this is according to Dan the gear man

I increased my Diesel engine power output. What can I do to make my NV4500 bulletproof?
It is relatively easy to increase the power output of late model Diesel engines. While the NV4500 is a pretty good transmission, it is not capable of taking extreme increases in horsepower on a sustained basis without sacrificing longevity.

The factory torque rating for NV4500HD transmissions is 460 ft/lbs. (About 235 hp). The design does seem to be capable of up to 600 ft/lbs with little loss of durability. However, as the horsepower and torque increases beyond 275hp/600ft-lbs torque, the size of the bearings, synchronizers, gears, and splines are no longer adequate. Rarely does a NV4500 fail catastrophically even at double the factory ratings, but the fact is most grossly overloaded 4500’s will not last even 100,000 miles without a rebuild.
Yes, we can eliminate typical fifth gear failures with the Torque King® mainshaft and gear sets, and yes, we can even supply larger input shafts, but there is no practical way to increase the capacity of the other components. Except for competition pulling, stock input shaft breakage with the NV4500 is extremely rare. Mainshaft breakage is even rarer.
When subjected to repeated overloads, accelerated bearing and synchronizer wear will occur. The NV4500 is a 195 pound transmission designed for stock vehicles up to 1 ton and 19,000 lbs Gross Combined Vehicle Weight. If your engine horsepower or towed loads are typical of medium and heavy duty trucks, you should, as a minimum, consider retrofitting a 6 speed transmission such as the 365 pound NV5600. Better yet, would be to buy a 2 ton truck made to take the power/loads you require.

 

To make it stonger the first thing i would do is up grade the input shaft to 1 3/8" (stock 1 1-4") but this will require u to upgrade ur clutch too. while u have it appart it would b a good idea to replace the 5th gear nut to be safe. you could change the counter shaft but iv never herd of one to fail. Other than that the nv4500 is a very strong gearbox.

 

 

i know mine is