Rope trick

Despite having just flown the airplane to warm the Continental O-300D engine, cylinder number two was leaking badly on the differential compression (leak-down) test. This test is one tool in a mechanic’s toolkit to determine engine health. You pump 80 pounds of air pressure into a cylinder through its spark plug opening and read how much compression the cylinder retains using a specialized pressure tester. Readings greater than 60 psi are considered normal. According to Continental’s guidance in Service Bulletin 03-3, readings as low as the mid-40s are minimally acceptable when the tester is properly calibrated. My reading was 40 psi.

We could hear air gushing out of the exhaust pipe, but a borescope inspection did not reveal a burned exhaust valve or other mechanical anomaly. “Probably a piece of crud preventing the exhaust valve from closing completely,” said my mechanic. I wasn’t concerned. I’d seen this before, and an engine runup or quick flight usually burns off the crud (often a chunk of carbon) and reduces compression leakage in the cylinder.“Try flying the airplane to burn off the deposits and seat the exhaust valve,” my mechanic said.

We hauled the airplane outside and performed a high-power ground runup. Rechecking compression on cylinder number two within two minutes of engine shutdown yielded a new reading of…30 psi. Now, air was escaping from both the exhaust and the engine breather tube.

“We’ll do everything we can to avoid removing this cylinder,” said my mechanic. “Try flying the airplane to burn off the deposits and seat the exhaust valve. During flight, you’ll have 1,000 psi pushing on the exhaust valve. That should do the trick.” He explained that removing a cylinder is rough on an engine. Removing and retorquing cylinder base nuts twists the engine case and creates potential problems. I flew my airplane at high cruise rpm for another hour, and we eagerly removed the cowling for another compression check. We saw…40 psi.

Now my mechanic and I were getting concerned. The $1,600 cost of a new cylinder was less of a concern than the time, effort, and potential detrimental effects to the engine itself if we had to remove and reinstall a cylinder.

My mechanic had one final trick up his sleeve before he removed the cylinder for inspection and repair. He would lap the exhaust valve in situ using the “rope trick.” I’d heard about the rope trick for years but had never seen it performed.

To restore compression, my mechanic would use valve lapping compound to smooth the mating surfaces between the valve and valve seat. But before this could be done, two strong springs pulling the valve against the valve seat had to be removed. And to remove the springs (and rocker arms), a retainer must be dislodged using a spring compression tool while two collets holding the whole assembly together are pulled out.

Now here’s the rope trick: A clever way to prevent the valve from falling into the cylinder during this process is to fill the cylinder with about 10 feet of quarter-inch nylon rope stuffed through the spark plug opening. Once the springs are released, the rope is removed from the cylinder and compound can be applied to the valve using a long acid brush through the spark plug opening.

My mechanic attached a drill to the valve stem sticking out of the cylinder using a small section of hose and two hose clamps. The drill spins the valve, thereby grinding smooth the mating surfaces between the valve and valve seat. This process is called lapping. After four applications of compound and valve lapping, my mechanic was reasonably confident the crud was gone and the cylinder would hold compression.

We reversed the disassembly process, stuffing rope back in the cylinder, compressing the springs, and replacing the rocker arms, retainer, and collets. Once the whole valve assembly was back in place, the rope was removed, and it was time to learn the result. We pumped 80 psi into the cylinder, and it held 64 psi—a win for minimally invasive engine maintenance.

The next time one of your cylinders is low on compression from a leaking exhaust valve, ask your mechanic about lapping the valve in place using the rope trick. It doesn’t always work, but it’s worth the hour-long procedure to see if it can restore compression without pulling a cylinder.

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