Specialized procurement consists of the following categories: Valve Replacement, Component Replacement, Obsolete Valve Replacement, Compliance with AML's (Authorized Material Listing), and Compliance with OSHA 1910.119 Management of Change.
Don't be lulled into complacency by shops who portray themselves as having the ability to supply reverse engineered parts, based on a sample you may provide. The two areas of concern should be materials of construction and dimensional tolerances.
Materials of Construction
Suppliers who are able to provide an accurate Positive Material Identification of the Component have only asked the first question, "What is the chemical composition?" What they are forgetting or ignoring can be equally or even more critical.
What is the grade or condition of the material? Has it been heat treated or case hardened to provide greater hardness or strength? This cannot be determined through PMI use, but only through extensive research and in many cases additional non-destructive testing.
Has the component been plated, and if so, why? This question left unanswered can have serious implications in parts manufacturing, and can lead to premature or even catastrophic failure.
Is the original component cast, wrought, or forged? Again, this is undeterminable by PMI. There is a time and a place for each of these material families to be called upon. Strength (both ductile as well as tensile), durability and toughness (hardness ranges), are key factors in the original material family selection by the manufacturer. Only through close examination, extensive research, or even additional testing can the material family be determined and only then can the proper substitute material be provided.
"Make it exactly like this" is a dangerous approach to reverse engineering. The sample provided in many cases has seen years, or even decades of service, resulting in wear and potentially an out-of-tolerance condition.
Even though valve manufacturing tolerances are considered loose by comparison to other engineered products; they do play an important part in the form, fit, or function of the component. Only a knowledgeable source can begin to ask the necessary questions such as "dimensional stack up", to determine if a component is potentially out-of-tolerance.
Machined finishes can also present a difficult question to the potential supplier. Is it a polished finish, a ground finish, or even a lapped finish? Questions such as these need to be addressed to make sure the proper finish is supplied. Absent of that, premature failure could be just around the corner.
Conversely, there are numerous component finishes where "as cast" is acceptable. In these cases the finish has no appreciable value to the function of the component, and efforts to attain better finishes and tighter tolerances on these "non-form, fit, or function" areas drives up cost for no reason at all.