Piston Pin

The piston pin on an aviation engine must possess maximum resistance

to wear and to fatigue. For this reason, the piston pin is considered,

from a metallurgical standpoint, the most important part on the

engine to produce in quantities and still possess the above

characteristics. The material used for the Liberty engine piston

pin was S. A. E. No. 2315 steel, which is of the following chemical

composition: Carbon, 0.100 to 0.200 per cent; manganese, 0.500

to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,

0.045 maximum per cent; nickel, 3.250 to 3.750 per cent.

Each finished piston pin, after heat treatment, must show a minimum

scleroscope hardness of the case of 70, a scleroscope hardness of

the core of from 35 to 55 and a minimum crushing strength when

supported as a beam and the load applied at the center of 35,000

lb. The heat treatment used to obtain the above physical properties

consisted in carburizing at a temperature not to exceed 1,675 deg.F.,

for a sufficient length of time to secure a case of from 0.02 to

0.04 in. deep. The pins are then allowed to cool slowly from the

carbonizing heat, after which the hole is finish-machined and the

pin cut to length. The finish heat treatment of the piston pin

consisted in quenching in oil from a temperature of from 1,525 to

1,575 deg.F. to refine the grain of core properly and then quenching in

oil at a temperature of from 1,340 to 1,380 deg.F. to refine and harden

the grain of the case properly, as well as to secure proper hardness

of core. After this quenching, all piston pins are tempered in oil

at a temperature of from 375 to 400 deg.F. A 100 per cent inspection

for scleroscope hardness of the case and the core was made, and

no failures were ever recorded when the above material and heat

treatment was used.

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