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Slow down tire wear.


Nowadays, people want things quick, fast, and in a hurry. But no one wants rapid tire wear on their M939-series 5-ton trucks.

Premature tire wear shouldn't be a problem IF:

* your -A2 truck's CTIS maintains the right tire pressure

* the front wheels disengage when the front wheel drive selector lever is moved to the OUT position

* you have a functioning linear valve that engages and disengages 6-wheel drive

* the front end is aligned

But if premature tire wear is still a problem, you may be able to slow it down by giving some attention to your truck's Cardan and CV joint's front internal axles. If the Cardan and CV joints are worn, they can cause premature tire wear, especially on older trucks. Get your mechanic to replace the Cardan and CV joint internal axles when necessary.

You may find different combinations of internal front axle shafts on the M939 FOV due to unscheduled maintenance. Determine which joints need replacing on these trucks. You can figure out which internal front axle to remove and replace by measuring the outside diameter of the seal surface of the axle.

For M939A2s, if the outside diameter measures approximately 2.44 inches (and no less), your truck has the Cardan joint internal axle, also known as the U-joint style internal axle.

The M939A2-series truck's front internal axles have Cardan joint axle left hand shafts, NSN 2529-01-271-8024, and right hand shafts, NSN 2520-01-271-8025. To replace the seal retainer assembly, order retainer assembly, NSN 5330-01-271- 9490. The seal that's used with the retainer assembly is NSN 5330-01-271-9362.

Note: If you want to change just the seal itself, you'll need a special tool that's called out in the TM for that procedure. If that tool's not available, the truck head-shed recommends that you just order the retainer assembly.

For M939s and M939A1s, if the outside diameter on the same seal surface area measures about 2.122 inches (and no less), your truck has Cardan joint internal axles. Order replacement shafts for M939 and M939A1 truck front axles equipped with Cardan joint axle shafts using NSNs 2520-00-734-6985 (left-hand) and 2520-00734-6984 (right-hand).

By the way, these NSNs will bring you CV joint internal front axles with the same seal surface outside diameter measuring 2.122 inches. They are a suitable replacement internal axle. Cardan internal axles measuring 2.122 inches have been discontinued, so you can't order them.


You can replace the seal retainer assembly using NSN 4320-00-734-6951. The seal itself that's used in this retainer assembly comes with NSN 5330-00-145-8355. You can change this seal using common tools.

Note that the measurements of the internal axle seal surface outside diameters mentioned are the only possible measurements you can have with the Cardan joint/ U-joint style internal axles.

M939, M939A1, and M939A2 front axles equipped with constant velocity (CV) internal axle shafts, NSN 2520-00-734-6985 (left hand) and NSN 2520-00-734-6984 (right hand), use retainer assembly, NSN 4320-00-734-6951.

The seal used in this retainer assembly is NSN 5330-00-145-8355. This seal can be changed using common tools.
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Title Annotation:M939/A1/A2-Series Trucks ...
Publication:PS, the Preventive Maintenance Monthly
Date:Aug 1, 2010
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