In the early days, Hektor's smiths had hand-fitted every valve and bolt, but with the expansion of the Deep-Rail and the Green Ring, this "bespoke" engineering was failing. A bolt forged in the East Reach wouldn't seat in a flange from the North Vault. Maintenance was becoming a nightmare of custom filing and mismatched threads. To scale to a city of thousands, Kael knew he had to move from craftsmanship to industrial interchangeability. He initiated the Master Gauge project—the creation of the absolute physical standards for the Protectorate.
The technical challenge was the "Zero-Point." To create a standard, Kael needed a measurement that wouldn't change with temperature or wear. He utilized the geothermal bore's constant-temperature chamber as his laboratory. There, he forged the Master Rod—a bar made of a specialized "Invar" alloy of iron and nickel (refined from the Salt-Spur meteoritic ores) that had a near-zero coefficient of thermal expansion.
"Precision isn't about being small, Elms," Kael explained, using a fine abrasive of crushed garnets to polish the rod's ends. "It's about being the same, every single time. If we define the Ashfall Inch today, it must be the same Ashfall Inch a hundred years from now, whether it's used in a steam piston or a medical lancet."
The grit of the project was the creation of the Working Gauges. From the Master Rod, Kael's most skilled toolmakers produced "Go/No-Go" gauges—hardened iron rings and plugs that represented the maximum and minimum tolerances for every screw thread, pipe diameter, and gear tooth in the mountain. These were distributed to the Foundry and the Expansion Brigades. A smith no longer "guessed" if a part was finished; if it passed through the "Go" gauge but was stopped by the "No-Go," it was perfect.
Socially, the Master Gauge project met with unexpected resistance. The veteran smiths, who viewed their ability to hand-fit parts as a mark of their mastery, felt the new system devalued their "eye." Kael addressed this by introducing the Tier 9: The Metrology Guild. He took the most experienced smiths and turned them from makers into "Arbiters of Quality." They became the guardians of the gauges, responsible for checking the wear on the working sets and ensuring that the "Ashfall Standard" remained absolute.
A technical failure occurred during the standardization of the Deep-Rail's axle-journals. A batch of fifty axles was produced using a gauge that had been dropped and slightly deformed, a deviation of less than a hair's width. When installed, the axles ran hot within an hour, the friction threatening to seize the entire transport line.
Kael utilized the Precision-Lapping method to recover the parts. He designed a "Master-Lap"—a lead cylinder cast from a perfect gauge—and used it with a fine oil-and-grit slurry to grind the oversized axles down to the true standard. This incident reinforced the guild's mantra: The Gauge is the Law.
The engineering of interchangeability allowed the population to spike. With parts that fit perfectly every time, the construction of Tier 8 and Tier 9 Residential accelerated by 400%. The population count rose to 870 as a group of clockmakers and instrument-smiths from the Western Provinces arrived. They were drawn not by the heat, but by the rumor of a mountain where the iron was as precise as silver.
Kael stood in the Metrology Vault, watching a young apprentice verify a gear for the Green Ring's ventilation fans. The gear clicked into the gauge with a satisfying, airtight precision.
"The foundation is set, Elms," Kael said, his hand resting on the velvet-lined box of the Master Rod. "We are no longer building things; we are building a system that builds things. But a standardized city of nearly nine hundred people needs more than just mechanical parts. It needs 'Standardized Time.' If the shifts in the Green Ring don't align with the Foundry and the Rail, the energy load will spike and blow our valves. We need to synchronize the mountain."
Kael began the designs for the Chronos-Column—a massive, steam-driven master clock that would pulse the exact time to every tier through the acoustic pipes.