Short splines on the 5th gear main shaft is the major design defect.  What Dodge owners have said:  "Mine came loose and we pulled the back cover off  and sure enough, there was parts in the bottom and the nut and 5th gear were backed off"."Hauling a heavy trailer cross country most of the year  my 1 ton Dodge pickup, it seems like every 1-2 years the main shaft nut backs off of the main shaft. The last rebuilt NV4500 lasted only 8 months before it started having fifth gear problem. The builders I bought it from claimed to have some sort of a lock to stop the problem.Our recommended fix is a replacement shaft that has a longer splined area, it is thought that most of the problems are as a result of lugging the engine, i.e. say when approaching a stop light thinking that it will turn green you slow down the light changes to green and with shifting down you power through the light!See the engineering drawing for the updated main shaft design for the NV4500.  This updated main shaft increases the spline length by 1" and we use a retaining clip to accomplish a 100% sure fix for this problem.   For the 2WD main shaft kit order our part number MWT4741468-R your cost is  $199.95.  The 4WD mainshaft kit is our part number MWT4882318-R your cost is $199.95 plus S&H.Our kit includes a newly designed main-shaft, retaining clip package and a new main shaft retaining nut.  When installed into the main case of the transmission, the main shaft tolerance should be .002" to .006".The NV4500 is capable of handling more HP with upgrade.

• Upgrade the input shaft to 1 3/8"
• We can provide cryogenic treating to shafts and gears and increase the strength by 30 %.