 25 MPH through the air cleaner. When this occurs, an

examination of the air cleaner assembly will reveal the presence of abrasives around the air cleaner stud and

on the engine side of the air cleaner.

From the air cleaner, abrasives travel through the carburetor and accumulate on carburetor parts. This can

cause excess wear of the throttle and choke shafts and bushings.

One of the most distinctive signs of abrasives passing through the carburetor is a unique wear pattern found

on the intake valve. Each time the valve opens and closes some of the abrasives are caught between the

valve face and seat. Soon the valve seat wears wider, toward the cylinder and a groove develops on the valve

face. Some of the abrasives are dragged down into the valve guide to eventually wear the valve guide.

Once through the intake valve the abrasives are drawn into the cylinder. At 3600 rpm the piston changes its

direction of travel 120 times each second. While abrasives are present, the rings wear rapidly resulting in

excessive ring end gaps. In addition, the cylinder also becomes worn. The cylinder’s crosshatch pattern is

erased, and at the top of ring travel in the cylinder, a ridge is developed. An engine which receives regular

and proper maintenance will continue to show crosshatch patterns even after hundreds of hours of rugged

use.

Abrasive grit can cause additional destruction on all internal parts such as, crankshaft journals and main

bearings.

The main symptom of failure from abrasive grit is scratched or pitted parts and wear beyond reject sizes.

Lack of Lubrication: Operating an engine with an insufficient amount of lubrication causes temperatures to

increase beyond what the engine can tolerate. Excessive temperatures cause discoloration of internal parts.

Parts most commonly found discolored are the connecting rod, crankshaft journals, main bearings, piston pin,

piston skirt, and the cylinder.

Besides discoloring parts, the increased operating temperatures also change the working clearances between

the crankshaft journals and their respective mating bearings, resulting in more scored bearing surfaces. When

two or more crankshaft journal bearing surfaces are scored, the engine has usually been run, at sometime,

with an insufficient amount of lubrication.

When scoring is isolated to a single crankshaft journal bearing surface, the cause can usually be attributed to:

a manufacturing defect involving that bearing surface, or the manner in which the engine had been mounted to

the application. For example, excessive belt tension, misalignment between engine and equipment, etc.

The main symptom of failure from lack of lubrication is discoloration of parts that require lubrication.

Overheating: An accumulation of debris within the cylinder fins prevents the circulation of air and the cooling

of critical engine areas. A partial restriction of the cylinder fins is sufficient to increase the operating

temperatures of the engine above normal. When this condition occurs, damage can result.

Excessive engine operating temperatures produce “Hot Spots” within the cylinder. These same higher

temperatures can also cause the cylinder to fluctuate in size, preventing proper conforming and sealing of the

piston rings. Excessive oil consumption and smoking result. Since metal expands with increased

temperatures, the excessive heat generated from plugged cylinder fins, especially in the valve area, can

produce sufficient expansion to loosen the exhaust valve seat. In some extreme instances, the exhaust valve

corner of the cylinder may warp. This condition can be detected by placing a straight edge across the head

gasket mounting surface on the cylinder. Loss of compression and a blown head gasket result.

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When the engine continues to run in an “overheated” condition, crankcase oil loses it’s viscosity because of

the continuous “cooking” it experiences at the higher temperatures. Besides losing effective lubrication, the

“cooked” oil is reduced to a tar like substance within the crankcase.

The main symptoms of failure from overheating are discoloration and warping of parts.

Over speeding: A connecting rod that breaks as a result of over speeding may not necessarily show signs of

discoloration, scoring or seizure. However, it is very typical of a rod which has failed due to over speeding to

break very close to the piston pin.

The real potential dander of an over speeding condition is the flywheel exploding. This occurrence is not

usually considered by most individuals, even those with many years of engine experience. As engine speed

increases, centrifugal forces working upon the flywheel also rapidly increase. Sufficient forces can be

obtained causing the flywheel to explode. Removing a flywheel by striking it with a hammer, or prying

underneath it with a screwdriver, increases someone’s chances of experiencing a flywheel exploding.

Major Engine Failure Cause and Effect Relationships

Cause Effect

Improper or Infrequent air cleaner service. Damage air

cleaner mounting gaskets or bent air cleaner stud.

A. Premature Wear

A & B B. Seizure

Insufficient amount of lubrication. C. Breakage

B, C, D, & F D. Scoring

Misadjusted Governor/Bypassing Governor E. Over speeding

E, C, B F. Overheating

Plugged cooling fins G. Excessive Oil Consumption/Smoking

F, E, C, & G  

Loss of crankcase vacuum  

G, B, C, D, & F  

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Major Engine Failure Analysis Quiz

Name_______________

1. Engine wear is normally associated with:

a. dirt or similar abrasive

b. insufficient amount of oil

c. Over speeding

d. Over heating

2. A cylinder measures 0.002" over standard at the top and 0.005" at the bottom. Abrasive grit most likely

entered the engine through the:

a. Air cleaner

b. PTO oil seal

c. Oil inlet when oil was added

d. Carburetor throttle shaft

3. When an engine becomes worn from abrasive grit, the complaint that usually comes from the engine owner

is:

a. the engine lacks power, smokes and uses too much oil

b. runs too fast

c. seizes

d. hunts and surges

4. An engine which has consumed and excessive amount of abrasive grit it will normally have:

a. scored and discolored bearing surfaces

b. a warped head

c. no cross hatching in the cylinder

d. a worn exhaust valve face

5. An engine that has run with and insufficient amount of oil will:

a. always break a rod

b. score only the cylinder

c. cause internal discoloration and scoring of two or more bearing surfaces

d. cause cylinder discoloration

6. If an engine has scoring at only one bearing surface the cause was likely:

a. Over speeding

b. Overheating

c. Improper engine mounting

d. Insufficient lubrication

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7. When a connecting rod breaks due to over speeding, it most likely:

a. will score and turn black

b. was partially broken or fractured prior to the over speeding

c. not discolor and break close to the crankshaft

d. not discolor and break close to the piston pin

8. Plugged cooling fins will cause:

a. over speeding

b. over heating

c. excessive oil consumption

d. broken connecting rod

9. If an engine is suspected of over speeding, which should be considered:

a. insufficient oil level

b. plugged cooling fins

c. misadjusted governor

d. abrasive grit entering an improperly service air cleaner

10. Warping and discoloration from over heating will most likely occur on the:

a. crank shaft

b. cam shaft

c. connecting rod

d. head

Major Engine Failure Analysis Quiz

Name_____KEY__________

1. Engine wear is most often associated with:

a. dirt or similar abrasiveb. insufficient amount of oil

c. Over speeding

d. Over heating

2. A cylinder measures 0.002" over standard at the top and 0.005" at the bottom. Abrasive grit most likely

entered the engine through the:

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a. Air cleaner

b. PTO oil seal

c. Oil inlet when oil was addedd. Carburetor throttle shaft

3. When an engine becomes worn from abrasive grit, the complaint that usually comes from the engine owner

is:

a. the engine lacks power, smokes and uses too much oilb. runs too fast

c. seizes

d. hunts and surges

4. An engine which has consumed and excessive amount of abrasive grit it will normally have:

a. scored and discolored bearing surfaces

b. a warped head

c. no cross hatching in the cylinderd. a worn exhaust valve face

5. An engine that has run with and insufficient amount of oil will:

a. always break a rod

b. score only the cylinder

c. cause internal discoloration and scoring of two or more bearing surfacesd. cause cylinder discoloration

6. If an engine has scoring at only one bearing surface the cause was likely:

a. Over speeding

b. Overheating

c. Improper engine mountingd. Insufficient lubrication

7. When a connecting rod breaks due to over speeding, it most likely:

a. will score and turn black

b. was partially broken or fractured prior to the over speeding

c. not discolor and break close to the crankshaft

d. not discolor and break close to the piston pin

8. Plugged cooling fins will cause:

a. over speeding

b. over heatingc. excessive oil consumption

d. broken connecting rod

9. If an engine is suspected of over speeding, which should be considered:

a. insufficient oil level

b. plugged cooling fins

c. misadjusted governord. abrasive grit entering an improperly service air cleaner

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10. Warping and discoloration from over heating will most likely occur on the:

a. crank shaft

b. cam shaft

c. connecting rod

d. head

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Clogged Cooling Fins

Discolored and warped head surface from overheating, caused by

clogged cooling fins in above picture.

Loose valve seat caused by expansion and contraction from over

heating

Discolored piston pin and damaged piston caused from

overheating

Scored and discolored crankshaft journals from insufficient

lubrication

Worn piston from abrasive grit

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Bent air cleaner screw allows abrasive grit to enter engine

Cracked breather grommet allows abrasive grit into engine.

Cracked air cleaner grommet allows abrasive grit into engine.

Broken connecting rod near piston pin from over speeding

Broken block from broken connecting rod

Scored crankpin bearing from insufficient lubrication.

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