131 – Drew Wright’s Cessna 182 Exhaust Valve Lapping Success!

Airplane Owner Maintenance - By Dean Showalter

During the recent annual inspection on Drew Wright’s Cessna 182, cylinder #1 had a low compression of 42/80. However, with the amazing technology of borescoping, he was able to take a look inside the cylinder. The nice concentric circle of deposits on the face of the exhaust valve indicates a healthy valve. Drew wondered if perhaps this valve would be a good candidate for lapping in place, without removing the cylinder.

Even though the valve appeared healthy, Drew noticed a significant amount of air leaking out the exhaust during the compression test. Once again, his borescope was very helpful in observing the condition of the exhaust valve and seat area. The valve looks good, except for the rough and uneven seating area.

The seat also looks good, except for the corresponding rough and uneven seating area.

With the supervision of his A&P, Drew was able to remove the rocker cover and the exhaust valve springs. After verifying the valve guide was in good condition with no excessive wear, and the valve was sliding freely in the valve guide, he was ready to “lap” the valve (to apply a small amount of valve grinding compound to the edge of the valve using a long Q-tip through the top spark plug hole, and spinning the valve against the seat to smooth out and improve the seating area.)

Drew used a piece of wood dowel rod, some clear tubing, and some hose clamps, for this task… I like his idea so much, I’m planning to use this technique in the future.

One of the most important parts of the lapping process, is cleaning out the residue after the job is finished. One method is to spray Av-gas into the top spark plug hole to wash the grinding compound off the valve and the surrounding areas, and let it run out the bottom spark plug hole. Multiple washings, along with some compressed air, cleans up the area very well.

An additional thing Drew did was to push a thin cloth into the top spark plug hole, and pull the valve back to squeeze the cloth between the valve and valve seat. Then, he could turn the valve against the cloth and clean the seating area even better. I’m also going to keep that little tip in mind for my next lapping project. Thanks Drew!

The seating area of the valve showed a nice smooth band after the lapping was done – very impressive!

After the lapping was completed, a new rotator was installed on the exhaust valve, everything was reassembled, and the compression immediately improved!

But the best result was the compression test that was done after running the engine… that same cylinder that was 42/80, now had a compression of 78/80! What a fantastic result, and now, hopefully that cylinder will run reliably for many more hours.

Be sure to listen to the audio for this episode for more details about Drew’s adventure. He also shared about an induction leak that was visible because of some blue fuel staining.

After removing the intake pipe, it was obvious the gasket had shifted out of position during the assembly process many years ago… perhaps this was part of the poor idling problem that was also corrected during the annual inspection. After installing a new intake gasket and adjusting the idle mixture setting, the engine idled much better than before.

Drew’s story is a true inspiration in so many ways.

So, if you, as an airplane owner, have an interest in learning and getting involved in the maintenance, just find an A&P who is willing to give you some supervision to keep things legal, and you’ll have all kinds of possibilities.

Well done Drew, on your exhaust valve lapping project! I’m thrilled with the result you achieved on this one!

Drew has a young family, and it makes me happy to know he’s taking such good care of his airplane… after all, he’s got some precious cargo to haul around!