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

david.perl

Well Known Member
My inspector has advised against exhaust valve lapping in place - to quote him he "would only do this if stuck somewhere remote or in the third world". He says as its not a lycoming approved procedure so it can't be right.

Savvy aviation have recommended this as my next step. My inspector is saying i am introducing contaminants into the cylinder and valve guide and that can't be good.

If done a ton of reading around this and cant seem to find many negatives. In medicine, when a new procedure comes along, its well researched and the findings published. I cant see to find any published research for airplane engine procedures of this nature.

the more i read, the more im confused.

What am i missing that my inspector is seeing?
 
lapping compound is a slurry of grease and silicone carbide or aluminum oxide grit, just like found on sandpaper. It is usually available in two different grits. I would recommend the fine for this job. Your mechanics concerns are well founded, as if this slurry gets onto contact with moving parts, it will do it's job of scratching and wearing the surface down. You may have noticed the most steel parts that move have a very smooth and polished surface and 120 grit doesn't leave that kind of surface. It is ok for valve faces as they only contact another part and don't slide against them.

To give you an idea of what sandpaper can do, when I machine round parts on the lathe, I usually take the last .001" off by holding sandpaper against the spinning part. However there is a whole bunch more grit there then you will be using.

The key is how confident you are that you can get it all removed. You will have access through the exhaust port and can shine a light through the plug holes. Also if you are able to go very sparingly on the compound the amount of wear that would occur to other parts that this gets on should be minimal as the grit eventually becomes smooth and no longer creates wear. It can only do so much damage before it wears out. You have probably seen this when using sandpaper; As it wears, it does a continually less effective job. I imagine it is not easy to remove and if you accidently drop a large blob of it into the cylinder, you won't be happy.

This all comes down to your confidence level in getting the parts clean and not accidentally getting the stuff in other places. I have never done it to an in situ valve, so can't offer advice. Getting a jug out is not too bad of a job, depending upon which one it is. Most of the time is in baffle removal and installation. Getting a jug off without baffling is about two hours work. Given the risks here, it is no surprise that Lyc does not recommend this, though I am confident that a good number of mechanics do this all the time.

You will also need to make a tool to spin the valve from the top of the stem, while applying pressure pushing the valve into the seat. Laping is usually done with a rod and suction cup adhered to the face of the valve. You will also need to buy or make a tool to compress the valve spring in order to get it off.

Larry
 
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If you do a really good job with a pressurized spray of varsol or equivalent after the job is done. I think the risk is negligible. Wash it really well draining through the lower spark plug hole or if you are really worried about it take that cylinder's exhaust pipe off and spray in there as well. When done squirt some oil in the cylinder and rotate the prop to distribute it and you will be fine. I have done this hundreds of times using a 6 inch rubber hose attached with a hose clamp on the valve head and a small debur tool shoved into the other end of the hose. Then using an air drill, on slow, to rotate the valve while lapping with an in and opt motion to keep the compoud on the valve face. I found doing it by hand often wasn't enough to clean the surfaces and provide a good surface to surface seal. Using course compound first and then fine as a finish worked for me.
Good Luck,
Mahlon
 
If you do remove the cylinder, try to leave the piston in place and not pull it out. Lycoming recommends re-honing the cylinder if piston comes out.
 
Removing a cylinder comes with risk as well which is why Saavy recommends lapping in place.

Mahlon above used to build engines for a living, I would trust his advice on something engine related for sure.
 
Don't be sloppy.

It is surgical procedure . . as stated - use compound sparingly, do not drop in the cylinder and clean^3. Too much and as the valve seats and rotates it will make pearls of compound. You can feel the grinding - when it stops, clean excess and reapply. 5 min of hand grinding and the job will likely be done.

The primary risk is carbide compound in the cylinder. Mahlon is the voice to listen to.

I like cotton swabs as they are tough & the void matrix will collect, wick and store material, use 100 if needed. Twirl once and pitch.
 
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So, for a thought exercise... what can go wrong here?
note) the grit breaks down fairly quickly - as you lap the valve you'll feel less grittyness as you spin the valve, which is partly the valve and seat getting smoother and partly the lapping compound breaking down to fine particles

So, let's say some is left in the cylinder. You start the engine... a fair amount will be blown out the exhaust. The oil in the compound will burn and the particles will get ejected.

OK, maybe some stays in the cylinder, and gets into the rings... what happens? OK, the rings and cylinder wear some as the lapping compound works. Worst case? Rings wear out? Well, your engineer wants to throw away this cylinder anyhow.

If he's really nervous, change the oil after a start and short ground run. Heck you could even do the ring flush proceedure too.
 
If you do remove the cylinder, try to leave the piston in place and not pull it out. Lycoming recommends re-honing the cylinder if piston comes out.

I think the point of removing the cylinder is so you can get the piston out to gain access to the valve seats. If you're going to leave the piston in, there's no point in removing the cylinder at all... Or am I missing something?
 
I think the point of removing the cylinder is so you can get the piston out to gain access to the valve seats. If you're going to leave the piston in, there's no point in removing the cylinder at all... Or am I missing something?

You are not. The piston goes up and down in the cylinder Millions of times across it usefull life. I don't see how fully removing it creates lasting damage. Sure it has to be dragged across the uconditioned wall surface at the bottom of the bore, but it goes in that way and over a couple of hours the rings polish up just fine. Dragging is across the unconditioned surface ALSO happens when you bring the piston to the bottom of the bore to get the pin out. Don't see how an extra 1/2" to take it all the way out makes a difference. Ring compressors are very smooth and should create no impact on the rings.
 
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Not Scary

I was skeptical when I first heard of this too, but I’ve lapped a few. Lapping a valve in place is far less intrusive and risky than pulling a cylinder. Made sure I removed the gritty stuff with cotton swabs and flushes of carb cleaner, then WD-40 followed by lots of 50 weight on the cylinder wall and valve guide and away they went. No problems I’m aware of. The last one was a DeHavilland Gipsy Major on a Tiger Moth. It’s flying happily now.
 
Just a thought here, but for folks worried about getting grit into the piston/ring assembly-----how about putting a lift under one side of the plane, so the cylinder being worked on is low so as to create a "sump" at the plug area. Any crud could be washed out easier that way.
 
lapping valves

Pulling a jug is asking for trouble. lapping in place is the solution.
Just suck it up and go for it. Of course watch Savy's aviation video 44 times before you jump in.
Not really, maybe twice will do.
Art
 
Great to read the responses.

Ive spoken to the local shop here and they are more than happy to help me out - the main guy knows of Mike Busch and approves of his methods
 
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Pulling a jug is asking for trouble. lapping in place is the solution.
Just suck it up and go for it. Of course watch Savy's aviation video 44 times before you jump in.
Not really, maybe twice will do.
Art

Pulling a jug asking for trouble, really?
Busch has everyone scared that actually fixing sh*t is something to be avoided.
Lapping valves IMO is a temporary fix.
 
So far the only risk I have read above is that all the valve paste might not be removed if one doesnt take care

If that’s the only risk sent I suspect it might be worth a shot as if it doesn’t work or is only temporary, the cylinder has to come off then.
 
Pulling a jug asking for trouble, really?
Busch has everyone scared that actually fixing sh*t is something to be avoided.
Lapping valves IMO is a temporary fix.

A new jug is a temporary fix too. :)

If lapping the valve lasts for 50 hours... I've gotten another year of use out of it. Maybe next year I can get a new jug in less than 6 months.

I've done both in the past 18 months... and had to use a reconditioned jug on the h2ad as there was no new stock available.
 
Ain't this a lovely can of worms We've opened up??? Tasty and enlightening.
See both ways as a solution and pollution of the situation.
Your luck will surely vary Art
 
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. No problems with paste or scratching of the cylinders as I do check before and after to see if this may be an issue.

Lapping fixes non-rotating valves.

Typically I clean the stem off via the spark plug hole, and clean off carboned up springs and seats also. Usually clean the guides as well.

Pulling a jug on a Bonanza for example is a huge effort so lapping makes a lot of sense. The key is to lap valves before there's low compression issues.

One Bonanza I annual is well past TBO running very strong thats had all of the valves lapped about 3 years ago. The owner was considering an overhaul at the time, and now there's no discussion of it.
 
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.
 
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...
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.

Great to hear, David! I've only watched the videos and listened to podcasts and am very interested to hear your results after the next engine run, and perhaps after a bit more lapping. I have never heard that it can make things worse, so I agree with your mech's assessment - don't panic!
 
So, at least $3000 saved (probably an under estimate of changing a cylinder).
For a cost of what... $20 in materials and some time? Not counting what you needed to pay your helpful mechanic.
 
So, at least $3000 saved (probably an under estimate of changing a cylinder).
For a cost of what... $20 in materials and some time? Not counting what you needed to pay your helpful mechanic.

Yes well pleased, we pay pounds to your dollars so likely saved even more. I was quoted GB£5,000 for cylinder replacement. The engineer said they used to lap in place all the time but now its not worth their time and they just remove the jug.

It cost £20 in materials plus purchasing the Ablescope 400 which for its price is a great bit of kit and will be useful at future inspections. Plus a few hours of time, £70 in thank yous and also my membership to Savvy QA who interpreted the borescope images within 1 hour over a weekend- im also gleaning some useful data from their flight data analysis tool.

I also learned a lot about valves and the importance of borescoping them at regular intervals.

I think if i was doing a lot of these, I would invest in a valve wizard as getting the springs compressed and the collets out and then back in was a fiddle. I did email about postage to the UK but never heard back.
 
My inspector does not appear impressed i did this. He said at the least ground run and change oil and filter. How would any grinding paste that may have been left get thought the oil filter?
 
My inspector does not appear impressed i did this. He said at the least ground run and change oil and filter. How would any grinding paste that may have been left get thought the oil filter?

So you’ve got guys over on your side of the pond that are like that too?!

Bottom line, I think, is that if you have to satisfy him before you can fly, then an early oil and filter change are cheap (compared to the amount you’ve saved over doing a cylinder change already….), and if it gets you back in the air, well - it might be stupid, but you can be “right” and grounded, or say “yes sir” and go fly…..

And no - you shouldn’t have any remnants of grinding compound survive to show up in the oil….

Paul
 
David, good the hear of the success - yea you have to lift and go back and forth. I experimented 40 yrs ago and that works best.

High confidence that you will get good readings after a flight hour or two, oil change and filter. Engine will still be happy, can not hurt and will make a happy (or less grumpy) inspector.

Good for you stepping this through. You will find easy/proper ways of getting past the reinstallation difficulty of the rockers.
 
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
 
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.
 

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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.
 
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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.
 
Another tip to keep grinding compound out of the engine

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.
 
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.
lifter.JPG
 
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