2nd piston ring always stuck- what's the story ?

[RFguy]

I'm overhauling a jabiru 6 cylinder engine, and it has on #1 and #2, the middle ring stuck

on both....... both stuck on the minor thrust side .

 

the last Jab 6 cylinder I overhauled, it ALSO had the middle ring stuck on 3 of 6 cylinders.

 

Does this 2nd ring in the jab  not get enough pressure across it (IE first ring does all the work )  to move , extend and seal  (and do something useful) ?

 

I've heard muttering about insufficient ring land clearance being an issue.

I am interesting in commentary  on this . and guesses

Can someone with a bit of experience particularly with seeing this on non aircraft engines please comment ?

 

has any one seen this on only unleaded petrol only jab engines ?, or is it just an avgas fun thing. 

 

-glen

Edited December 1, 2022 by RFguy

[facthunter]

Normally the top ring lands wear the most. Light rusting of the bore will pug up the rings with rust residue. Also I'd consider the lead or the antifriction additives. Less likely with aviation oils like W-100 plus.   Nev

[old man emu]

14 hours ago, RFguy said:

both stuck on the minor thrust side .

I'm not an engineer, but that statement arouses my curiosity since it meant enough to you to mention it. You gunna explore the significance of that?

[dlegg]

My 3300 at 1000hr overhaul had no stuck rings, always run on premium 98. Everything actually looked quite good. Gen 2.

[RFguy]

dlegg -Yes, 95/98 ULP is the best fuel for Jabirus. no argument from me on that. keep them clean and they wont get much buildup, which means less /no stuck rings, no red hot buildup coals, etc etc less of everything undesireable.

 

OME : It's significant that the sticking wasnt random. they're stuck within a few degrees of precisely the centre of the minor thrust side. ther eis significant temperature difference between the top and underside of the barrel, so its not that.  So was it an event that causes this to occur, or is the clearance such that the buildup will just kill it in time. 

 

An event might be... sudden barrel cooling where the piston goes almost interference fit ? 

Or ...is the lower pressure on the piston  on the minor thrust side, make the scraping of the barrels is not as effective / different and it scrapes junk into the ring lands ? something goes on here.  If I can get it out without damaging the ring , I'll take a look under the microscopes. 

 

 

 

Edited December 2, 2022 by RFguy

[Bruce Tuncks]

I'm not clear just what fuel you were on rfguy. At the ASC, we had a LOT of rings sticking on avgas, so much so that we wondered if they had changed the formulation.

 

[Bruce Tuncks]

And, how did you notice the sticking? was the engine being stripped down anyway?  and what cht's had you noticed?

 

[kgwilson]

My initial thoughts when reading the first 2 paragraphs of Post 1 were it is Avgas and lead that has caused the problem. Having seen a number of Avgas fuelled Jab engines pulled apart & dleggs comment, I'd say it is 100% an Avgas issue.

 

Don't use Avgas despite what the Jabiru manual says. They had to say this for their certified engines. Their manuals now say Unleaded RON 95 & above is a recommended fuel but also have a caution note regarding quality control & shelf life. JSL007-7 is the latest fuel guidance service bulletin & is worth a read. 95+ RON unleaded petrol shows to be the best fuel with the same quality control and shelf life cautions noted so that reduces the star rating they provide.

[facthunter]

Heat the piston to free the ring. If it's too tight in the land you will have to do something about that. That's not easy to set up and do accurately. IF the aluminium has been contacting the cylinder there it could have burred the edge over a bit and trapped the ring. That's happened before. A slight chamfer with a fine file will help prevent that. Nev

[onetrack]

The experts provide the precise answer as to what the 2nd piston ring does. Its job is to transfer heat to the cylinder wall, and not actually provide additional compression seal.

 

https://www.enginelabs.com/engine-tech/answers-on-the-second-ring-the-science-of-the-second-piston-ring/

[turboplanner]

3 hours ago, RFguy said:

OME : It's significant that the sticking wasnt random. they're stuck within a few degrees of precisely the centre of the minor thrust side.

A lot of things are happening inside the combustion chamber which is alternatively heating to around 2000 deg C, then cooling, piston going up then reversing and coming down piston faces pushing against ro bore on opposite sides alernately, rings banging on each adjacent land every rotation, ring to barrel pressure in different directions, gugeon distorting the piston, and mixture entering at all sorts of different richness in each cylinder and each owners engine and each make of engine. and each hour/hundred hours of service.

 

The result of this is that your engine on your application with your fuel richness, your timing, your exhaust, your rpm behaviour cool down behaviour wil produce a different visual/mechanical result.

 

Some people glaze bores, some burn holes in pistons, some have sticking rings, multiplied by different makes, models, sizes,layouts.

 

In my racing engines, I get to see the results often and at early hours and now and again through the open barrel hole in the side of the cylinder. You get to expect the characteristics of normal cylinder temps, and a sharp rise due to the cylinder being given a very heavy load perhaps due to a spark plug lead coming of another cylinder.

 

So if this pattern of stuck rings you're seeing is common across the engines you're pulling down, pick one of the above; it will relate to some common thing being done to those engines.

 

 

[Thruster88]

I have seen about 20 lycoming/ Continental pistons following cylinder removal due low compression. Non have had stuck rings if I remember correctly. What is the difference with these engines designed to and all would have been run on avgas.  

[RFguy]

thanks all the interesting observations and comments . Nev before I extract it, I will get it under the microscope and see if I observe any trapping of the ring.

Thruster what's the ring land gap on the 2nd ring in a O320 ?

[turboplanner]

22 minutes ago, Thruster88 said:

I have seen about 20 lycoming/ Continental pistons following cylinder removal due low compression. Non have had stuck rings if I remember correctly. What is the difference with these engines designed to and all would have been run on avgas.  

Lycoming/Continental are shut down by pulling the mixture to zero, so relatively dry cylinder shut down.

RA 4 and 2 stroke usually have on/off switches so oil up on taxy to parking area, full rich shut down?

[Thruster88]

3 minutes ago, turboplanner said:

Lycoming/Continental are shut down by pulling the mixture to zero, so relatively dry cylinder shut down.

RA 4 and 2 stroke usually have on/off switches so oil up on taxy to parking area, full rich shut down?

I don't think that is it, your typical lycon is probably more rich at low power setting unless the pilot leans on the ground.

[turboplanner]

3 minutes ago, Thruster88 said:

I don't think that is it, your typical lycon is probably more rich at low power setting unless the pilot leans on the ground.

Maybe not but I'm not thinking of flight, just the last few minutes or just the shut down itself. I've pulled pistons out that were bone dry and others soaked in fuel/oil.

[facthunter]

Diesels have more compression rings than petrol engines because of the higher pressures they work at. You will have a lot more blow by with just a single ring.  Some popular diesels have a piston kit with more comp rings than originally supplied.  Some race engines have a single "L" section ring . The main purpose of  it is to reduce friction. OIL SCRAPER rings can have a lot of friction. IF the oil groove has holes drilled in the base of it there will be no compression/blow by effect whatever. It's just removing oil.  Nev , 

[jetboy]

replaced rings on my 2200 @ 500 hrs because I had a spare set and leakdowns were becoming poor - engine had blowby but otherwise running fine.

 

one ring was broken and another stuck, the pinch points mid span of the open slot under the oil control ring assembly if memory serves.

dont remember which side of the skirt this was but I had difficulty getting a new ring to fit it was tight in the land in that area

saw no rust or gunge to clean out and followed Jabiru instructions with an old ring to help clean out the lands there was no sign of burring over it seems to me the lands had narrowed in this area due to heat and shrinkage around the unsupported thermally isolated slotted portion of the piston crown.

#1680 from  2003 the pistons may be the original version - using Avgas

dont know if current pistons use slotted skirt or have holes so is this stuck ring thing only for the older engines?

Ralph

 

[facthunter]

Slotted pistons are not suitable for air cooled engines that tend to run a bit hot and put out above about 40 hp/ litre. Nev

Edited December 2, 2022 by facthunter

[RFguy]

Hi jetboy, thanks for the input. Jabiru have used 'various' pistons. 

Interesting on your thoughts of the heat narrowed lands hypo.

 

When I cleaned out the lands of the last engine , the middle ring lands had what looked like black glass in them... it was tough like diamond.

took a month of sundays of scraping with the end of a ring to clean all the lands pristine. 

[facthunter]

Because the top of the piston has been so hot.  Nev

[RFguy]

diamonds are made under super temperature and pressure , I guess we have the right conditions....

(yeah I know its not real diamond- but has glass appearance.)

Edited December 3, 2022 by RFguy

[facthunter]

It's not diamond. Much higher temps and pressures are needed. Some dust is usually part of the deposits especially where the aircleaner is not that good. Nev

[RFguy]

so, if the ring becomes stuck - (and the stuck rings are all the way in against the inner radius) IE flush with the piston  outer-   then it would stop effectively conducting (its share of the )  heat to the barrel on that side of the piston , and just get hotter.

[Carbon Canary]

Melting point of silica is around 1700C and theoretically a smidgen of silica could come from quartz type dust getting through the air filter.  Melted (and then frozen) silica would look like and is closely related to glass. I can't recall what the combustion temp inside a mogas ICE is, and too lazy to look it up, but it's probably in the 1700C ballpark.  I've seen glass like deposits in a hog-fuel (tree bark) fired boiler from sand entrained on the logs that go through the de-barker, so it can happen. 

The issue becomes that glass is a reasonably good insulator, so you could have a differential heating influence on the piston ??  Is the assumption that the rings are seized onto the piston at operating temperature as well ?

 

Anyhow, I should butt-out here - I'm barely a backyard mechanic, so risk leading you down a rabbit hole through speculation.

CC