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

[RFguy]

Hi Carbon. your insight is useful. yes glass like indeed.
" Is the assumption that the rings are seized onto the piston at operating temperature as well ?"

you basturd (french accent) you just introduced another unknown ! 🙂

it would be useful to measure BMP etc on each cylinder eh instead of in a lab. I would have thought spark plugs could have done that .  well you can of course with measuring breakdown voltage of the plug etc, and various breakdown emission corona modes..

[Carbon Canary]

I always wondered why a particular type of file was called a 'flat bastard'....until I accidentally filed the top of my knuckle with one.

 

Again theoretically, the differential thermal expansion of the piston and ring should release the ring at op temperature, but the glass-like deposit may act as a glue. 

 

I dunno !

 

[facthunter]

I've had anti friction additives in some particuar oils jambing exhaust  valve guides in cast iron head motors. It's very likely they would get in ring grooves on  hot running pistons as well. If  it's the same spot each time the piston is probably warping. The original pistons were designed for a touring car, not an aircooled aeroplane motor.  Slipper pistons with slots are not strong and don't get max heat transfer to the cylinder walls. Nev

Edited December 5, 2022 by facthunter

[Thruster88]

On 02/12/2022 at 4:32 PM, RFguy said:

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 ?

The book says 0 to .004 with a service limit of .006 inch. So not much clearance at all. I think lycoming rings are tapered so not sure how it is measured. Pistons on lycoming and Continental are definitely not slotted, they are built like an old mack truck. 

[facthunter]

They are strong so they don't break up and also transfer heat better.  Jabiru doesn't try very hard to cool the steel cylinders where they're used.  You'll often see blue in the cyls near where the  exhaust valves are and scratches in the bores indicating too hot and breakdown of the oil film. Nev

[RFguy]

agreed Nev.
There is a tenuous relationship between piston expansion and barrel expansion....

I did run some numbers for another when considering  scenarios of go around and say, top of climb WOT  >> flight idle.
**************************************************************

Jab cromo 4140bores : 175deg C rise (25C >> 200C) (12.2ppm/mK up to 400 deg C)

assumes unifrom temp , Though I would expect the bore to go non circular  because the undersides  are far hotter than the top sides (40 deg difference haphazardly measured) . IE pear shaped

however, if uniform,  is about 0.2mm growth. so...say start at 97.6mm >> 97.8mm

4032...

I know pistons have irregular expansion (due to lots of things.)

worst it would be 19.4ppm/m/K . pretty steady rise in CTE with temperature. linear to 300C...

20ppm at 100C, 21ppm at 250C

we'll call it 20.5 ppm . so 97.5mm room temperature  piston worst case assuming uniform temperature

 (is NOT- bad assumption !!) would be 97.85mm - a 0.34mm growth at 200C (175degC rise)

---------------

Now,  2618. that has quite a different CTE curve

CTE 22.3ppm 25-100C, 23.2ppm 20-200C, 24ppm 20-300C.

We'll use  23.2ppm. so above numbers for 2618 is ---- 97.89 or 0.39mm growth.

Therm conductivity  for 2618 is 146. for 4032, is 138 (lower due to Si).
 

[facthunter]

The temper colour blue does have a defined Temperature which I'm sure is way over where the oil film will do it's job.  4140 and 3 piece stainless oil rings are not compatible as well. That causes the scratches. Bores unevenly heated distort, the hotter areas migrating from a chosen datum as if the material was in fact solid unless some other factor (local  stresses)applies to alter it. Nev

[RFguy]

Jab are  iron. (I thought) .  or a stainless type I am not familiar with. 

[RFguy]

pistons have been sitting in gasoline for a few days. (last time I concluded that MEK, xylene, acetone etc which I have all wasnt really any better than unleaded petrol....)

Observations (since this is my 2nd 6 cylinder jab rebuild)  - top ring land the black stuff, the black diamond is hard, quite  resistant to the broken ring tool.  The #2 ring, the black gunk appears not to be hard glass carbon, more just gunk that can be pushed out within a few dozen revolutions of the broken ring tool.  The oilcontrol lands are just soft gunk. brass brush/toothbrush fodder.

 

There's also gunk in the circlip grooves, which the previous rebuild , similar hours, did not have. 
would be useful to be able to get the piston in the end of a large lathe chuck at 30 RPM  to do some of the work for me. 
 

Edited December 31, 2022 by RFguy

[Bruce Tuncks]

It sure sounds like you have been running avgas on that engine and also controlling the cht's too well. The combined effect is to have lead and gunk deposits like that.

[onetrack]

Glen, this is the stuff I use for hard-to-remove carbon deposits ...

 

https://www.supercheapauto.com.au/p/sca-sca-throttle-body-and-carby-cleaner-400g/0238.html

[Bruce Tuncks]

One of my pet gripes is how hard it is to find just what the active ingredient is on something like that cleaner from Supercheap.

I am only slightly chemically challenged, and one day I worked at finding just what the chemistry of flue cleaners was. It turned out that the cheap ones were common salt and the expensive ones were an aluminium salt. Some said that putting old ( emptied!) beer cans in the fire was just as good.

A current problem is this: Is it necessary to use a glycol-based coolant in your car if it never gets frozen or near boiling? And why?

Anyway, thanks onetrack for telling us that the supercheap stuff works good.

 

[RFguy]

Thanks onetrack. I will try that. also, its been suggested I try selleys BBQ cleaner. Selleys BBQ cleaner will be for #3 and Supercheap carb cleaner for #4. The fronts are more gunked, the rear pots are the hot ones with the harder carbon. 

like I said,  (last time I concluded that MEK, xylene, acetone etc which I have all wasnt really any better than unleaded petrol....). I have a pressure cooker which I clean stuff in soap, but I am not game to try 1.5 bar 120C boiling MEK etc 🙂 . Large ultrasonic cleaner isnt bad, either. 

given the top ring land diamond-like carbon was deposited at >250C, I cant see that coming off at much less temperature, but it's worth a go to see what the chemists come up with.

I'm just remembering what a bastard job this is. (Bruce, previous engine I  rebuilt  was sold - this is a 310 h TSO engine I got to refurb ) . 

Seeing gunk in the circlip grooves is new to me. anyway, parts tank and toothbrush job for that. for me , suggests dirty oil/ oil filter not in circuit . wil check, disassemble PRV , clean etc. Filter didnt have any nasties in it. 

[Carbon Canary]

Bruce,

a ‘good’ coolant also increases the boiling point but more importantly should also contain an azole-based copper corrosion inhibitor, possibly a phosphate and/or molydate steel alloy corrosion inhibitor and maybe a smidgen of silicone-based anti foam. 
How you determine what’s in an off the shelf coolant is a good question and frustrating unless you request an SDS sheet and it has CAS numbers on it for every active.

The short answer is I wouldn’t go back to straight water unless you add the inhibitors.

I was an industrial chemist in a former life and routinely formulated this kind of stuff, but have now forgotten more than I ever knew.

Now back to seized piston rings………

[RFguy]

Hear say is a that 100% ethylene glycol in a pressure cooker makes a great carbon remover for parts  also... Carbon Chemist, what's going on there ?

[turboplanner]

32 minutes ago, Bruce Tuncks said:

One of my pet gripes is how hard it is to find just what the active ingredient is on something like that cleaner from Supercheap.

I am only slightly chemically challenged, and one day I worked at finding just what the chemistry of flue cleaners was. It turned out that the cheap ones were common salt and the expensive ones were an aluminium salt. Some said that putting old ( emptied!) beer cans in the fire was just as good.

A current problem is this: Is it necessary to use a glycol-based coolant in your car if it never gets frozen or near boiling? And why?

Anyway, thanks onetrack for telling us that the supercheap stuff works good.

 

As one who has traditionally cleaned everything with two stroke mix and accumulated decades of carcinogenic load up to the eyeballs apparently, and also blown out brake components with an air compressor, with its carcenogenic dust invading my nostrils and skin and watching how many professional mechanics use gloves, I recently did a brake reline following the safe procedure of spraying the work area with genuine Brake Cleaner, and was mesemerised by all that dusty area flowing down to a tiny puddle of mud on the ground and the components coming up clean with no dust to blow up my nostrils.

 

It killed degreaser and was about to become my favourite cleaner.

 

My next job was a rough running Chev 350, and the diagnosis was a dirty MAF (Mass Airflow System) where the sensor is a wire, and Chevrolet has big warnings Do Not Use Brake Cleaner (obviously for people like me who'd just become addicted to it.) This wire sniffs the air, takes its temperature and sets the air/fuel mix. Another warning was Do Not Touch It. Mine was in a tunnel with a mesh grille each end in case someone wanted to see if something else would clean it or give it a rub with a finger or rag to clean it. I bought the CRC Mass Airflow Sensor and sprayed the hell out of it. The airflow then proceeded in to the throttle body, and for that Chevrolet specified Throttle Body and Air Intake cleaner, so I bought a can of CRC Throttle Body and Air intake Cleaner and sprayed the hell out of that.  We live in sophisticated times and my gloved prevented any carcinogenic spray getting to the fingers. They both must have been clean because that didn't work.

 

The MAF spray has two characteristics; it dries very fast and it leaves no residue on the wire which, to be fair, is a very fine gauge.

 

Getting on to coolants, I lived through a period when my customers who worked on their truck engines usually replaced the exotic coolant with swamp water, tap water or whatever came out of the hose, which was fine for most of the history of the automobile, but we were the clever generation and had started doing amazing things to engines like using Dry Ice shrunk sleeves in aluminium blocks to provided a cheaper repair than reboring and a lighter vehicle, and we had solved things like corrosion by using very specific coolants. We'd got the engines out to about 1.4 million km to in-frame rebuild, and even the little four cylinders out to 700,000 but we started getting warranty claims for pin holes in the liners. When we checked the radiators we found the coolant had been replaced by water, or a different coolant.  With some big fleets, it wasn't unusual to have 100 engines with pinholes, so after those painful years my answer on coolant is, while the engine might not get near booing or might not be parked up in the snow, always replace/top up with the coolant specified in the owners manual (there are a lot of different coolant mixes).

 

 

 

[facthunter]

Most oven cleaners are just caustic. I wouldn't put that anywhere near aluminium.  There are cold bath aluminium tolerant degreasers that need a water seal to keep the volatiles from escaping similar in principle to what gasometers used to have incorporated in the lid Since they are dangerous you'll find them only from the makers   (Gibsons) or someone in the engine reconditioning trade working with alloys. Nev

[RFguy]

yes, always try a test piece.

Dishwasher tablets- Sodium Hydroxide - deadly on aluminium. but   good for cleaning up valve steams....

[Carbon Canary]

34 minutes ago, RFguy said:

Hear say is a that 100% ethylene glycol in a pressure cooker makes a great carbon remover for parts  also... Carbon Chemist, what's going on there ?

Its possibly acting on what’s binding the particles of carbon together- like a bit of residual hydrocarbon or other general muck, rather than ‘dissolving’ the carbon.

In the bad old days chlorinated solvents we’re the mainstay for this kind of job - methylene chloride (paint stripper) or carbon tetrachloride etc but these are particularly nasty to humans.

Carbon tetrachloride was even used as sheep drench back in the first half of last century, but it probably killed more sheep than worms.

 

Just remember to never put holy water into a pressure cooker - you will blow Christ out of the kitchen !

[facthunter]

Watch out for  alloys containing magnesium. They need special care and chromic acid treatment.   Nev

[Ian]

Piranha solution will dissolve the deposits. However it will probably also continue to dissolve the entire piston 🙂

 

But it is good on glass.

[facthunter]

Diamond hard carbon is most unlikely but silicon carbide and aluminium oxide could be there. There's also the possibility of some antiscuffing additives building up and DISTORTION  of the piston being part of the second compression ring problem. Some deposits in ring lands are so difficult to remove it's better to just replace the pistons.   Nev

[Blueadventures]

Don't use brake cleaner on anything you are going to apply oxy flame heat to; the white smoke it will knock you down or worse.

Edited January 1, 2023 by Blueadventures

[facthunter]

Usually the top ring groove wears the most. IF the second one is consistently grabbing increase the side clearance to .002" on the upper face but it must be done accurately. Rings  seal on the bottom face where the pressure pushes them.  Nev

[RFguy]

thanks Nev. I will do some accurate measuring...
ironically, it's cheaper for me to just go and buy a new set of pistons than refurbishing this set of 6, however, then I wouldnt get much benefit about learning the process- although I've already done that once. pressure cooker, MEK soak, ultrasonic cleaning, nylon and brass rotating brushes then finally ring lands with broken ring...  that was last time, probably half a day a piston.