On the last NV4500 (left) we rebuilt, we added an updated shaft, updated thrust washer and retaining nut, and welded the nut to the main shaft for good measure. We are currently running this transmission behind a 450 hp and 975 ft lb. We will keep you posted. If you have an NV4500 and don't want to hassle with this upgrade, it's best not to tow heavy loads in 5th gear. We should also point out that testing on this truck has shown that significant heat buildup in the transmission fluid begins to occur when towing a heavy load in 5th gear and turbo boost pressures exceed 13 psi.