Factory Mopar cranks were...
Factory Mopar cranks were rugged, whether cast-iron or forged. A regrind on the original piece will suffice for most street buildups.
Milling cylinder heads is a machine-shop operation similar to decking the block, but is done on the flathead gasket surface of the heads. Milling can be just a clean-up cut to ensure flatness and surface finish, or a heavier cut to decrease the combustion-chamber volume for a higher compression ratio. If a heavy mill job is going to be performed, the heads will sit lower on the block, and intake alignment becomes an issue. The intake-manifold surface of the heads (or the intake itself) will also need milling to compensate. Another consideration is that heavy milling will move the rockers in closer to the cam a like amount, which may cause the pushrods to have a less than ideal fit, since they will effectively become too long by the amount milled.
For high-performance or race use, the list of tasks to do on cylinder heads just keeps growing. If dual valvesprings are required by the cam selection, the stock-style valve seal will no longer fit, requiring machining for a guide-mounted seal. If high valve lift is also part of the plan (i.e. a large camshaft), the tops of the valveguides will need to be cut down to gain clearance; although fortunately, this machining operation is typically done in conjunction with the valve-seal operation. Depending upon the valvespring selection, the spring seat may also need machining to properly locate the spring. Now, for a real performance effort, the cylinder heads do more than seal-they flow air, and airflow is power. Porting heads quickly adds to the complexity and cost of cylinder headwork, depending upon how far it's taken. With the availability of new aftermarket cylinder heads, replacement is often a much more economically viable path.
The amount of time, effort,...
The amount of time, effort, and cost to fully prep a set of stock heads for performance use can add up quickly. An aftermarket head such as the Edelbrock on this test engine can provide the performance of a fully race-modified stock head out of the box. Compare the costs before going overboard with old iron.
The crankshaft is what conveys the reciprocating motion of the piston to the rotary motion that burns rubber at the rear wheels. In a simple rebuild, the crank is inspected for wear, and if in good condition, the journals can simply be polished and the crank reinstalled. If the journals are showing wear, they can be ground undersize by a crank grinder, creating a fresh machine surface for the bearings to ride on. Oversize bearings are available to match the undersize journal diameter, typically to .030 inch.
Mopar engines were equipped with either forged or cast cranks. Forged cranks are made by forging steel in permanent molds, while cast cranks are sand castings poured from cast iron. Forgings are higher in strength and vastly superior in terms of ductility. That being said, Mopar cast cranks have also proven themselves to be reliable in service, including typical street/strip and even sportsman race applications. Steel cranks require hardening to provide sufficient surface hardness for journal life. And for the most part, the factory used a deep penetrating induction-hardening process, which remains after an induction-hardened steel crank is ground. Some rare factory race cranks, however, were Tuftrided, meaning they underwent a shallow hardening process; a regrind on one of these cranks will require the part be reheat-treated.