I was playing with putting a cross inside a small ring yesterday and found out that steel expands and shrinks when heated quite a bit more than I thought it would. Made the cross fit and hold by heating the ring and making it shrink around the cross.
Then today, I was drawing out the ends of a rod to equal lengths by marking the center and then measuring how far I drew each taper out so I was sure they were the same. Again, I was surprised to see how different the measure was between when it was hot and cold.
So I just went out to the shop and did a little more careful testing with digital calipers and a 1/2" square rod of mild steel.
What I found was that a 10" rod will lengthen about 1/8" when heated to a bright orange (around 1000C or 1800F). That's a simple enough number to remember and adjust for.
That value matches the documented thermal expansion numbers of steel I found on the net.
It never occurred to me that it would stretch that much. I assumed the expansion was more like a few thousands - something that would be hard to measure with a ruler and below the level of accuracy a typical blacksmith project would need to care about (aka one tap with a hammer to adjust). But it's a lot more than that becuase we are working with such large temperature changes. At these amounts of change, trying to make hand forged parts a specific length (like fitting a hot scroll inside a cold frame) becomes a fun challenge if you don't keep in mind the difference in length between hot and cold. Make the inside part snug when it's hot, and it will be very loose once it cools if you don't adjust for the change correctly.