Mazda Training manual - part 236

 
3 – SHORT BLOCK 

 

29

Piston Engine Fundamentals

TC010-05-01S

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

MAIN BEARINGS

 

 
The crankshaft main bearings are split circular sleeves that wrap around 
the crankshaft main journals. The upper half of the bearing has one or 
more oil holes to allow lubricant to coat the inside surface of the bearing, 
as shown in Figure 18. The upper bearing fits into a main support on the 
bottom of the cylinder block. The lower half of the bearing fits into the 
bearing cap. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

FIGURE 17. The 
torsional damper 
controls twisting 
of the crankshaft.

 

 

Torsional 
damper 
(crankshaft 
pulley)

 

Rubber

 

Timing belt 
pulley

 

Crankshaft 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
 
 
FIGURE 18. 
Crankshaft main 
bearings wrap 
around the main 
journals. 
 

Upper main 
bearing 

Oil hole 

Lower main 
bearing 

 
 

 

 

 

 

 

 

 
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Piston Engine Fundamentals

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FIGURE 19. 
Bearing 
clearance allows 
oil to circulate 
between the 
bearing and 
journal 
surfaces.

 

 

Oil passage

 

Upper main 
bearing

 

Oil film

 

Lower main 
bearing

 

Crankshaft 
main journal

 

Bearing 
clearance

 

 
 
 
 
 
 
 
 
 

The wear surfaces of the bearings are made of softer material than the 
crankshaft. The softer material reduces friction, and it tends to mold itself 
around any uneven areas on the main journal. In addition, if wear does 
occur, it will affect the bearing, which is cheaper to replace than the 
crankshaft. 

 
 
 

Lubrication

 

 

In most engines, the upper and lower bearings are not interchangeable 
because the upper bearings have an oil hole. Pressurized oil is forced 
through the hole, allowing lubricant to flow around the bearing as the 
journal turns. 

 

To prevent the bearing from seizing on the crankshaft journal, the inside 
diameter of the bearing is slightly larger than the journal diameter, usually 
by a few thousandths of an inch. The gap between the bearing and the 
journal allows oil to circulate freely between the two surfaces, as shown in 
Figure 19. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

 

 
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Piston Engine Fundamentals

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Clearance

 

 

The gap between the bearings and the main journal is called the bearing clearance. 
Clearance is one of the most critical measurements in the engine. 

 

The oil that lubricates the bearings does not actually stay in a continuous film. As the 
crankshaft turns, the oil works its way to the outer edges of the bearings, where it is thrown 
off into the crankcase. New oil constantly feeds in through the oil hole to replace the oil 
thrown off. 

 

The constant flow of oil over the bearings helps cool them and flushes away grit and dirt 
from the bearing surface. If the clearance is too small, not enough oil will be allowed in to 
lubricate the bearings. The resulting friction will wear out the bearings quickly. On the other 
hand, if the clearance is too large, too much oil will flow through the bearings. Oil pressure 
will drop and the crankshaft journal may start to pound against the bearing rather than spin 
inside it. 

 

The oil pump in an engine has a limited capacity, and if all the oil is being pumped through 
a few bearings close to the pump, then other engine parts will be robbed of lubrication. For 
example, bearings further away from the oil pump will not get an adequate supply of oil, so 
they will wear out quickly. (The major early symptom for loose or failing bearings is low oil 
pressure.) 

 

To prevent damage to the bearings — and possibly to the crankshaft — the bearing 
clearances must be set precisely whenever the bearings or crankshaft are repaired. 
 
 

 
3 – SHORT BLOCK 

 

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Piston Engine Fundamentals

TC010-05-01S

 
 
 
 
 
 
 
 
 

FIGURE 20. The 
thrust bearing 
controls the 
front- to-rear 
movement of 
the crankshaft.

 

 

Upper thrust 
bearing

 

Oil grooves

 

Lower thrust 
bearing

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Thrust Bearings

 

 

In addition to rotating, the crankshaft also tends to move back and forth. 
This movement is called end play. To limit end play, one of the main 
journals on the crankshaft is machined to accept a thrust bearing. This 
bearing keeps the crank- shaft from moving back and forth. The upper and 
lower thrust bearings have oil grooves that allow oil to flow around the 
journal, as shown in Figure 20. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
REVIEW EXERCISE 3

 

 
Fill in the words that correctly complete these sentences. Check your 
answers with the answer key on page 35.

 

 
1.    The ___________________ is the main supporting member of the 

engine. 

 
2.    The crankshaft is attached to the cylinder block with U-shaped parts 

called ___________________ . 

 
3.  The term main refers to parts used to mount the 

___________________ . 

 
4.    The bearing that controls crankshaft end play is called the 

_____________________ .