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42 & 90 Degree Gearbox Caps

A couple of weeks ago I was asked to give some basic information to the maintenance man the pickup team brought with them. He hadn’t worked on Huey for 30 years. Despite this, he had retained a lot over the years.

They were taking the UH-1H from California to Illinois. In the process of going over the recent maintenance it was mentioned that there was an oil seep/leak from the 42 deg. gearbox during ground runs.

They had replaced the cap O-ring and the sight glass O-ring. The aircraft had not been run since then. Something clicked for me, so I asked them to check the filler cap. It was vented. This was a 204-040-003-37 gearbox see enclosed photo figure 181.

This gearbox has a vent installed, the cap has to be non-vented or the oil will be siphoned out in operation and with less than 2/3 of a pint it goes fast. It was a vented cap. The cap was changed to a non-vented one and no leaks. The 212-040-003-023 gearbox figure 88 from the Cobra doesn’t have a separate vent so the cap must be vented.

If this cap is not vented it will force oil out of the seals. The 90 deg. gearbox is always vented. From the outside the caps look the same so when replacing caps always check to see if it is vented or not. All of this is in TM 55-1520-210-1.


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