Deposits on underside of piston crown

[Jens]

The two pistons of my 20/2 with around 1200 hrs run time showed a substantial accumulation of something or other on the underside of the piston crown. Is this a known phenomena and if so, what causes it and can it be prevented. I have visions of this junk breaking off and making a bee line towards the nearest oil hole for big end lubrication.
This is not casting sand but rather material that appears to be depositing in layers, possibly from overheating of the piston   I have not had any trouble with deposits on the top of the piston (at least not anything I would consider abnormal). Thickness of the deposit is hard to gauge but I would guess an average of 1/16" or so. It seems to affect the center of the piston crown backside more so than the edges but that is a general observation and I am not sure if it is valid.

[mobile_bob]

was it not there to start with?

its hard to imagine that the piston crown temps are high enough to coke oil, but i suppose it is possible.

if you are going with plain upper brgs and hollow dippers, there will be no problem with anything getting into
the big end brg,  in my opinion.

if the piston is getting hot enough to coke oil i would be concerned with the piston over  growing the available
clearance between it and the liner, some scoring might occur

did you have any scoring or scuffing of the liner or the pistons?

i guess at this point it all comes down to if you recall how clean the pistons innards were back in the beginning.

a slight discoloration should be normal, a 1/16" in 1200hrs is not normal in my opinion.

bob g

[Geno]

I had my piston out a few weeks ago and I had a quarter size deposit right in the middle of the underside. My engine has never overheated and the oil gets to 130°F only on the hottest days. I know it wasn't there when I put it together. My cylinder wall was fine. A WAG is that that is where the pre chamber gasses hit the top of the piston.

Thanks, Geno

[mobile_bob]

thanks Geno, i hadn't considered the prechamber exiting and impinging on the head of the piston
it would be very hot and that would explain a small patch like you describe.

the way i took it was pretty much his complete piston underside was coated with the back stuff.

but i may have read more into his report than was there.

bob g

[Henry W]

Hi Jens,

It looks like the piston was overheated. And the black deposits look like coked oil.

Henry

[Henry W]

Jens,
I think you found your answer. You wrote that the pistons might not get enough oil splashed up on the bottom of the piston to cool it. It seems this could be it.

[Henry W]

Looking closer at the pictures I notice that there is a spot on the piston skirt that has a blue discoloration. This also looks like overheating due to the piston being to tight in the cylinder. This happens at times when the piston expands. The pistons could of been to tight in the cylinder when assembled.

Another thing that could cause this is the piston is not ground right and once heated the piston gets out of round.

What are your pistons made out of?

If they are aluminum who knows what alloy was used.

Most aluminum pistions are made from a combination of both aluminium and silicon.  It is the amount of silicon though, which determines the pistons overall strength verses wear resistance properties.  Silicon also controls the rate of expansion of the piston as the material becomes hotter (the less expansion then better!).  Silicon content also markedly affects actual material hardness.  More Silicon makes the piston much easier to machine-in the manufacturing phase.  There are traces of many other metals in cast pistons, including copper, nickel, manganese and magnesium, all of these adding somewhat to the overall behaviour and strength of the piston.

From my experiance in building Ford 427 and 428 FE big block engines for high performance use I noticed the following.

Some forged pistons fitted to the clearance that the manufacture recommends slaps on a cold startup and it sounds horrible until the engine comes to operating temperature. I would avoid most forged pistons for every day use because of excellerated wear when cold.

On the other hand Hypereutectic Pistons are great for every day use and mild performance applications because of low expansion rate and they can be fitted with less clearance in the cylinder. The draw back of Hypereutectic pistons is they cannot handle detonation very well. But I like Hypereutectic Pistons for making an engine that will go 100 of thousands miles.

There are so many differant alloys used by piston manufactures and some work better in certain applications.

Hope this helps.

Henry

[Henry W]

Jens,
I think you found your answer. You wrote that the pistons might not get enough oil splashed up on the bottom of the piston to cool it. It seems this could be it.

The thing that bothers me about that is the layering of the deposits. This would imply that even if there is oil splashing happening, it is not sufficient to cool the piston. Does this mean I need to modify the internal lubrication with a directed spray to the piston crown? Why would I run into that issue at all seeing that the design is well proven ?

Putting in oil coolers (Oil Sprayers) will help to keep the piston cooler. This could be very expensive to do.

About proven design, Well it all depends who and how it was assembled. There could of been clearance issues or material issues. All manufactures run into this from time to time.

Please go over what I wrote on my last post. it might help explane some things.

Henry

[Henry W]

Some other things that crossed my mind is that could of caused this is: Fuel type and Timing.

One or both could cause piston top heating.

Henry

[Henry W]

Ok, Cast iron pistons do not displace heat as well as aluminum pistons. This is most likely why you have cokeing on the bottom of the piston. The only way to cool it is get more oil on the bottom of the piston.

Time to figure out a way install oil jets and a oil pump.  I don't know if you can just install aluminum pistons in and be done with it. You would have to find out the weight of aluminum pistons vs cast iron pistons.

Henry

[Henry W]

Jens,
You mentioned that you almost seized the engine. This explanes a lot. Replace the liners and pistons with new rings. Make sure the pistons have the right clearance in the liner once the liners are installed. Make sure rings are gapped properly in the liners also.

Make sure oil is getting splashed and live with it.

We can get to worried about things at times.

Henry

[wormshoe]

Hi Jens !

I have no direct, hands-on experience with Listers, so I am way over my head even suggesting anything to this group. It appears from the piston pictures that you posted, that your connecting rod might be twisted.  It would be a shame not to check its alignment before you reassemble your twin.  Just a thought.

I enjoy reading about this group's adventures.  Thank you for allowing me to sit on the sidelines and watch you guys.  Good luck Jens.

wormshoe

[Crofter]

Usually lack of clearance for the piston or tight ring gaps etc will show up trouble early in the game, not later. I am suspicious of oil dilution by the veggy oil being burned. Dilution perhaps to the poiint of near actual replacement as consumption by the engine is also occurring. Lots of tales about how veggy oil turns to undisolvable varnish on heated surfaces. Not good for heat transfer and could block space behind rings needed for compression activated ring/cylinder contact. blowby then starts and that really causes runaway heating, piston growth and scoring. You might have reached the tipping point that led to the final chain of events where the oil level got sucked low and lubrication ceased and piston seized.

All conjecture of course but cast iron pistons dont often seize up without something obvious causing overheating. Personally I am only aware of a situation where a dislodged muffler baffle was the cause and the original suspect problem was head gasket failure and coolant loss. Found the real culprit after a replacement engine was installed and nearly repeated the meltdown.

There does seem a bit of non symetrical scoring on the piston but could be lighting in the photo. As you fit the bearings make sure that the side clearance in con rod to crank cheeks exists all 360 degrees crank rotation, not just at bottom dead center. That should indicate that the con rod is workably free of cock and twist.

Make sure that bottom sleeve O rings are not balled, twisted or half sheared in their grooves in the cylinder casting. That has caused sleeve distortion though perhaps on less substantial sleeves than the listeroid. It is a pain to do but there is a lot of advantage to fitting up bearings and setting cylinder deck height in a mock up without the rings installed. The drag they create can mask other problems like piston cock or tightness in bearings.

[wormshoe]

Hi Jens !

Crofter was seeing the same thing that I saw in your photos.

"There does seem a bit of non symetrical scoring on the piston but could be lighting in the photo. As you fit the bearings make sure that the side clearance in con rod to crank cheeks exists all 360 degrees crank rotation, not just at bottom dead center. That should indicate that the con rod is workably free of cock and twist."

Crofter is probably right.  It may just be the lighting, but it's worth a check. 

Jens, Re your question: If a rod and piston assembly are in alignment, the wear pattern on the skirt will be centered on the skirt ( 90 degrees to the axis of the piston pin).  If you notice that the pattern seems to be shifted on the skirt ( the centerline of the skirt and the centerline of the wear pattern are not one in the same), then I would immediately suspect a twisted rod.  (This wear pattern will be most apparent on the major thrust side of the piston.) Depending on the application, a rod with a twist can normally be brought back into alignment if the mechanic is comfortable with all of the known variables. His experience might dictate that a high rpm machine that is under heavy loading is not a candidate for this procedure, while a Lister at 650 rpm might be a perfect choice for this fix.  I can't stress this enough: The mechanic's own experiences and beliefs must rule the day.

A bent rod will present itself differently.  Instead of a vertically centered wear pattern on the skirt (normal), the bent rod will cause a diagonal pattern to be present on the skirt.  Yes, both 'twisted' and 'bent' conditions can be present on a single piston skirt.

Jens, I hope that my explanations are clear.  I look forward to hearing THUMPER at speed.  Good luck to you sir.

wormshoe

[Crofter]

Jens, I am going to replace the con rod when I pull the 10-1 down to do the plain bearing install and hollow dipper There were a couple of issues with the original rod, probably no more than cosmetic for and indian clone but it gave me the opportunity to prefit things a bit and John F. fixed me up with one for a very good price. The original rod was straight in horizontal alignment of the eyes and no yaw either but the little end was offset making the top end off center. Also a chip from an edge that I opted to blend in with the die grinder. I rather blueprinted layouts so a bit more critical than just making it run so no slur on the Jkson.

In your case, with a premature failure of your "thumper" it is good to be extra careful and suspecting of every possible gremlin until you find if there really was one or not.

A 4" exhaust run would seem like plenty large for that engine displacement but a trick resonance could see one piston facing a pressure wave every exhaust opening at a certain rpm. Highly unlikey but very powerful magic when evoked. Have heard of it happening when twinning two large compressors into a  common line. My son builds tuned pipes for two strokes. Your twin engine has that weird heartbeat that comes from 180 and 540 deg. firing. Lots of other things though in that engine to account for favored oiling to one or the other piston and rods. Lots of fun!

Pic of offset rod