They set the casting beneath a work light and began cleaning it. The block carried a long fracture along a water jacket, but a coat of old oil and dirt hid almost everything around it. Jonah worked patiently with brushes and rags until the rough surface of the casting showed through. After several minutes he pointed to a hairline mark and asked whether the crack ended there.
Silas leaned over without touching the iron. "What do you think?" he asked. Jonah said the line appeared to stop near a raised rib, although he had noticed a discoloration beyond the rib that might be connected. The old man handed him the inspection light and told him to look again from another angle. What had seemed like a neat ending dissolved into a much longer path, thin as a pencil mark, continuing into an area that would have escaped attention unless somebody had been looking for it.
"That's why you brought all this junk home," Silas said, pointing toward the wall. "Not because every piece can be saved. Because every one can show you something a good piece won't." Jonah followed the dark line until it disappeared beneath another layer of grime, and the joke about his collection suddenly seemed very distant. There was no room in his head for the farmers laughing at the salvage yard; he was trying to understand how a crack could travel farther than the eye first allowed.
Silas made him start with a notebook. On the first page Jonah wrote the date, the casting's approximate model, the location of the visible damage, and the reason the part had been discarded. He drew the fracture before touching it, added measurements as the surface became clearer, and left space for the results that would come after a repair attempt. Silas said a man who kept only the record of his successes could fool himself into believing he had never made mistakes.
Jonah took that advice more seriously than the old mechanic expected. He had a black composition book for the first group of castings and soon needed another because he was recording observations that other boys his age might have considered useless. A clean hole that wandered slightly from its intended line, a thread that felt different from the others, a wet patch that appeared only after the pressure rose: he wrote all of them down. He also wrote down the questions he was too embarrassed to ask out loud and brought them to his grandfather after supper.