They are the same gears which are at the center of a lawsuit the Justice Department has filed against Boeing for allegedly installing defective parts in the Chinook.
As CBS News National Security Correspondent David Martin reports, beyond that lawsuit is a long history of problems which have plagued the giant helicopter.
"There were serious technical problems. The crews were experiencing in-flight fires about every four to six weeks," said Ron Williams, who was once in charge of buying the Chinook and is now retired from the Army.
"We met as often as four to six times a month for the specific purpose of deciding whether or not to recommend the grounding of the aircraft over these problems," said Williams.
When asked if it was always sort of touch-and-go, Williams replied, "There were always doubts in my mind, yes."
A 1988 crash in Texas killed 10 soldiers and left several more with disabling injuries. While Boeing has not been accused of wrongdoing in that or any other fatal crash, air crews began to view the Chinook as an accident waiting to happen.
In a home video, Captain David Sullivan was able to joke about an emergency landing he was forced to make in Honduras in 1988, saying, "...another day in the life of a Chinook pilot. There's really not that much to it. It happens a lot."
But it turned deadly serious when five of his men were killed in the crash of another Chinook.
Many of the problems, including the in-flight fires, were eventually solved and the Chinook went on to compile an excellent safety record. But then, earlier this year, the Army discovered it still hadn't gotten all the bugs out of the Chinook.
In a two-week period, five sets of gears were found to have cracks or divots in the hard metal surface that spins at 12,000 to 15,000 revolutions per minute.
"The effect of urning that fast is that you have a lot of energy when this thing is operating if there should be a flaw, it could experience a failure in either the pinion or the gear," says engineer Mark Jeudi.
The army is spending an estimated $8 million to replace 143 sets of gears on the Chinook. But for all its problems, the Chinook is still the workhorse of every Army deployment.
In fact, the Army plans to spend another $2.5 billion to keep it flying for 20 more years. The initial contract has already been awarded to Boeing.