[LEWISMATKIN]

Only one weak point I know of on these engines is the crankshaft. The crankshaft on the 637 is not flame-hardened(called Tocco hardening or armasteel), instead, it is TUFF-RIDED, a chemical washed on hardener only penetrating .006" into the surface of the shaft. In order to provide undersize bearings, the shaft must be rehardened by either re-TUFFRIDEing the shaft, or chrome plating the journals. On this engine, please use a high-quality SAE30 or SAE40 engine oil (preferably diesel approved). The 305-351-379-401-432-478 gas engines can be resurfaced w/o going to the expense of this process.