This is a subject of intense interest as the value of collector cars soars. I wanted to provide some insights about this and hopefully clarify some misunderstandings.
I've attached the National Corvette Certification Board's (Bloomington Gold judges) policy and it pretty much mirrors my policy and thoughts over the past 20 years.
First of all, many people believe that a Corvette must have its original engine to qualify for a Bloomington Gold Certificate. In reality, that is not necessarily true.
If the engine stampings and the rest of the engine appear to be within 95% of they way they would have appeared at the day of production, there is no problem. In other words, if a restorer has accurately reconstructed the engine and stampings to the point that it appears no different from factory, it will pass. But it needs to be accurate to the gnat's eyebrow.
For example, our "Last" Corvette has an accurately restored engine (which is no secret) and has easily passed the Bloomington Gold standards.
On the other hand, if the inspectors can easily detect differences in the configuration or finish of the engine block, it will not qualify for a Gold regardless of its overall score or cosmetic perfection. Therefore, even though "the numbers might match", the differences in shapes of the stampings (compared to factory) will be detected by experts. And Bloomington Gold has the judges, technology, and records to do this very reliably.
Unfortunately, some less experienced judges (or over zealous "experts") can out-fox themselves by not knowing how to distinguish between inaccurate stampings and factory "flukes". If a judge makes such a mistaken identity, it may defame the automobile and engine in question. This is a disservice to the owner, but more importantly to the automobile and even the industry in general.
So what is the difference between accurately restoring an engine stamping and counterfeiting one? For this answer, I'll defer to Bloomington Gold's definition which has been their policy for decades. It is also one that I strongly support.
From Bloomington Gold's Procedures and Protocols, page 4/5.
RESTORATION vs. COUNTERFEIT
It is not Bloomington Gold's policy to "look for counterfeits". However, if a counterfeit engine, trim tag or VIN is discovered, our policy subjects the owner to immediate disqualification and ineligibility to participate in future Certification events. If a Bloomington Gold judge is aware of such a counterfeit, he is required to advise the Division Director. Furthermore, if any Bloomington Gold judge is found to be associated with a Corvette entered in Certification which is determined to be counterfeit, he is subject to immediate and permanent dismissal from the National Corvette Certification Board.
Bloomington Gold does not necessarily consider "re-stamping" to be counterfeiting. If the "re-stamp" was done with the intention of restoring or replacing the original numbers rather than deceiving, Bloomington Gold does not consider that to be counterfeit. Bloomington Gold uses Webster's Dictionary to define:
restoration: "To renew; to put back into existence or bring back to a former or original state." For Example:
1. Repainting an original Tuxedo Black Corvette with Tuxedo Black paint.
2. Restamping an original block with the same CID/Suffix/VIN as that block had when it left the factory.
counterfeiting: "To make an imitation of something else with the intent to deceive or defraud." For Example:
1. Repainting an original Glen Green Corvette with Riverside Red paint and changing the trim tag so it appears it left the factory as a Riverside Red Corvette.
2. Replacing the engine in a small block Corvette with a big block and making the CID/Suffix/VIN stampings appear that the Corvette left the factory as a big block.
3. Replacing the carburetor on an engine with a fuel injection unit and altering the block numbers and suffix code to make it appear it left the factory as a fuel injection Corvette.
Copy from existing material excerpted from Certification Guidelines
Note: Reprinted with copyright permission from Bloomington Gold