It’s never a good sign when during your aircraft annual inspection the mechanic calls to report an issue. It’s sort of like getting a call from your doctor’s office following blood work and the person on the line says the doctor would like to see you within the next 24 hours! Depending on who is calling, either you or your plane has a problem!
Recently I got such a call from my trusted mechanic regarding my Cirrus SR22T with 660 hours total time. During the annual inspection he was surprised to find on a hot engine compression test that two cylinders had very low compression (32/80 and 38/80). Potential options for addressing the problem included putting the plane back together and flying it for another 15-20 hours and retest the compressions, replace the two cylinders, or do a “rope a dope procedure” as he called it to see if the compression would improve by grinding smooth the back of the exhaust valve and the face of the valve seat. The low compression might be due to carbon or rust or lead between the valve and valve seat preventing full closure of the exhaust valve. If successful, the down time and expense of two new cylinders would be eliminated. The other potential leaking site could be stuck piston rings failing to hold pressure within the combustion chamber. If non functioning rings were the problem there should be high oil consumption due to oil passing by the rings and entering the cylinder to be subsequently burned in the combustion process. Oil consumption was minimal at 1 quart every 10 hours There was no oil coating on the spark plugs and when performing the compression test air was heard hissing out the exhaust pipe. With this strong evidence that low compression was exhaust valve related the decision was to try the “rope a dope” trick.
After watching a YouTube video “Lapping an Exhaust Valve in Place” a can of Laser 2561 Grinding Paste was ordered from Amazon along with a package of 6” long Foam Cleaning Swabs.Two days later my mechanic was ready to perform the surgery under my expert supervision! The spark plugs and the two rocker box covers were removed to expose the exhaust valves and the rotocoils surrounding the exhaust valve stems.
By placing an ink mark across the rotocoil and its adjacent housing, the rotocoils function was tested by repeatedly hitting the valve stem with a rubber mallet and a piece of driftwood. The in and out motion of the valve caused rotation of the valve stem thanks to the rotocoil working properly.
Replacing a functioning rotocoil would not be required. Next, the valve spring had to be removed without allowing the now free valve to drop into the cylinder. To prevent this potential calamity the cylinder was packed with 25 feet of rope threaded in though the top spark plug hole.
The propellor was rotated compressing the rope against the valve as the piston rose within the cylinder. With the valve securely restrained the exhaust spring were removed. While holding the valve stem with one hand the rope was removed from the cylinder. The rope was pulled out of the cylinder. The valve was carefully pushed inward a bit to expose the space between the back of the valve and the valve seat. A borescope exam showed a normal looking valve without hot spots or obvious valve erosion at its edge.
The underside of the valve and valve seat were discolored with products of combustion (possibly carbon, rust, or lead). My kindly mechanic offered me a view through the scope and that’s when I pushed the valve stem in a bit more to get a better look at the valve seat. A sickening clunk was heard! I had pushed the valve into the cylinder and made removing the cylinder a near certainty in my mind. Undaunted (at least externally) my mechanic set about to maneuver the valve stem into the valve guide and pull the valve back into its original safe resting place. “No way” I thought! This involved roughly 30 minutes of adjusting the valve position to line its stem with the valve guide by using a magnet and a copper wire with a hook to grasp and maneuver the valve stem. This repositioning was monitored by use of the borescope. Once aligned the propellor was slowly rotated to elevate the piston which pushed the valve stem into the valve guide and the rescue was complete!
So, with the valve stem grasped securely it was pushed slightly inward to create a space between the valve and the valve seat. Under boroscopic guidance a foam swab liberally coated with grinding paste was introduced through the top spark plug opening and the paste was applied between the valve and the valve seat . The valve stem was then coupled to an electric drill by means of a 3/8” vinyl tubing connecting the two elements. Several minutes of rotating the valve against the valve seat both clockwise and counterclockwise while reapplying the grinding paste several times was accomplished. After a solvent cleaning of the area to remove the grit the borescope view showed a shiny valve seat and a clean underside of the valve. (exhaust valve open 9.2 KB). The valve springs and rocker boxes were reinstalled.
Once the annual inspection had been concluded it was time to see if success had been achieved. The engine was run for a few minutes and a hot compression test was accomplished. To my amazement the two cylinder pressures had climbed from 32 and 38 to 67 and 68 on both cylinders). The patient had been saved! Try as I may to screw up the surgery the “rope a dope” trick employed by my mechanic saved me from potentially having to deal with the removal of two cylinders.
So, if down the road you face a low compression issue along with a suggestion to replace one or more cylinders consider performing the ”rope a dope” trick. It may save you down time, money, and the potential of a maintenance induced adverse event as well. Alternatively, if the low compression is thought to be due to sludge causing stuck oil rings then do a ring solvent wash to hopefully unstick the rings and perhaps avoid cylinder replacement. (Youtube “Oil control ring solvent flush and custom tool”)
In consideration of my mechanic’s magic I have renamed him Houdini and I am the “dope” of the “rope a dope” trick!
The rope trick warrants serious consideration before using . On my Franklin it was tried with more than slightly disappointing results. The piston was taken to the bottom of the stroke and the nylon rope was inserted through the plug. So far so good. But and you knew that there was a but. The d@#$%@#d rope looped over the valve and refused to come out or close the valve. The end result was the removal of the cylinder which in it’s self was an epic as it was the rear lefts side and likely would have been easier to have pulled the engine.
Comments are closed.