Staking exh valves, MMO or other help for low comp test

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docmirror

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I had two jugs that previously read in the low 70s on compression test come back in the low 50s. Plane is running well, plenty of power in all right regimes, I looked at the valves and see no evidence of burning. I can hear air coming through the exh valve on the the two low jugs.

I've heard two fixes that have some success not related to taking the jug off and reaming the guide, and cleaning the valve shaft. Staking is pretty well known in old Jag and VW circles for engines which develop a modest leak. Also heard that adding some MMO to the fuel helps.

Would like to know of someone with actual experience in staking an exh valve, or has run MMO in the fuel to help minimize the issue with build up on the valve and guide?

Please answer question prior to moralizing. If you have only moralizing to share, I'm just not interested.
 
I tried staking the exhaust valve on my A75 last year with no success. Finally pulled the jug and it turned out the exhaust valve seat on the cylinder had a crack in it.
 
When you were in there with the cam, could you get good shots of the valve seats? Any pitting? Do the ex. valves in the low cyls travel straight up and down, or is the some lateral motion as they settle in?

I've had success with MMO freeing up stuck rings with a wash, but have no experience with running it in fuel.
 
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What was your comp reading? I looked pretty carefully, and didn't see any defects in the seat, but it could happen.
 
Did you try borescope first? Taking off the jug is not a trivial job on the plane. I know you can see things better once the jug is off, and the valve is out.
 
docmirror said:
I had two jugs that previously read in the low 70s on compression test come back in the low 50s. Plane is running well, plenty of power in all right regimes, I looked at the valves and see no evidence of burning. I can hear air coming through the exh valve on the the two low jugs.

I've heard two fixes that have some success not related to taking the jug off and reaming the guide, and cleaning the valve shaft. Staking is pretty well known in old Jag and VW circles for engines which develop a modest leak. Also heard that adding some MMO to the fuel helps.

Would like to know of someone with actual experience in staking an exh valve, or has run MMO in the fuel to help minimize the issue with build up on the valve and guide?

Please answer question prior to moralizing. If you have only moralizing to share, I'm just not interested.
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I released an engine for two years in a row with marginal compression checks, there was no smoking gun and the airplane flew about 30 during those years. The next annul the compression was even lower and time to throw in the towel. It wasn't visible till cleaned of al the combustion products in the head but there were several cracks in the "crown" of the head between the valve seats.

Did you do the soapy water leak check with air pressure on?
 
bnt83 said:
I released an engine for two years in a row with marginal compression checks, there was no smoking gun and the airplane flew about 30 during those years. The next annul the compression was even lower and time to throw in the towel. It wasn't visible till cleaned of al the combustion products in the head but there were several cracks in the "crown" of the head between the valve seats.

Did you do the soapy water leak check with air pressure on?
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We didn't, and I'm thinking that's the next step. These jugs are notorious for leaks around the crown of the head. However, I did clearly hear air hissing out the exhaust, so it could be a combination of the two. I'm careful with operations and usually only run 50% or a bit more. I tried running at 75% for an hour to see if there was some glaze or unseated rings, but my oil consumption is very low and it didn't change much.

The first test we did was cold, and the readings were miserable, so I flew it hard and they came up to 51 and 53 which is typical and in line with Conti SB03-3. I don't really want to pony up for the calibrated orifice fitting but I may break down. The real answer is to pull the jugs, but they are on opposite sides, and that's a ton of work. Hoping for a miracle fix. :D
 
Was the engine warm when you did the test? Did you use a "Master Orifice" compression gauge?
 
There are so many misconceptions out there concerning this and neither Lycoming nor Continental has helped much with their bulletins.

Here's what I'm going to tell you from my 40 years of experience. You can stick a borescope or a dental camera in there but you're not going to see anything. If you do see something then your compression reading is going to be ZERO. The compression test, as crude as it is, is the de-facto standard for determining the preliminary health of that cylinder. A low reading is not however cause to immediately yank it off and replace it or send it to the shop.

But - as far as MMO and staking the valve goes, yea it might work but it is only temporary. The valve is not seating properly because of either lead/carbon build up on the stem or worn guide and mis-alignment.

You ain't gonna "fix" that with some solvent. Maybe if you had used MMO from the very beginning when the engine was new? I don't know but there are people out there who say it's so.
 
Silvaire said:
There are so many misconceptions out there concerning this and neither Lycoming nor Continental has helped much with their bulletins.

Here's what I'm going to tell you from my 40 years of experience. You can stick a borescope or a dental camera in there but you're not going to see anything. If you do see something then your compression reading is going to be ZERO. The compression test, as crude as it is, is the de-facto standard for determining the preliminary health of that cylinder. A low reading is not however cause to immediately yank it off and replace it or send it to the shop.

But - as far as MMO and staking the valve goes, yea it might work but it is only temporary. The valve is not seating properly because of either lead/carbon build up on the stem or worn guide and mis-alignment.

You ain't gonna "fix" that with some solvent. Maybe if you had used MMO from the very beginning when the engine was new? I don't know but there are people out there who say it's so.
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I've had a bunch of different engines and this is my fourth big bore Conti. I've had four jugs with issues in the past, and it's never been the exh valve. Two of the prev were head leaks, and one was a cracked ring, the other was a plug fouling with high oil use. The fuel consumption is low, I lean aggressively, don't use much oil, and I'm trying to figure out why two of them are dropping. The only thing I can think of is what you said, the carbon build up on the valve stem, or mistake in the guide bore.

I'll just bite the bullet and yank them off maybe when it cools down. I'm going to install a autopilot in Oct, I'll have them off then and do it right.

Grrrrrrr

(do not have the master orifice tool. If I did it may show within spec, but just barely)
 
If the valve/seats are leaking and you don't see signs of burning yet, you can pull the exhaust pipe off as well as the exhaust valve spring and assembly and clean and lap the valves and seats in place by putting a drill motor on the valve stem and applying compound through the exhaust port. If there is a significant build up of carbon, go ahead and spray some X-66 Carbon Cleaner from GM in there first.
 
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Henning said:
If the valve/seats are leaking and you don't see signs of burning yet, you can pull the exhaust pipe off as well as the exhaust valve spring and assembly and clean and lap the valves and seats in place by putting a drill motor on the valve stem and applying compound through the exhaust port. If there is a significant build up of carbon, go ahead and spray some X-66 Carbon Cleaner from GM in there first.
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When I did that on my A75 I left the exhaust attached and applied the compound with a long tip through the spark plug hole. "Rope trip" was used to remove the rocker arm.
 
Minimum checks I do for each cylinder:

(1) Visual inspection of cylinder in general (no scope)
(2) Differential Compression Test Reading
(3) Soapy water check
(4) Listen for exhaust valve leakage --- > If leakage, stake valve. Still leaking, then lap valve via rope trick. Still leaking, then pull cylinder.
(4) Listen for intake valve leakage --- > If leakage found, stake valve. Still leaking, then lap the valve via rope trick. Still leaking, then pull the cylinder.
(5) Listen for piston ring leakage at the crankcase breather or oil filler, or both. Seem excessive?
(6) Note the difference in the amount of fouling and garbage found in spark plugs between each cylinder as they are pretty telling on what is going on.
(7) Visual inspection via flashlight thru spark plug hole


I don't mind doing a quick comp check at oil change either, in an effort to catch something amiss since I seem to fly about 30% of the time at night.
 
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Hey, we are on the same page. Last night I was thinking of working through the plug hole to clean the stem and lap with a drill motor. I even ordered these; http://www.amazon.com/Astro-Pneumat...id=1405346025&sr=8-81&keywords=wire+brush+set

I'm going to go in with a dremel on moderate speed and work the stem from the plug hole. Will need to put a rubber sleeve on the brush shaft to protect the plug threads though. I'll attach a vacuum to the exhaust and pull any bits out of the jug. Then lapping compound and check again.
 
docmirror said:
Hey, we are on the same page. Last night I was thinking of working through the plug hole to clean the stem and lap with a drill motor. I even ordered these; http://www.amazon.com/Astro-Pneumat...id=1405346025&sr=8-81&keywords=wire+brush+set

I'm going to go in with a dremel on moderate speed and work the stem from the plug hole. Will need to put a rubber sleeve on the brush shaft to protect the plug threads though. I'll attach a vacuum to the exhaust and pull any bits out of the jug. Then lapping compound and check again.
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Uhhh, ok? Google "aircraft rope trick". You can pull the valve stem out thru a spark plug hole, but what is the point? That's not necessary for lapping valves :confused:
 
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bnt83 said:
Uhhh, ok? Google "aircraft rope trick". You can pull the valve stem out thru a spark plug hole, but what is the point? That's not necessary for lapping valves :confused:
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I'm not pulling the valve stem out through the plug hole. Going to work the tool through the plug hole, and the valve stem stays in the guide, just pushed back into the jug where it can be attacked by the small round brush.

I've used the rope trick before. Sorry for the confusion.
 
Remember the GM X-66 Carbon Remover, I haven't come across another product quite like it. Any GM dealer used to carry it at their parts counter.
 
Henning said:
Remember the GM X-66 Carbon Remover, I haven't come across another product quite like it. Any GM dealer used to carry it at their parts counter.
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Called three, none have it. I have two different part numbers, but GM is phasing it out in metro areas because of the toxicity of chemicals. It's a petro naptha mix with some other VOCs. I'm wondering how to get it through the engine? Really not fond of mixing stuff with my auto fuel and flying around. I could add some to that, but I'm thinking I'll just use the aerosol version on the valve itself and not run it through the fuel system. I have rubber bladder tanks, and it has Ethanol as well as Penatanol 4 which is pretty icky stuff.
 
I had a similar problem with my 150. You could also hear the air through exhaust when you turn the prop over. Ended up being a stuck valve and the machine shop found a crack in the cylinder. Luckily we have the original cylinder which has very low hours which will be rebuilt.
 
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