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  #1  
Old 08-14-2022, 07:25 AM
david.perl's Avatar
david.perl david.perl is offline
 
Join Date: Dec 2010
Location: Watford UK
Posts: 154
Default what are the risks of lapping an exhaust valve in place?

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?
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  #2  
Old 08-14-2022, 07:53 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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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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Last edited by lr172 : 08-14-2022 at 08:26 AM.
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  #3  
Old 08-14-2022, 08:12 AM
mahlon_r mahlon_r is offline
 
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Posts: 1,085
Default

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
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  #4  
Old 08-14-2022, 08:36 AM
Bavafa Bavafa is offline
 
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Location: Sacramento, CA
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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.
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  #5  
Old 08-14-2022, 08:42 AM
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jcarne jcarne is offline
 
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Location: Worland, Wyoming
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Default

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.
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  #6  
Old 08-14-2022, 08:57 AM
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BillL BillL is offline
 
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Location: Central IL
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Default 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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Last edited by BillL : 08-14-2022 at 09:33 AM.
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  #7  
Old 08-14-2022, 11:18 AM
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Mike S Mike S is offline
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Quote:
Originally Posted by jcarne View Post
Mahlon above used to build engines for a living, I would trust his advice on something engine related for sure.
Yep, Mahlon is the man!!
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  #8  
Old 08-14-2022, 10:31 AM
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Snowflake Snowflake is offline
 
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Quote:
Originally Posted by Bavafa View Post
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?
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  #9  
Old 08-14-2022, 10:36 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Snowflake View Post
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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Last edited by lr172 : 08-14-2022 at 10:44 AM.
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