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  #1  
Old 05-17-2019, 03:45 PM
n567vb n567vb is offline
 
Join Date: Feb 2008
Location: MN
Posts: 81
Default Questions on staking and Lapping a valve

I have a intake valve that seems to have some gunk under the valve which is causing a low compression (50/80). I ran it for about 10 hours and got the same thing. However, I could never get the temps very high as it was winter in MN. I'm thinking of trying to either stake or lap the valve to try to get this out.

When staking the valve, should the valve be closed? I'm a little worried about hitting it too hard. How hard do I need to smack it? (I know to use a block of wood and mallet)

I've read a little bit about trying to lap the valve while on the engine. Are you able to get grinding compound on the valve through the spark plug hole, or can you go up through the intake pipe? How do you get the grinding compound back off completely when your done?

Can anyone tell if this looks like carbon or lead buildup? How hot do you need to get the cylinder to burn each off?

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  #2  
Old 05-17-2019, 06:23 PM
BobTurner BobTurner is online now
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 7,866
Default

I once watched an A&P do this on a 182. I have no idea if this was "right", just what he did:
1. "staking. Should the valve be closed?" It will be, since you need to remove the rocker arm. But position the piston so that there's no danger of the valve hitting it.
2. The A&P I watched really hit it hard. (rubber mallet)
3. Lapping in situ. Remove the valve keeper and the spring. Do not let the valve fall into the cylinder, but lower it enough to be seen thru a plug hole. The A&P used a Q-tip on a long wooden stick to put some lapping compound on the underside of the valve, working thru the spark plug hole. Use sparingly. He chucked up the valve stem, rotated the valve with a drill, while pulling outward modestly so the valve was engaged with the seat.
4. He used more Q tips to clean the lapping compound off. I can't recall, I think he moistened each Q tip with alcohol or something, to help make the compound stick to the cotton. After that, he tried to vacuum out the cylinder with a piece of vinyl tubing he had hooked to a vacuum source. Finally, with both plugs out, he tried to blow out any remaining stuff, applying air thru one plug hole, hoping it would blow out the other.
5. He used the "rope trick" to reinstall the valve spring and the keeper (piston at bottom; push rope thru the spark plug hole, as much as you can.
Bring the piston up on the rope (and valve) until there's resistance. You can now push on the valve stem, without the valve dropping into the cylinder).

Bear in mind, this is an inexpensive trick, which sometimes works. But sometimes it doesn't, and then you need to pull the jug to do a proper fix.
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  #3  
Old 05-18-2019, 12:15 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,735
Default

I don't know what staking is, at least aside from it's use on threads and fastners, so won't comment. I would concur that sharp blows may release the debris, but you must assess the risk of damage from doing so. Too much force (i.e. excess compression) could damage the valve springs,as well as the keepers if they are hardened. Lapping is for finely mating closely fitted parts and is not a method for removing debris that is keeping the valve from seating. Lapping compound can damage the seat if there is any meaningfull clearance in the guides and the valve is not in full contact with the seat. This is especially true with course grit compound. This is normally done while pressing the valve head directly into the seat. This is hard to control when chucking the stem in a drill and pulling. The debris could make the valve hit the seat at an angle, causing excess wear 180* from the debris, as it wears the debris away. 100% contact is important for longevity of the exhaust valve, as this contact transfers heat from the valve to the head.

Larry
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Last edited by lr172 : 05-18-2019 at 12:33 AM.
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  #4  
Old 05-18-2019, 07:25 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,329
Default Staking

This sounds like just striking the valve step to open the valve and let it accelerate back to its seat. I never heard a name for it.

A valve never "hits" the seat anyway as it runs, the cam allows it to settle, not land with any velocity. Flecks of something, a lead ball, is stuck to the face, then striking the top of the valve with an impact tool (not too hard) to allow it to jump back a few times may help reseal the surface, may.

It is a crude shade tree trick, but sometimes effective, and not damaging if done right. Brass, copper, bronze hammer . . . ahhh errr, impact tool. It moves maybe 3/8". After a valve job, I always did this to allow the valves to settle themselves in the correct position before leak testing.

If that does not work, then lap. Take extreme care to contain the compound. It is silicon carbide. Clean it very well after lapping, it is an oil suspended paste, so some hydrocarbon should work to clean. Light oil might work best to dissolve the carrier and collect the particles. Cotton swabs are good to pickup particles in the matrix, clean both valve and seat.

Opening the valve and doing another camera inspection would be warranted before disassembly.

BTW, I have not done this insitu, but a thousand on the bench.
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Last edited by BillL : 05-18-2019 at 07:32 AM.
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  #5  
Old 05-18-2019, 09:15 AM
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JonJay JonJay is offline
 
Join Date: Jan 2007
Location: Battleground
Posts: 4,348
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I believe it?s called ?staking? because you use a wood ?stake? against the valve and strike the stake with your hammer. The wood protects the valve.
I have done this a few times but the issue ended up never being a piece of carbon.
Easy first shot if you have a low compression cylinder show up. However, it can also mask a partially stuck or intermittently sticky valve.
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  #6  
Old 05-18-2019, 11:54 AM
DNeufeld DNeufeld is offline
 
Join Date: Aug 2016
Location: Coeur d Alene
Posts: 96
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If staking doesn't work...
As Bill wrote in his response, the lapping compound must be cleaned off thoroughly. Valve lap compound left in the intake port or on the seat can and will destroy the cylinder in short order. I have done this with success.

Piston bottom dead center (BDC). Lift the opposite side of the plane so the cylinder can be flushed out the bottom plug hole without the thinner running into the bottom of the cylinder. Apply the compound to the valve through the plug hole as said before. Use a drill chucked up to the valve stem at slow speed 100 rpm or less pulling against the seat and lifting just off the seat when the compound wears away. You'll hear it. Don't go crazy. If it takes more than a minute or so to clean it off, it's not going to work anyway and the cylinder will need come off for repair. Clean the valve and seat best possible with pieces of rag through the plug hole. Then, thoroughly, wash the valve and seat with a hose using a bendable wire inside the hose so the flush can be directed through the plug hole. Don't use pressure at first. Just a gentle wash siphoning from a can of thinner valve and seat first. Then position the flush hose at the piston and flush so it all runs out the bottom plug hole. Don't be skimpy on the flush. When you think you have it all out, flush it again! A can of thinner's cheaper than an engine! After the flush, shoot some engine oil with air blast into the cylinder.
The intake tube should be off while this is done for cleaning access.

Last edited by DNeufeld : 05-18-2019 at 12:02 PM.
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  #7  
Old 05-19-2019, 07:16 PM
Boyd Birchler Boyd Birchler is offline
 
Join Date: Sep 2014
Location: IN
Posts: 254
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Generally the valve that is causing the problem is the exhaust, but your question about removing the intake makes me think you isolated it as an intake valve.

I have staked a few valves but it never cured the problem. I have "done the rope trick" and removed the keepers and lapped the valve quite successfully.

I have also successfully fixed exhaust valves that were sticking in the guide by using a drift to drive the valve stem into the cylinder, dropping the valve stem out the bottom plug hole, cleaning the stem and using a reamer to clean bore of the guide. Worked like a charm...
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  #8  
Old 05-19-2019, 08:01 PM
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rocketbob rocketbob is offline
 
Join Date: Jun 2006
Location: 8I3
Posts: 3,769
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Staking: doesn't work.

Lapping: pull keepers and springs off. Spin stem with an electric drill. I've done this a few times and it works fine.

There was a write-up in the ABS magazine on a procedure the Bonanza guys use to lap in-situ applying lapping compound via a q-tip attached to wire thru the plug hole. No cleanup, burns away quickly.

Stuck valves: I recommend an appropriately sized ball hone from Brush Research. Drop the valve as my friend Boyd describes. Zip in and zip out 2-3 times and you're done. Guides wear oblong on both ends so a reamer is not the best tool for the job.
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  #9  
Old 05-19-2019, 09:36 PM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Louisville, KY
Posts: 750
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Drift, but clarification questions-

Brush Research- 1/2"/12.7mm is right in the middle of the service limits.

Which compound and grit do you like?

silicon carbide 240 grit? $20

What about the Macfarlane $160 reamers? Worth it?


https://www.mcfarlaneaviation.com/ar...oming-engines/

https://www.lycoming.com/special-ser...tion-no-1776-4

http://www.brushresearch.com/brushes.php?c1=1
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