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

[onetrack]

Supercheap Throttle Body and Carburettor Cleaner has the ingredients listed on the can - unlike many others, who regard their recipe as secret as the Coca-Cola recipe.

 

I don't have a can of STBCC handy, but from memory, it contains Methylbenzene (Toluene), Methanol and Acetone. You need a well ventilated space when using it. It also takes paint off, so use with care.

 

I use a 5-10% solution of Citric Acid for all my restoration work. It's safe, economical and pretty effective, when coupled with a 2000psi or greater pressure washer. Carbon deposits are best softened with plain cold water.

 

Interesting article from Hemmings in the link below, pushing a carbon removal chemical that you use to clean up a running engine. The results are quite impressive, and the engine deposits information is insightful.

 

https://www.hemmings.com/stories/article/the-lost-art-of-chemical-carbon-removal

 

Edited January 1, 2023 by onetrack

[facthunter]

Don't like to move stuff in running engines. Often there's heaps of muck in oil galleries in high time motors, In an 0-200 it's easy to remove the bottle shaped sump and fully clean it out. Not so easy in most others. Nev

[RFguy]

So....
IF  there is material being removed from the cylinder wall, (removed by contact with the skirt region on the four sides of the skirts next to the piston pin bosses (these are split skirt)) - IE the four point wear is visible at 50% of the stroke distance so the rings do pass it- and if the material is removed/conact on the power stroke (down) ..

then... Will it be the 2nd ring region that scoops up the material ??? and thus assist caking/choking or even welding up the ring lands /rings ???

Edited January 14, 2023 by RFguy

[Bruce Tuncks]

Is the material removed measureable with a micrometer?  or just because the cross-hatching is visibly worn there...  and you haven't said about avgas or mogas

[Bruce Tuncks]

I reckon we need some "certified" mogas which is of guaranteed quality and properties. I dunno where you get that .

[facthunter]

RFguy,I really think you're barking up the wrong tree here. Split skirt pistons are no good in air-cooled motors and it's most likely piston warpage and original fit too close to counter it.  To Bruce, nearly all bore wear is in the top  inch  Rust is the biggest risk in steel and iron lined cylinders' Doesn't happen with nikasil or chrome. Nev

Edited January 15, 2023 by facthunter

[Bosi72]

Did you buy new pistons or cleaned existing?

 

Last MR I personally cleaned piston grooves on 4 out of 6 cylinders that we pulled from o-300. It took me roughly half a day per piston to clean all sticked carbon using sandblaster and a pick tool to make it clean to the metal.

 

No apprentice would clean that far.

 

I would suspect if the ring isn't sitting perfectly in the groove, e.g. sitting on top of carbon residue, it wouldn't be sitting good on the cylinder wall neither?

 

 

 

 

[facthunter]

I would NOT be sandblasting ring grooves unless you wanted to make ashtrays out of them. Sandblasting is not permitted on any aero engine part. Walnut shells are all that's allowed and the ring lands must SEAL and be a true finish to do it..  Nev

[RFguy]

Bosi - these are 300 hour pistons. you'll need to read the first few posts in this thread to get context.,

Nev- "I really think you're barking up the wrong tree here. Split skirt pistons are no good in air-cooled motors"
----But that's what the Silvolite jab pistons are in Gen 3.
Anyway, I just wanted an opinion from (perhaps vehicle)  engine builders  to whether the removed material from half way down the cylinder (during an interference situation)  gets swept up in the the 2nd ring land, and that is perhaps the propensity to stick the middle rings. (but them you'd expect the top ring to sweep up the burned oil on the way back up the TDC)

I've been learning all about pistons, shapes, alloys , expansion control techniques, heat control techniques, shaped cylinder bores, ring designs and techniques... etc etc etc. (Bruce micrometer yes- most jab engines are like this) 

Edited January 15, 2023 by RFguy

[facthunter]

What Material are you referring to? Rust is possible. I've reconditioned and built engines all my entire life. Slotted pistons are for low performing engines and make the top of the piston run much hotter than with a 'solid" type where the heat flow is not restricted. Solid pistons will require a bit more clearance but the Al cylinder helps that way... Nev

[RFguy]

Nev this is removed cylinder material . I wont repeat the whole thing here .  (croMo bores) 
To these slotted pistons. I did some thermal analysis and step heat appication (heat applied to the crown)  with a thermal camera to the jab pistons to verify my analysis/  and I
measured the hell out of them in and out of an oven to verify my suspicions.

The skirt is max diameter, makes for peace and quiet . now that's fine if you consider the heat cant get across the slot to  heat the skirt, BUT given sustained power input, the heat  soaks through the (large) pin bosses and gets around into those classic four points. So, vehicles that WOT for 10 seconds might be fine. That location is is when the  piston is travel  half way between BDC and TDC.

The air cooled bores also have their own temperature profile. cold at the bottom.  the bores also show inteference between the centreline of the skirts and the bore region with the piston at BDC  (cold region). If the skirts get within 50 deg C of crown temp, they'll likely interfere down there. 

The $64 question is is the material removed because the rings stuck and then lubrication was not as good (blow by), (IE it tolerated the intereference UNTIL the lubrication suffered due to rings sticking and blowing off/cooking the oil film)  or did the material from the interference get in the ring lands and THEN they stuck. ????

Edited January 15, 2023 by RFguy

[RFguy]

#5 piston. step crown thermal application shows same distribution as that colouring !

 

There is negligible scuffing /marking of the bores at the back cylinders  , the unhappy bores are the pair at the front- the air blast cooled bores...

 #1 piston. (has ben through the MEK) 

BORE 1 - LOWER

 

BORE 1 UPPER

 

 

[facthunter]

Well you've been pretty thorough . Many aero bores are tapered (Choked) smaller at the top) to allow for the change of temperature along them. NEW LYConts in the tropics often burnish the pistons on initial operation as they are too tight if that happens. This will cause a lot of frictional heat and may create a destructive cycle.  Most engines run their best and are most reliable at about 1/2 life.. Some pistons come well out of the cylinder spigots at BDC. I would see high crown temps as part of the problem. It can change some of the anti scuffing additives in the lube oils but if you are going on Jabs oil recommendations. (Using aero oils), that issue should be covered. If this is happening continuously just increase the ring land clearance by removing some more metal on the TOP side of the 2nd groove to about .0025" careful to keep the width even. Nev.

[facthunter]

I was posting while you were. That's a mess. It's even gone through the coating where the skirt is at most load below the ring travel area. Most conrod angularity. Calls into question whether that coating is up to it..  Nev

[RFguy]

HI Nev. it's not uncommon either.....

The Mahle 4032 forged piston is a option. the dish CC is about the same, compression height is about 0.8mm  higher than the jab piston which puts compression up from 8:!  to 8.6 ish. exactly how tight it gets between head and top of piston at full smoke I dunno yet. (check) . uses round wire clip I think.  This is not a split skirt of course, and my guess is that the skirt will be narrower because it cops more crown heat 

But before I go swapping pistons  I want to know ... everything. (These pistons are used by others in jabs with varying mods...) 

The bore region mid depth (the four points) if they show more than 0.01mm out that'll be no good for ring seal. (Jab limit  in book is 0.02mm)
(I have ordered a really good bore micrometer to improve on my crappy one) . 
The lowest regions of wear where the skirt is with piston at BDC - I can feel the wear depth with my finger.

I'll likely get a honing shop to overbore them out to get back to round at the 4 pointsd in the middle

IE only about half a thou (IF HALF A thou is what is required to get round) . I will need to consult gurus. IE I would consider half a thou mid way, then maybe 7 tenths of a thou at the bottom. Dont think I need to touch the top,  ALTHOUGH going to Liquid heads from air cooled heads takes 50 deg C off the top inch, so perhaps take the half though down to 2/3 depth, then 7tenths thou from there.  Still all miiles and miles within the (quite wide) Jab tols. (or lyco for that matter) . Different to the tols on track racing vehicles I observe. 

I think there IS a problem with the lack of taper in the cylinder i think . There are many with hourglass shapes. . hot in the middle, colder at the ends, and so narrowerin the middle , wider at the ends- is fairly common for air cooled cylinder bores it seems. 

 

 

Edited January 15, 2023 by RFguy

[facthunter]

Harley Davidson in the 20's GROUND choked bores at some expense and discontinued it in 1929 to save money. The taper was.0015"  Nickasil bores rarely show any wear and only get marked when something abnormal happens. Loss of oil etc.. The distortion limits on Cont Lyco cylinder s are quite small.. The nitrided types retain the hone marks for a long time.  I always thought the original Jab Hone finish was too fine  Once it's polished,  it doesn't retain oil . Nev

[RFguy]

well, these bores wont be refurbished by me with my long hone stone tool at home , it will be a shop most likely that does this stuff. We'll see how the measurements with the new micrometer come out when it arrives. 

comes back to my mantra  of that jab engine should not fly  if leakdowns are < 65/80 because the engines cannot tolerate  the ills..... ( unless you really know what you are doing).  Flying with low leakdowns in a Jab is a bad idea IMO because the engine cannot tolerate the ills that occur because of all the many reasons bad leakdowns are occurring.  . Lycos seems to have enough in hand with design and materials to put up with poor condition without breaking.

 

If I put the pistons back in, its likely they'd be OK since this engine didnt yet break and I am not hamfisted with power application.  And jabs dont go bang very often. Still plenty to learn.  Set of Mahles (x6) and rings (x6) is only $800 aussie....one could argue that would be a better use of my time but until I understand everyhting, that's too much of a experiment to run. 

 

 

[onetrack]

Don't forget there are "cam-ground" pistons, too, that are oval or elliptical, so that when they expand with heat, they fit the bore perfectly. Are the Jab pistons cam-ground?

 

Mahle also have a proprietary piston skirt anti-friction coating called GRAFAL, which is graphite-impregnated, and designed to last 100,000 miles in road vehicles.

 

GRAFAL greatly reduces the harsh contact between skirt and cylinder bore, and greatly improves piston life.

 

https://www.us.mahle.com/en/motorsports/forged-pistons-features-and-benefits/

[facthunter]

If there's anywhere where the cylinder coating is gone don't use the Barrells. Measuring blow by is a good test for the engine  So is turning it over and feeling the compression works if you know what you are looking for. 

( CAUTION with Impulse magneto's) A good condition motor should "Bounce back" off compression  as it stops and should NOT be stiff in any way..  Nev

[RFguy]

Jabs rarely bounce off compression. when stopped (after run) , they're usualy tight. There is conjecture about this whether it is pistons tight in the cylinders or crankcases goign out of alignment. 
Jab bores are plain cro-mo.
Onetrack- certainly the cast Jab pistons are shaped . In research Ive come across about 6 different 'webbing's under the piston crown each requiring their own ovality type  and characteristics so that they are round when hot..... It's quite a science and likely an equal amount of testing that results in cylinders  that are worse for wear and unhappy pistons. 

 

Edited January 15, 2023 by RFguy

[facthunter]

No one builds a race engine  using "W"  or "T" slot pistons.  The login behind them causes a lot of stress in the metal and is a design weakness.. If you cut the engine at full chat it would probably lockup as it requires a supply of heat to the top keeping the top hotter than the skirt . 

 I  agree about the stiffness' and crankshaft but some do behave normally. No motor should get stiff to turn when hot.  Nev

[RFguy]

Well that is an interesting point Nev- the stress with power down between top and skirt.

 

The two instances I have personally been told about,  not 2nd hand, the pilot and plane  at WOT was at top of circuit  climb, had done  or was about to reduce power , and high RPM..    (exact timing not known or remember) ......... Other people have  reported go around breakages, which is a hard one on the engine if power applied 'instantly' to chilled bores. 

I am used to watching the local school' Pipers  engine stop and prop rock at shutdown.

My previous 3300 did rock at shutdown, but then after 1 or 2 minutes became tighter... The older engines tend to rock and not get so tight  I am told.   

 

Edited January 15, 2023 by RFguy

[facthunter]

Logic not Log in. 

  With so many bearings the crank becomes less torsionally stiff especially on the six. Uneven heating to IT and the cases and who knows what's happening. Some cases fret.. No other flat motor has that number of bearings. It causes more cylinder offset also ruling out any twin version.  Nev

Edited January 15, 2023 by facthunter

[Bosi72]

Typo, should've said "blasted" only..

 

RFguy, assuming you used mineral oil (eg. red band) for the first 25hrs ?

[RFguy]

that's an engine I got used, 320h TSO, latest Gen3. I assume it went through the usual process.  I have simply pulled it apart to think about it.