I was a engine tester at the Tonn,NY

Each time we make a batch of iron, a 10 ton crane will move across the metal staging area and pick up certain metals and dump them into the cupola. Each batch is done this way, and the irom metalurgy between each batch can differ slightly. Certain iron batches are made for certain parts and heated to certain degrees. The only true way to tell the nickel content is by part number. But that alone is only a guideline, and cannot be relied on 100%. Most high nickel blocks will also have a brighter sheen after a chemical cleaning. It would be the same sheen if a standard block was shotblasted 2-3 times.

Iron properties will differ from each run. Some will be harder due to longer mold line time, and some are slightly harder due to a multi travel through the shot peening booth.

But don't be fooled that harder iron is better. GreyIron (blocks, heads) under a microscope look like a slivered chip, whereas nodular iron (cranks, carriers) look like perfectly round *****. Grey iron that is hard will easily crack. Grey iron that is soft will easily wear. Nickel blocks are nice to have, but they will also be subject to mold line time. In the end, it'just a brag to a select few if you have a nickel block.

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You need to understand how sand cores are created & assembled to have an idea of what I am talking about. The timing cover front slab core is a different sand core than the side slab cores or the rear slab core.

Each sand core has an identification number, so we can trace it back to the core number if there is a problem with the finished product. Because once the block has been poured, and mold line time has expired, the entire block mold goes through a shakeout process where the sand mold is destroyed and the iron block is extracted. Once this happens, all of the sand cores have been destroyed, and the numbers on each facet of the block is all we have left to trace identification.

Other identification areas are just inside the waterpump holes. You will see water jacket numbers there. You will also see numbers on the underside of the block, next to the first cam journal. This is the barrel core number. The large single digit number near the part number is the drag core pttern number. Alongside that, you might see CFD or GM-D, which means "central foundry Defiance, or GM-Defiance. SMCO means Saginaw metal casting operations.

Every facet of a block or head will have identification marks to tell me what particular core was used to create that particular part.

And as in your case, some cores are universal. Even all the big blocks & small blocks we make today use similar iron recipes. Some may have hotter iron batches such as the 3.8 cylinder head, but the ingredients are the same.... unless it's a bowtie run.

Hope this helps.

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When we pour iron, the only reference to what we are making is the last 3 digits of the part number. "We're making 781's today!" We never refer to any other identification such as core numbers. The iron blend is purely based on the casting part number.

Think about going out to buy a GM block. Try and ask the parts guy that you want a part number "xxx", BUT with a 010 side core or a 020 front core. He will look at you strangely, and then laugh at you, because the differences are indexed by part numbers only, not by core numbers. His parts book is indexed by part numbers only as well. GM indexes all the differences with their parts by different part numbers.

Once all the cores are screwed together to make a core package, the package assemblies can sit on shelves for days before they are placed in the mold for an iron pour. And... once all the core pieces are screwed together to make a core package, there is no possible way for us to see inside to verify what core numbers were used to make the core package.... Because of this, we cannot follow the myth that 010 ID's mean "high nickel", because it's impossible to view the 010 ID once the package is screwed together & shelved. The only ID we have is the part number.

What you wrote was how it was supposed to be. But once it was put through the system, engineering realized that their intent could not be efficiently tracked. So the plan was abandoned, but it would have been too costly to rework all the patterns in the core machines, so those were left alone.

When engineering originally developed the plan, they needed a way to identify the metallurgy after the block was poured, thus the fabled "stamp" But engineering soon realized that the same block part number could posess one of 3 different mettalurgical blends. Because of this, it was near impossible to trace or efficiently logistically locate castings from pourtime to end customer use, as well as service parts orders.

So it was abandoned, because with the original idea, you wouldn't be able to call any GM parts office & locate a "nickel 010 block", because there was not a specific part number associated with it. Parts are organized by part numbers, not core stamps.

Now, to straighten this out, they did change the casting part number to correspond with the metallurgical content. This is true for only older blocks (pre 71) Blocks after that just used the same sand cores as the older ones until pattern changes were made, which will give false hope to many.

Hinging on that, many cores were interchangeable. Today, error proofing measures have been installed, and it is impossible to assemble mismatched pieces without destroying a section of the core.

Processes were brutal years ago, and it was very common to grab the pallet of cores with the 010,020 stamps to keep the mold line running (even though the current run was not supposed to have the 010,020 stamps. The wost thing we could do, was to stop a mold line. If we did, the iron would cool, and we could freeze up the system with solidified iron. Always keep the mold line running! So, you cannot base your "nickel" block, solely on the 010 stamp on the timing face. Desperate times call for desperate measures, and cores have been switched in the past.

Today, most owners do not understand that the 010 on the side of the block has nothing to do what the metallurgical content is, or if it's a 4 bolt main.

Like I mentioned before, we have poured iron to what the part number is, not what the core stamps say. If I had 2 assembled core packages next to each other, you would not be able to tell if one had 010 on it, unless you destroyed & dissected it or poured iron into it & removed the iron to see. The one thing that is consistently 100% validated is the part number, which is part of the drag mold and is the seat for the upside down core package to rest in, before the cope is placed on the mold package, sealing it off.

In the end, if you have a 60's block with these stamps, there is a 95% chance that it is what you think it is. But it can be your luck that the 5% of mismatched core packages could be one of yours. If you have a 70's & up block, don't count on it, unless it's a bowtie (but only some bowties had different metallurgical qualities)

Hope that helps explain it.

a 1996 or later L31 truck block