Figure 6-23. Checking Cylinder Bores.

TM 10-3930-633-34

first to determine the maximum oversize. If the

cylinder will not clean up when refinished for the

maximum oversize piston recommended, replace

the block,

(3) Refinish the cylinder to within ap-

proximately 0.0015 inch of the required oversized

diameter. This will allow enough stock for the

final step of honing so that the correct surface

finish and pattern are obtained.

(4) For the proper use of the refinishing

equipment, f o l l o w the instructions of the

manufacturer. Only experienced personnel should

be allowed to perform this work.

b. Honing. H o n i n g is recommended for

refinishing cylinder walls only when the walls

have minor scuffs or scratches, or for fitting

pistons to the specified clearance. The grade of

hone to be used is determined by the amount of

metal to be removed. Follow the instructions of

the hone manufacturer. If coarse stones are used

to start the honing operation, leave enough

material so that all hone marks can be removed

with the finishing hone which is used to obtain the

proper piston clearance.

(1) Use a motor-driven, spring pressure-type

hone at a speed of 300-500 RPM. Hones of grit

Figure 6-23. Checking Cylinder Bores.

sizes 180-220 will normally provide the desired

e. Refinish cylinders that are deeply scored

bore surface finish. When honing the cylinder

and/or when out-of-round and/or taper exceeds

bores, use a lubricant mixture of equal parts of

the wear limits:

kerosene and SAE No. 20 MOTOR OIL.

(2) Operate the hone in such a way as to

Maximum out-of-round . . . . . . . . . . . . . . . . . . . ...0.001

Wear limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...0.005

produce a cross-hatch finish on the cylinder bore.

Maximum taper . . . . . . . . . . . . . . . . . . . . .,.....0.001

The cross-hatch pattern should be at an angle of

Wear limit . . . . . . . . . . . . . . . . . . . . . . . . . ...0.010

approximately 30 degrees to the cylinder bore.

Cylinder bore surface finish RMS . . . . . . . . . . ...15-35

(3) After the final operation in either of the

two refinishing methods described and prior to

f. If the cylinder walls have minor surface

checking the piston fit, thoroughly clean and oil

damage, but the out-of-round and taper are

t h e cylinder walls. Mark the pistons to

within limits, it may be possible to remove such

correspond to the cylinders in which they are to be

damage by honing the cylinder walls and in-

installed.

stalling new service piston rings providing the

piston clearance is within specified limits.

6-25. Replacing Core Plugs

6-24. Refinishing Cylinder Walls

a. Removal.

(1) To remove larger core plugs, drill a -

a. Machining. Cylinder walls that are severely

marred and/or worn beyond the specified limits

inch diameter hole in the center of the plug and

remove with a pilot bearing puller or large drift

should be refinished. Before any cylinder is

refinished, all main bearing caps must be in place

punch.

and tightened to the proper torque so that the

(2) On smaller plugs, drill a -inch diameter

hole in the center of the plug and pry out with a

crankshaft bearing b o r e s will not become

small pin punch.

distorted from the refinishing operation.

(3) Clean and inspect plug bore. Check

(1) Refinish only the cylinder or cylinders

carefully for any damage that would interfere

that require it. All pistons are the same weight,

with the proper sealing of the plug. If the bore is

both standard and oversize; therefore, various

damaged it will be necessary to true the surface

sizes of pistons can be used without upsetting

by boring for the next specified oversize plug. See

engine balance.

TM 10-3930-633-34P.

(2) Refinish the cylinder with the most wear

6-22