Table 3-2. Crankshaft Dimensions

TM 10-3930-634-34

shafts on which regrinding is unnecessary.

(12) Measure all of the main and connecting rod

(19) If the crankshaft is to be reground, proceed as

bearing journals. Measure the journals at several

follows:

places on the circumference so that taper, out-of-round

(a) Compare the crankshaft journal measure-

and bearing clearances can be determined. If the

ments taken during inspection with the dimensions in

crankshaft is worn so that the maximum connecting

table 3-2 and figure 3-135, B, and determine the size

rod journal-to-bearing shell clearance (with new shells)

to which the journals are to be reground.

exceeds 0.0045 inch, or the main bearing journal-to-

bearing shell clearance (with new shells) exceeds

Table 3-2. Crankshaft Dimensions

0.0040 inch, the crankshaft must be reground. Also, if

the journal taper or out-of-round is greater than 0.003

inch, the crankshaft must be reground. Measurements

of the crankshaft should be accurate to the nearest

0.002 inch.

(13) Carefully check the crankshaft for cracks

(fig. 3-135, C) which start at an oil hole and follow the

journal surface at an angle of 45 to the axis. Any

(b) Measurement of the crankshaft journals,

crankshaft with such cracks must be rejected. Several

and comparison of these measurements to the diam-

methods of determining the presence of minute cracks

eters required for various undersize bearings shown in

not visible to the eye are outlined below.

table 3-2, will determine the size to which the crank-

(14) Magnetic particle method: The part is magne-

shaft journals must be reground.

tized and then covered with a fine magnetic powder or

(c) If one or more main or connecting rod jour-

solution. Flaws, such as cracks, forma small local mag-

nals require grinding, then grind all of the main jour-

net which causes the magnetic particles in the powder

nals or all of the connecting rod journals to the same

or solution to gather there, effectively marking the

required size.

crack. The crankshaft must be demagnetized after the

(d) All journal fillets must have a 0.130 inch to

test.

0.160 inch radius between the crank cheek and the

(15) Fluorescent magnetic particle method: This

journal and must not have any sharp grind marks (fig.

method is similar to the magnetic particle method, but

3-135, E). The fillet must blend smoothly into the

is more sensitive since it employs magnetic particles

journal and the crank cheek, and must be free of

which are fluorescent and glow under "black light".

scratches, The radius may be checked with a fillet

Very fine cracks that may be missed under the first

gage.

method, especially on discolored or dark surfaces, will

(e) Care must be taken to avoid localized heating

be disclosed under the "black light".

which often produces grinding cracks. Cool the crank-

(16) Fluorescent penetrant method: This is a

shaft while grinding, using coolant generously. Do not

method which may be used on non-magnetic materials

crowd the grinding wheel into the work.

such as stainless steel, aluminum and plastics. A

(f) Polish the ground surfaces to an 8-12

highly fluorescent liquid penetrant is applied to the

R.M.S. finish. The reground journals will be subject to

part. Then, the excess penetrant is wiped off and the

excessive wear unless polished smooth.

part is dried. A developing power is then applied

(g) If the thrust surfaces of the crankshaft are

which helps to draw the penetrant out of the flaws by

worn or grooved excessively, they must be reground

capillary action. Inspection is carried out under "black

and polished. Care must be taken to leave a 0.130 inch

light".

to 0.160 inch radius between each thrust surface and

(17) A majority of indications revealed by the

the bearing journal (fig. 3-135, E).

above inspection methods are normal and harmless

(h) Stone the edge of all oil holes in the journal

and only in a small percentage of cases is reliability of

surfaces smooth to provide a radius of approximately

the part impaired when indications are found. Since

/32 inch.

inspection reveals the harmless indications with the

(i) After grinding has been completed, inspect

same intensity as the harmful ones, detection of the in-

the crankshaft by the magnetic particle method to de-

dications is but a first step in the procedure. Inter-

termine whether cracks have originated due to grind-

pretation of the indications is the most important step.

ing operation.

(18) In addition to the standard size main and

(j) Demagnetize the crankshaft.

connecting rod bearings, 0.002 inch, 0.010 inch, 0.020

(k) Remove the plugs and clean the crankshaft

inch and 0.030 inch undersize bearings are available.

and oil passages thoroughly with fuel oil. Dry the shaft

NOTE

with compressed air and reinstall the plugs.

The 0.002 inch undersize bearings are used

(20) Inspect the shells visually for scoring, pit-

only to compensate for slight wear on crank-

ting, flaking, cracking, loss of babbitt, or signs of over-

3-85