what are the risks of lapping an exhaust valve in place?

[abwaldal@gmail.com]

I really don't think Mike Busch has scared us into not tearing an engine apart. It's done everyday. But a much greater expense. And Oh just remember the hassle. What a pain in the elevators.
But again, the lapping really does work, and not just temporarily.
Ask some of the people who have done/had it done.
I always say "it's just money" if you have it. Pull the jug and do the break-in all over again.
My three cents worth. Art

[rvanstory]

Originally Posted by rocketbob

I've lapped more valves than I can count. At least 10 in the past year. Never have had any issue as I usually have owners do full power runs down the runway before rechecking compressions and borescope.

Bob,

Need some advice from a more experienced person than myself. I have one exhaust valve in which the burn mark is not the perfect "bullseye" (see pic). Savvy suggested I should lap it in place to prevent a problem down the road. Current compression check is good (74/80).

In order to learn more about how to do this, I helped a friend lap a leaking exhaust valve today on his plane (O-360). Prior to lapping, his compressions were 65/80. After lapping, they went WAY down to 25/80!!!

Though we are confident we cleaned the paste out of the cylinder, is it possible some grinding paste is causing the low compression? We did NOT do an engine run after the procedure and I'm thinking this may be the root cause of the lower compressions. Should we do an engine run before we re-check compressions?

Though the procedure seemed pretty straight forward, I don't want to create a problem with my engine. Right now, it all seems fine other than a less then perfect bullseye on the borescope.

Any advice would be appreciated.

[rocketbob]

Originally Posted by rvanstory

After lapping, they went WAY down to 25/80!!!

Though we are confident we cleaned the paste out of the cylinder, is it possible some grinding paste is causing the low compression? We did NOT do an engine run after the procedure and I'm thinking this may be the root cause of the lower compressions. Should we do an engine run before we re-check compressions?

Correct there is no way to really clean the lapping compound other than go fly it. I do minimal cleaning with a Q-tip. Check the compression after an hour or so of flying. If it doesn't come back up after flying there's another problem. I have uncovered various other issues after lapping when it doesnt work. For example recently I lapped a valve and it came up a little bit after a flight. Pulled the cylinder and found some pitting on an incorrectly cut valve seat, almost difficult to see but enough to cause a very low compression reading. I re-cut the seat and the compressions came back up to normal.

[WiscoMick]

I lapped one of my exhaust valves yesterday on an O-320 for a minor exhaust leak found on my annual CI per Mike Bush’s YouTube video. It worked perfectly bringing up my compression to 73/80 psi and eliminating any air sound coming from the exhaust. I used NAPA water based compound and used it very sparingly; just applying two Q-Tip dabs. Took my time with clean up using water dampened Q-Tips. Have an assistant with steady hands to help hold the bore scope to visualize the whole process.

[chaskuss]

Originally Posted by david.perl

Pressure came backup to 78/80 on retesting
phew, no need for new engine

David,
I'm late to this thread. Glad to hear that this worked for you. For those who worry about leaving grinding compound in the engine, I offer this tip. Decades ago, a customer brought me a Triumph Bonneville motorcycle which he had tried to "tune up". He had cross threaded the right spark plug which ruined the threads in the cylinder head. Normal fix is to remove the head and install a HeliCoil thread repair insert. I was able to successfully do the job without removing the head. I've also used this trick to lap the valves on a number of motorcycle engines.

Using a can of cheap shaving cream [foam style] and a rubber [vacuum hose] tube, I filled the affected cylinder with shaving cream. I then tapped the hole for the HeliCoil and installed it. HeliCoil inserts have a tang which must be broken off to lock the insert in place. This would normally fall into the cylinder.

The purpose of the shaving cream is to catch any aluminum shavings from the tap and to catch that SS tang. The shaving cream and assorted debris is then removed using a shop vacuum. You need to fabricate an adapter tube so that it will fit in the spark plug hole.

As shaving cream is merely soap, any residual cream left in the cylinder will do no harm. For anyone planning on using the valve lapping in place trick, I suggest doing the same. You even have two spark plug holes to aid in removing the shaving cream and any lapping compound from the cylinder.

Charlie K

PS I've used this trick successfully many times over the past 40 years. Also works great when someone has damaged a valve cover bolt hole on an engine. On cars, I cover the area with duct tape and cardboard. I then use the shaving cream to fill in any gaps in the cardboard. Then drill, tap and install HeliCoil as needed. Vacuum debris when done.

[gasman]

Originally Posted by david.perl

Mike's video makes it look so easy! I watched it at least 4 times to try and absorb as much as i could. I also read some articles on the topic.

I had a go today - I've never messed with aircraft engines before so good I had some help from some experienced mechanics!

the rope i purchased was too compressible, good job maintenance had some thicker stuff. Getting the rocker arm off was the next challenge - I would have given up at that point had i not had some help. The shaft that goes through the rocker arms was in tight and needed some persuasion to come out.

The valve guide felt good according to the mechanics with the valve sliding smoothly within it.

Trying to get the valve paste in the right place was a challenge. Cleaning up was a challenge. Fiddly but achievable with some patience.

Getting the rockers back on was hard because the hydraulic tappets on the intake valve had filled with oil and didn't want to move.

Pressures before lapping were 58 and after it dropped to 20! I was thinking of ordering a replacement engine at this point.

The engineers said dont panic, may be a speck of paste has been left in site. Go do a full power ground run to see if that moves anything and then recheck. I managed the ground run and then rain and time stopped play. Ill recheck in the next day or 2.

Im not convinced i lapped long enough (i did just a couple of mins with coarse then fine compound using a slow drill speed)) and only changed direction once and did not move the valve in and out whilst lapping. I've only realised all this after doing more research on how to lap valves - I should have done that before.

Can lapping make the valve worse? If not, i'll give it one more go as now i know how to get the valve springs off and back on, it should be a bit easier.

Thanks for any critique of my technique and how it can be improved.

This may help next time. This process collapses the lifter and makes the rocker and pin go on very easy.