The X was added to the block to distinguish a car 409 engine from a truck block that had lower compression. Also during the closing years of the 409's production the factory addressed complaints of owners taking their 409 in for repair because they missed a shift at 8,200 and it over reved.
Great story but when the tech who took the 409 back to his shop (because that was a perk of line tech's that they could selflessly step forward to claim an engine to destroy it on behalf of Chevrolet). Back then the factory had the dealership throw warranty claim engines in the dumpster, or other wise destroyed them (interpreted by the techs to mean not as being not available for resale until they were rebuilt).
Because back then Chevrolet didn't want the motor back (it cost more to ship than it was worth) the tech got it. He took it home and pulled the heads off only to discover that the 409 they had warranted was in fact a 348. Even an experienced tech and a warranty claim officer can not tell if a 409 is a 348 except by the oil dip sticks location. Unfortunately $37 bought you a 409 oil pan and oil pump pick up. (there were other indicators but who would suspect an honest customer?)
Hence the factory getting wise and marking the 409 blocks toward the end of the production run with a very visible X (if they had done the same thing with the 427 block it would have saved me a fortune in money spent on 396 motors that I bought with a 427 decal on it that you could buy for $3.85). It also gives you an idea how far back people have been pulling oil pan swaps, and restamping of cheaper less desirable blocks (the 348 wasn't stamped with a VIN, but the 409 was).
Last edited by Big Dave; 02-25-2014 at 08:16 AM.
Reason: typi fron fat fingers