TOPIC 4. VALVES-INTAKE AND EXHAUST

TM 10-3930-644-14&P

TOPIC 4. VALVES-INTAKE AND EXHAUST

A. GENERAL

C. REMOVING VALVES

Valves require grinding at various intervals during the

The cylinder head as well as the valve tappet cover must

engine service life. These intervals cannot be exactly

be removed to gain access to the valves and valve

specified because many variable factors are involved,

springs. The end of each valve stem is fitted with a

often without the operator's knowledge. Of these factors

shallow steel retainer that surrounds the end of the valve

the following have been found, to a greater or lesser

spring, and is held to the stem by a pair of wedge

degree, to cause reduced valve life:

keepers or retainers. These retainers must be removed

before the valve can be removed.

1. Fuels that break down to farm deposits that

impair seat contact and prevent heat induction.

With a valve spring lifter (Figure 4-1) compress the

2. Deposits from either fuels or oils that

springs and remove the locks or pins from the valve

accumulate on the valve stems, and cause

sterns which are in a closed position. Close the other

burning and sticking.

valves by rotating the crankshaft and remove the locks

3. Oil not reaching valve guides due to dirt or

(or pins) from these valves in the same manner.

sludge.

Remove all valves and place them in order, in a rack

4. Shutting down a hot engine without idling for a

with holes. numbered for both intake and exhaust

few minutes. Exhaust valves that happen to be

valves, so they will be reinstalled in their original

off the seat when the engine stops may warp; so

positions.

that, burning occurs on restarting.

5. Tappet clearances not correctly maintained.

6. Lean mixtures due to incorrect carburetor or

incorrect carburetor adjustments.

7. Pre-ignition due to incorrect timing, wrong plugs,

carbon deposits, or excessive operating

temperatures.

B. CHECKING COMPRESSION

A compression check is the best method of determining

whether or not valves need grinding. Since different

pistons will develop different cranking compression

Figure 4-1. Valve Removal

pressures due to compression ratio variations, no

specific figures can be given for this test.

Most

D. SERVICING OF VALVES AND COMPONENTS

significantly, the pressures in all the cylinders must be

within a 10 p.s.i. variance. If it is felt that compression is

Upon removing each valve, examine it carefully.

leaking past the piston rings, inject some heavy engine

Remove all carbon and burned oil, and check the valve

oil through the spark plug hole before making the test.

stem and its fit in the guide. Excessive wear in either the

This will temporarily seal the rings. in addition, a quick

stem or guide will make it impossible to Secure a tight

knowledge of valve condition may be gained by listening

seat by grinding, unless the valve or guide, and possibly

at the carburetor entrance (remove air cleaner

both, are replaced.

Check valves and seats for

connection) and the exhaust outlet while the engine is

excessive burning, cracks or pitting. Check the valve

cranked over. Piston ring blow-by may be heard at the

guides by inserting a valve and noting the amount of

oil filler opening as the pistons are slowly brought into

side play. Worn valve guides should be replaced. Refer

compression and the air allowed to seep past. If valves

to following subparagraph 1, Valve Guides.

are leaking badly, the piston ring leakage may not be

noticeable. Another indication of leaking valves is an

Whenever the valve chamber cover is removed, the

unsteady vacuum reading, particularly at idling speed.

valve and spring mechanism should be examined for

evidence of inadequate lubrication due to sludging.

Excessive sludge in the valve spring area is an

indication of too low oil operating temperature, poor

filtering action, or an oil that breaks down and is unsuited

for the operation involved.

R-104-1

3-7