Chapter 293: The Hidden Recession

Samuel Croft smiled triumphantly, tapping the brass dial with his knuckle. "Found it! A broken piston ring. The iron snapped across the slot and scored the bore."

Benjamin looked at the gauge, then at Mercer, who was standing twenty feet away, ostensibly reading a technical manual while listening to every word they uttered.

"Do not assume, Samuel," Benjamin warned.

"The cylinder has no pressure, Benjamin! Where else does compression go except past the piston?"

"Through the valves," Patrick suggested quietly from the rear of the stand. "Or through a blown copper head gasket into the water jacket."

Benjamin tapped his chin with the pencil.

Yesterday morning, during the general lecture, Mercer had mentioned an old workshop trick used on marine oil launches when the cylinders ran weak.

"Andrew," Benjamin said. "Bring the hand oiler from the lathe bench."

He took the brass can, slipped the long curved spout through the spark plug hole of Cylinder Two, and pumped three full squirts of heavy 40-weight lubricating oil directly onto the top of the piston crown.

Croft frowned. "What is that for? You’re drowning the chamber."

"If the piston rings are worn or broken," Benjamin explained, his confidence growing, "the heavy oil will sit in the ring lands for thirty seconds and form a temporary seal against the iron cylinder wall. The compression will jump fifteen pounds on the dial."

He threaded the gauge back into the head. "Patrick, turn her four times."

Patrick heaved on the crank.

The needle shuddered: twenty-six pounds.

Not a single pound higher.

Benjamin straightened his back, a slow, genuine smile spreading across his face. "The rings are holding what little they have. The leak isn’t going down into the sump."

"Then it’s the valves," Patrick said, pointing to the rocker gear.

They pulled the stamped sheet-metal rocker cover.

Benjamin reached into his waistcoat pocket, pulled out his personal feeler gauge, and slipped the ten-thousandths steel leaf between the rocker pad and the valve stem of Cylinder One’s intake.

Snug. A light, smooth drag.

He moved to Cylinder One’s exhaust.

Snug. Exactly ten-thousandths.

He checked Cylinder Two’s intake.

Snug. Ten-thousandths.

Then he slipped the thin steel leaf toward Cylinder Two’s exhaust valve.

The leaf struck a solid iron wall.

Benjamin blinked. He tried to force the leaf under the rocker pad, but the blade buckled against the gap. He tried an eight-thousandths leaf. Nothing. A four-thousandths leaf.

It wouldn’t even start into the joint.

"There is no clearance," Benjamin whispered, running his thumb over the rocker arm. "The rocker is resting dead against the valve stem even when the cam is on its base circle."

Patrick leaned in close, inspecting the pushrod below. "If the rocker is riding the stem while the engine is cold, what happens when the iron warms up?"

"The valve never hits its seat," Benjamin said, the mechanical logic snapping into place like an ash mortise. "The pushrod holds the valve head three-thousandths off the iron casting. Every time the piston rises on the compression stroke, the air blows straight out the exhaust pipe."

Croft seized a twelve-inch forged wrench from the tray. "I’ll back the adjuster nut off three flats and give it ten-thousandths."

Benjamin caught Croft’s sleeve with both hands. "No!"

Croft pulled against his grip. "Benjamin, the valve is jammed open! Let me turn the nut!"

"Mercer’s third question, Samuel!" Benjamin snapped, dragging the wheelwright back from the stand. "How do you confirm the leak before you turn an adjustment? If we turn the nut now, Mercer will fail the bench before dinner!"

He turned toward the tool rack, spotted the compressed air line used for cleaning lathe beds, and grabbed the rubber hose.

"Andrew, thread the air fitting into the plug hole," Benjamin ordered. "Patrick, turn the flywheel until Cylinder Two hits top dead center on the compression stroke so both valves are supposed to be shut."

They brought the piston to the top of its stroke. Andrew cracked the brass air petcock, forcing sixty pounds of shop air into the sealed cylinder.

HSSSSSSSSSSSS.

A loud, steady whistle of escaping air hissed out of the open exhaust manifold.

Benjamin held his open palm two inches from the exhaust flange. A cold, high-pressure jet of air blew against his skin, carrying the faint smell of burned soot.

"Confirmed," Benjamin said, stepping back with his arms crossed. "The exhaust valve is off its seat. Now you may take your wrench to the adjuster, Samuel."

Croft grunted, fitted the spanner to the rocker adjuster, backed the locknut off, and turned the square-headed screw out until his ten-thousandths feeler slid through the gap with a clean, oily drag.

He locked the nut with a sharp pull.

Patrick gave the starting crank three solid lifts.

TUP-TUP-TUP-TUP-TUP!

The twin cylinders caught with a sharp, echoing roar.

Fault Engine Three danced on its timber trolley. The blue exhaust pulsed into the overhead collection duct, the cooling water swirled through the rubber pipes, and the engine settled into that deep, rhythmic tup-tup-tup that meant two healthy cylinders working in harmony.

Croft let out a wild shout, slapping the timber stand. "We did it! Thirty minutes flat!"

Patrick grinned, wiping sweat from his forehead. Andrew began packing the wrenches into their canvas roll.

Benjamin stood beside the roaring engine, feeling the warm pulse of air against his knees.

He had found the symptom. He had isolated the pressure loss. He had set the clearance with a feeler gauge, and the machine was firing on all four strokes.

He looked toward the central aisle.

Mercer was walking toward them, his hands in his pockets, his face an unreadable mask of iron and chalk.

Mercer reached out, pulled the brass ground switch on the magneto, and killed the engine.

The workshop fell back into silence, leaving only the soft tick of cooling cast iron.

"The engine runs," Mercer acknowledged quietly.

Benjamin nodded, trying to maintain an expression of modest professional dignity. "Low compression on Cylinder Two caused by zero clearance on the exhaust valve rocker. Adjusted to ten-thousandths. Air seal confirmed."

"Very neat, Mr. Hale," Mercer said.

Then the instructor leaned his shoulder against the timber post, his gray eyes narrowing.

"Now tell me, Mr. Hale... why did that valve clearance disappear?"

Benjamin’s smile froze on his face.

"What?"

"Why did the clearance disappear?" Mercer repeated. "A rocker adjuster does not tighten itself down against a hardened valve stem. It is locked with a twenty-foot-pound jamb nut. The screw did not move."

Silence descended over Bench Three.

Croft stared at Mercer. "Someone assembled it wrong at the factory."

"Did they?" Mercer asked, his voice entirely unyielding. "Assume this engine belongs to an apothecary in Oriel. You back the screw off three flats, take his forty riels, and send him down the road. Three hundred miles later, the clearance disappears again, the valve hangs open, and he breaks down in a thunderstorm."

Mercer tapped the cold iron cylinder head with the knuckle of his forefinger.

"You fixed the symptom, gentlemen. You did not touch the disease. Pull the cylinder head off."

Benjamin felt a dull, heavy knot form in the pit of his stomach.

He didn’t argue. He picked up the breaker bar, signaled to Patrick, and began backing off the ten head studs in the proper diagonal cross-pattern.

They lifted the cast-iron head from its dowels and turned it upside down on the wooden bench under the gas lamp.

Benjamin leaned over the combustion chambers.

Four circular valve heads faced him, their faces coated in black carbon.

At first glance, they looked identical.

Then Benjamin looked closer, bringing his eyes level with the machined deck of the casting.

On Cylinder One, the exhaust valve sat flush with the cast-iron chamber roof, its beveled edge rising barely a sixty-fourth of an inch above the surface.

On Cylinder Two, the exhaust valve was sunken. It sat down inside a dark, beveled crater, recessed nearly thirty-thousandths of an inch into the cast-iron head.

"The seat," Benjamin whispered, his fingers touching the recessed rim. "The valve has dropped into the iron."

"Valve-seat recession," Mercer said, stepping up beside him. "The engine was run on an experimental lean mixture during fuel tests in Belfast. The combustion temperature rose four hundred degrees over standard. The hot exhaust gas turned the gray iron seat soft, and the valve hammered itself thirty-thousandths deep into the metal."

Benjamin stared at the sunken valve.

The mechanical sequence unfolded in his mind with absolute, terrifying clarity:

The seat softened.

The valve sank deeper into the head.

The valve stem rose upward toward the rocker arm.

The clearance disappeared.

The valve hung open.

The compression blew out the pipe.

"If we had only adjusted the screw," Benjamin murmured, "the valve would have kept sinking until the spring collapsed or the head pulled through the seat."

"Exactly," Mercer said. "A mechanic who only turns adjustment screws is a quack doctor giving laudanum to a man with a broken leg."

The instructor picked up a small tin of Prussian blue marking paste from the shelf and set it on the timber bench beside a hand-operated valve-seat cutting tool.

"Now," Mercer said with that quiet, terrifying smile of his. "You are going to cut a fresh forty-five-degree iron seat into that combustion chamber, lap the valve face with clover paste until it holds liquid paraffin for ten minutes without weeping, and then set your clearances."

He checked his pocket watch.

"You have two hours before the afternoon bell. Don’t drop any valves into your trousers, Mr. Hale."