In 2002, I did something unfortunate: pulled a perfectly good cylinder off of one of the engines of my Cessna 310. If I had it to do over again, I wouldn’t have touched the cylinder. But at the time, I thought I was doing the right thing.
It was the usual story. I had just downed the airplane for its annual inspection, and the first items on my checklist were performing a hot compression check, draining the oil, sending oil samples to the lab, and cutting open the oil filters for inspection.
All the cylinders had compressions in the low- to mid-70s. All but one, that is. That one measured about 60/80 with air leaking past the exhaust valve.
At the time, the engine manufacturer’s guidance on compression tests was Continental Motors service bulletin M84-15, which instructed mechanics that a jug could leak considerably past the rings and still be considered perfectly airworthy. However, any leakage at all past the valves was considered unacceptable, according to TCM, and required the cylinder to come off for repair or replacement.
So off it came.
Pulling a cylinder is a real PITA. I spent two hours removing cooling baffles and the exhaust and induction plumbing. It took me another hour to remove the rocker cover, rocker shafts, rocker arms, pushrods and pushrod housings. Finally, I used a cylinder base wrenches and a big breaker bar to coerce the eight cylinder base nuts loose. About four hours into the project, I held the offending jug in my arms and carried it over to my workbench to survey the damage.
I inspected the cylinder closely, with special attention on the exhaust valve. Surprisingly, I couldn’t see anything wrong. The valve looked normal, as did the rest of the cylinder. Yet it must have been bad, I thought, because it had clearly been leaking air past the exhaust valve.
I sent the cylinder out for re-valving and honing, installed new rings on the piston, then spent another four hours reinstalling them on the engine and replacing the exhaust, intake and baffles.
Like I said, it was a PITA. It cost me more than $500 plus a full day of sweat equity. (Had I not been doing the grunt work myself, the tab would have been at least $1,500.)
Continental pulls a switcheroo
That episode turned out to be a classic case of bad timing. Had my annual inspection come a few months later, that cylinder would never have been yanked. That’s because not long after my jug came off, Continental radically changed its guidance to mechanics regarding cylinder inspection.
On March 28, 2003, the wizards in Mobile issued service bulletin SB03-3 titled “Differential Pressure Test and Borescope Inspection Procedures for Cylinders.” This 14-page document is arguably the best guidance ever provided to mechanics on the subject of when a cylinder should be pulled. (SB03-3 was recently incorporated into Continental Motors Standard Practice Maintenance Manual X-0, and is no longer a service bulletin.)
Continental’s guidance in SB03-3 differed from its predecessor M84-15 in two crucial respects. First, it reverses Continental’s previous position that even small amounts of leakage past the valves during a compression check is unacceptable and grounds for pulling the cylinder. Many experienced A&Ps considered the “zero leakage past the valves” standard as being unrealistic and after 19 years Continental finally agreed with that assessment.
The other difference is arguably even more important: For the first time, Continental directed mechanics to perform a borescope inspection of the cylinders at each annual inspection, 100-hour inspection, and any other time a compression check is done. Continental’s language about this is quite emphatic: “Continental Motors REQUIRES a cylinder borescope inspection be accomplished in conjunction with the differential pressure test.”
This was huge.
Although SB03-3 officially applies only to Continental engines, the guidance it offers makes good sense for Lycomings, too.
At the time in 2003, borescopes were expensive and exotic devices whose use was pretty much limited to turbine engine inspections. Relatively few piston GA maintenance shops and A&P mechanics owned a borescope, and even fewer had a clue how to use one or what to look for. In SB03-3, Continental specifically recommended a particular make and model of borescope: the “Autoscope” from Lenox Instrument Company in Trevose, Pa. This was a simple, low-cost rigid borescope developed in the mid-1980s for use by auto mechanics, and cost about $1,000. The Lennox Autoscope had excellent optics and provides a remarkably clear view of what’s going on inside a cylinder. However, it was purely optical, and offered no way to take photos or capture digital images.
Since then, borescopes have become much less expensive and feature-rich. For years, I recommended the BK8000 digital borescope (also about $1,000) which provides excellent image quality and the ability to view images on a screen or capture them as JPEG files on a computer.
Last year, I purchased a Vividia Ablescope VA-400 from Amazon for less than $200. This is an amazingly inexpensive rigid digital borescope with the unique ability to adjust its viewing angle to anything between 0 and 180 degrees. The unit doesn’t come with an imaging device, but it has a USB cable that can be connected to any notebook computer or Android tablet or phone with a micro-USB port. It comes with both PC software and an Android app. This thing is so cheap that it’s now practical for every aircraft owner to have one.
What to look for
Also A&Ps tend to have little or no training in how to use a borescope, it’s certainly not rocket science. Here’s a photo what valves normally look like. The smaller valve on the left is the exhaust valve, while the larger one on the right is the intake valve. The reddish deposits on the exhaust valve and the brownish ones on the intake valve are typical. These deposits should appear reasonably symmetrical, indicating that the valves are rotating in service as they should be.
By way of contrast, here’s a photo of a burned exhaust valve. Note the asymmetrical appearance, especially the highlighted region (white arrows) where the deposits are minimal or absent (because that portion of the valve is running too hot). This is the classic visual signature of a burned valve. If the cylinder leaks air past the exhaust valve during the compression check and if the borescope shows this kind of asymmetrical deposit pattern, you can be relatively certain that the valve is burned and that the cylinder has to come off. But if the valve looks normal under the borescope, some leakage during the compression check is not grounds for removing the cylinder. (Now they tell me!)
The borescope is also a great way to check the condition of the cylinder barrel. Ths photo shows two borescope views of the upper cylinder bore—the so-called “ring step area.” The left view is normal; the right one has abnormal wear and scoring—possibly due to a broken compression ring—and probably needs to come off.
Next time you put your airplane in the shop, ask your mechanic what kind of borescope he uses. If your A&P doesn’t have a borescope or doesn’t know how to use one, educate him (and let him know that it’s now required equipment for any mechanic that works on Continental engines)…or find another mechanic.
Pulling a cylinder without first borescoping it is a lot like performing major surgery without first getting a CT or MRI. Don’t let any mechanic do that to your engine.