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0 compression on cylinder 2

Some more happy news. A&P chopped out my front baffle air dams, worked on my baffling, adjusted my idle mixture, took off on a full power steep climb 2000fpm and highest cht was 389.
 
"0" compression.....

I'd check for a sticking valve before doing anything else. Did you notice where the air was escaping from during the compression test?

I agree with the note that a true "0" compression would be unusual except for a wide-open exhaust valve but, even then, might not be zero.

If your mechanic is familiar with the Rope Trick I would drop that (likely) exhaust valve out and check the valve itself (carbon fouling) and the guide (carbon fouling). Both can be cleaned up and the valve put back in.

I also might agree with the "zero cold, normal hot" might leave you in a field somewhere. That also sounds like a sticky valve that will some day stick open and stay. Ask me how I know that.....:eek: My compression was not zero until the day I brought it back after a near off-field landing and checked the compression. Stuck exhaust valve with NOISE coming out of the exhaust. This was in the Cub, not SuzieQ.

Or you can just fly and see what happens......;)
 
I guess the question is, what do you do in this scenario? Seems all noninvasive techniques are kind of exhausted, so do you pull the cylinder and open a new can of worms in terms of risk? I guess I could carry my extra cylinder around with me.
Did you do a Ring Flush? That's about as noninvasive as it gets, and a cylinder with rings that stick occasionally is a perfect candidate for the procedure.
 
I agree with the note that a true "0" compression would be unusual except for a wide-open exhaust valve but, even then, might not be zero.

If your mechanic is familiar with the Rope Trick I would drop that (likely) exhaust valve out and check the valve itself (carbon fouling) and the guide (carbon fouling). Both can be cleaned up and the valve put back in.

I also might agree with the "zero cold, normal hot" might leave you in a field somewhere. That also sounds like a sticky valve that will some day stick open and stay. Ask me how I know that.....:eek: My compression was not zero until the day I brought it back after a near off-field landing and checked the compression. Stuck exhaust valve with NOISE coming out of the exhaust. This was in the Cub, not SuzieQ.

Or you can just fly and see what happens......;)


Definitely no noise coming through the exhaust according to A&P. He said something about having to pump it up to 100psi to hear it even coming through the oil neck. I just spoke with him, he's flown the plane a bunch dialing in the CHTs, and he said the plane ran great and he suspects it was a fluke.

I'm going to fly it around, re check compression in 10 hours or so, and keep my spare cylinder around in case I need it.

Thanks for your reply!
 
Did you do a Ring Flush? That's about as noninvasive as it gets, and a cylinder with rings that stick occasionally is a perfect candidate for the procedure.

I have not, but I have considered it. Plane's at A&P still, I'm going to fly it around a bit and see how things go. If I run into compression issues in 10 hours or so, I will try the ring flush before putting in the new jug.
 
Did you do a Ring Flush? That's about as noninvasive as it gets, and a cylinder with rings that stick occasionally is a perfect candidate for the procedure.

Were the front cylinders the hot ones originally? [#1 and #2] ?

They were all hot. I actually think #3 was the hottest by a hair, but they all ran pretty close. He did the washer trick on #3 just in case. Either way, he's been sending me videos of aggressive climbs and still not getting to 400, so I am happy on that front.
 
Ring End gaps

I had an o-360A1A cylinder that would line up the ring end gaps in about 150 hours. It did it twice and all measurements were in book limits. Changed out cylinder and piston and it has not happened since. Could not figure out if it was the cylinder or piston because all measurements were per the manual.
 
I had an o-360A1A cylinder that would line up the ring end gaps in about 150 hours. It did it twice and all measurements were in book limits. Changed out cylinder and piston and it has not happened since. Could not figure out if it was the cylinder or piston because all measurements were per the manual.

cylinders do not lose compression when the ring gaps line up. Air is very clever and does not need a straight path to move effectively under pressure. If this were a real issue, manufacturers would have devised a way to keep the rings in the desired alignment many decades ago. Yes, forcing the air to turn does slow it down just a bit, but the difference is quite insignificant at 80 PSI. Even less so at the 150-200 PSI in dynamic compression. Even less so at the hundreds of PSI during combustion. OTOH, If someone filled down the rings too much, leaving an excessive gap, that can reduce static compression.
 
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You need to change A&P's. If I skimmed all the post correctly the plane has been fine, and the engine monitor says all temps are fine. Then you took it in and he said 40, then 0, then 75, but now there is an overheating problem???

0 is almost impossable. I would say either a bad gauge, or he was not checking it on the compression stroke. Now the timming is off to cause overheating and he performs surgery on previously working baffels??? - run like ****. Either he does not know what he is doing, or the timming marks on your flywheel are not indexed correctly. Either way, your A&P sucks.
 
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cylinders do not lose compression when the ring gaps line up. Air is very clever and does not need a straight path to move effectively under pressure. If this were a real issue, manufacturers would have devised a way to keep the rings in the desired alignment many decades ago. Yes, forcing the air to turn does slow it down just a bit, but the difference is quite insignificant at 80 PSI. Even less so at the 150-200 PSI in dynamic compression. Even less so at the hundreds of PSI during combustion. OTOH, If someone filled down the rings too much, leaving an excessive gap, that can reduce static compression.

And they did...

In the 70's, my 2 stroke Tohatsu motorcycle piston had a pin in the gland to keep the ring in one place so it would not rotate and catch on a port. The ring was designed to seal around the pin.
 
They were all hot. I actually think #3 was the hottest by a hair, but they all ran pretty close. He did the washer trick on #3 just in case. Either way, he's been sending me videos of aggressive climbs and still not getting to 400, so I am happy on that front.

So if they were all hot, that points to baffle sealing problems.
Removing the dams in front of #1 and #2 will increase airflow on those two cylinders... stealing air from the aft cylinders.

Given your location, these are aggressive climbs in what 50F air temps?

I mean, I'm happy progress is being made, and your cylinder issue seems to have been some sort of fluke, but I'm still a bit bothered by the approach. If temps are all hot, fix the leaks FIRST. Then decide about the dams.

I see lots of A&P's rip out those dams, with the logic that you don't see them on Cessna 172's and Piper Cherokees. Planes with much bigger inlets.
 
You need to change A&P's. If I skimmed all the post correctly the plane has been fine, and the engine monitor says all temps are fine. Then you took it in and he said 40, then 0, then 75, but now there is an overheating problem???

0 is almost impossable. I would say either a bad gauge, or he was not checking it on the compression stroke. Now the timming is off to cause overheating and he performs surgery on previously working baffels??? - run like ****. Either he does not know what he is doing, or the timming marks on your flywheel are not indexed correctly. Either way, your A&P sucks.

Hey There,

I took it in because of high CHTs. Baffles were def not working like they should based off of evidence of mating on the cowl and the baffles.
 
So if they were all hot, that points to baffle sealing problems.
Removing the dams in front of #1 and #2 will increase airflow on those two cylinders... stealing air from the aft cylinders.

Given your location, these are aggressive climbs in what 50F air temps?

I mean, I'm happy progress is being made, and your cylinder issue seems to have been some sort of fluke, but I'm still a bit bothered by the approach. If temps are all hot, fix the leaks FIRST. Then decide about the dams.

I see lots of A&P's rip out those dams, with the logic that you don't see them on Cessna 172's and Piper Cherokees. Planes with much bigger inlets.

Yes it was cold. There are quite a few RVs I’ve seen out here with those dams removed and two people with 7s told me they got cooler CHTs once they were removed. I’ll keep an eye on differences in CHT.
 
This is a monstrous thread you linked me to. Are you focusing me on the page with the link with the intense baffle mods?

Yes. Point being Dan, who has studied the cooling air flow EXTENSIVELY has added a lot more dams to the airflow in search of good cooling + efficient use of cooling air.

I'll be interested to see if your CHT's are still balanced front to rear without the dams. I've put dams IN on two RV7's and improved the front to rear balance. Let us know how it is when you get it back.
For experiment purposes aluminum duct tape works great for creating a temporary dam if you want to try with & without.
 
Yes. Point being Dan, who has studied the cooling air flow EXTENSIVELY has added a lot more dams to the airflow in search of good cooling + efficient use of cooling air.

I'll be interested to see if your CHT's are still balanced front to rear without the dams. I've put dams IN on two RV7's and improved the front to rear balance. Let us know how it is when you get it back.
For experiment purposes aluminum duct tape works great for creating a temporary dam if you want to try with & without.

Okay will do, appreciate the words.
 
For temporary dams just put a piece of velcro in front and you can adjust to virtually any size fast.
Art

I used metal tape (stainless, I think; might have been aluminum) and experimented with heights until I got good balance, then cut the stock dams to that size (I had manufactured the baffles with nutplates so I could remove them). Got to where most flights the front-to-back and left-to-right deltas are < 5 degrees F.
 
And they did...

In the 70's, my 2 stroke Tohatsu motorcycle piston had a pin in the gland to keep the ring in one place so it would not rotate and catch on a port. The ring was designed to seal around the pin.

But that is unique to 2 stroke engines with the ports cut into the cyl wall. I have seen them as well on dirt bike engines. Pretty sure the Lyc is a 4 stroke, but will need to check my notes:D
 
If piston rings are sticking, you might try some top-end oil (Marvel Mystery Oil). Maybe pour an ounce into top spark plug hole, replace spark plug and turn engine through several compression cycles. Let oil work its magic for a week or so and then start engine.

I run 2 ounces Marvel Mystery Oil mixed in 10 gallons gas. I’ve been doing it forever...
 
Per MMO:

"ADDED TO FUEL: At every fill-up add 4 ounces of Marvel Mystery Oil or Marvel Ultimate Oil to every 10 gallons of diesel or gas."
 
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