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Here is a little video that shows the driveline noise made in 3rd and 4th gear. Also at about 50 seconds, you can hear what appears to be noise from the synchronizer and it has a little trouble going down to 3rd.
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Here is the NP420 ready for tear down. First, take the bottom plug off and drain the oil in a pan.
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Here are the material needed for the rebuild: ball pein hammer; plastic and rubber-faced hammer; steel and brass drift pin; wescott; 1/2" wrench (for the PTO); socket driver, breaker bar and sockets; breaker bar and 1 5/16" socket (to take off the output yoke) and a bearing puller. You will also need a caliper, feeler gauges, notebook, pen, camera and plastic bags (to put bolts in). A repair manual for the transmission as well as a master rebuild kit is also needed.
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The NP420 master rebuild kit comes from Transmission Exchange Company in Oregon and costs 50$. It includes all bearings as well as a gasket set.
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Here is the kit again shown with the oil seal for the output yoke that has to be purchased locally.
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For a service manual, you can use a Dodge shop manual or, like I did, a military NP420 maintenance manual (TM9-2520-232-35). The military manual, being intended for field service, includes much more detailed instructions that makes this job idiot proof.
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Then take the PTO lever of and unscrew the six bolts holding the top cover. Note that the two middle bolts are pilot bolts to properly register the cover on the transmission. Also note that the bolts are not the same lenght.
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Take off the shifter tower and set it aside for later inspection and repair, if required. Note the three forks: 3rd and 4th gear fork (on the left), the reverse idler fork (longer one) and the 1st and 2nd gear fork (on the right).
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Now we can inspect and find what was wrong with the transmission. It turns out that the 3rd and 4th gear synchronizer is out of specs. This is most likely due to a input or main shaft bearing being shot.
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Take off the input shaft bearing retainer. We can see that the cages, that hold the balls evenly spaced in the input shaft bearing, have separated and parts of rivets were found at the bottom of the retainer.
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To take off the output yoke and input bearing retaining nut, we must first lock the gears so the shafts can't turn.
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Then we pull the cotter pin out of the crown nut and unscrew it with a 1 5/16" socket and breaker bar. Pull the yoke off.
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To take the input bearing retaining nut off, use a brass drift pin and hammer to unscrew it in a clockwise direction. In the foreground, you can see one of the bearing cage that fell off.
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Return both the synchronizer and 1st and 2nd gear in neutral position. Turn the input shaft so the space where teeth are missing alines with the countershaft at the bottom and pull the input shaft out of the case. At the end of the main shaft, you will find a pilot bearing composed of 14 needle bearings. You can also see all the parts that fall of the input bearing. To take the input bearing off, take a brass mallet and tap it off in a rotating motion.
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Next, unbolt the the output shaft bearing retainer. Use a drift pin and hammer to drive the oil seal off from the retainer.
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Use a gear puller (shown here is a puller and bearing separator) to pull the output bearing off from the main shaft.
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Slide the main shaft to the rear and take out the synchronizer assembly. Then rotate the main shaft upward and pull it out of the case as shown.
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We are now going to take the reverse idler gear out. First, unscrew the idler gear shaft retainer...
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Then, while holding the idler gear from the inside, pry out it's shaft from the outside (use a prybar in the shaft's notch).
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If you haven't done yet, take the PTO out. Note the number of gaskets used, this will be required during reassembly.
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Unbolt the the countershaft front cover and take it off. Take the retaining wire off so the countershaft bearing retainer can be unbolted.
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Using a softwood dowel jammed between the case and one of the countershaft's gear (to keep it from rotating), use a breaker bar to unscrew the two bolts that hold the front bearing retainer.
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Unbolt the rear countershaft bearing retainer. Using a round wood dowel and or a small brass hammer, tap the countershaft wowards the rear untill it's gear contacts the reverse idler gear boss inside the case. This will serve two purpose: 1- to drive the rear bearing retainer out of the case and 2- drive the countershaft out of the inside of the front bearing.
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You can then tap or push the rear of the countershaft to push the front bearing off. Pull the countershaft back and rotate it while lifting the front to take it out at the top of the case.
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