Munched Piston (PICS) - What the hell caused this?

[mrshin]

No, I wouldn't use any of that engine again, but by saying less damaged, I mean that the block isn't split, it hasn't kicked a leg out of bed, and you didn't pull all the pistons out in 30 pieces.

[stradlater]

Well that's the thing.

I do however need to use the pistons, or some 1g-gte pistons..The block and so forth doesn't worry me, infact the block itself is in pretty good condition, the bores are great.

As for how old the bottom end was, well the bottom end is in pretty good condition, nothings gone wrong. However it was probably original, so what, 15 years?

[Bored?]

What condition are the valve guides in? Any broken bits?
(Another possible source of metal entering the combustion chambers)
Has the engine ever been really hot or run low on oil?
(This might account for the piston scraping)

[YelloRolla]

Scuffing could also be caused by seized rings. With the electrode gone from one of the plugs it is fair to say that the engine has been detonating, most likely from a lean condition and from that point on everything else would be subsequent damage.

What spark plugs do you run? The reason that I ask is that it is normal to go to a colder plug when increasing boost. IE when the combustion temps are going up you need to run colder plugs (kind of like wearing shorts on a hot day).

edit - one other thing with regards to the excessive blow by could also mean that you don't have enough radial ring tension.

[brett_celicacoupe]

this browny yellow tan indicates combustion leakage between the bore and piston.

ie rings not sealing 100%

nothing uncommon for a engine with 100 000+ k's

[hypo23]

on the topic of srewed 1g pistons heres mine when it decided it had enough, just to compare the pics arent that clear, but a big chunk missing on one side and on the other side the edge of the pistons is like round worn away sort of thing, detonation?

this was piston #6 all the others were fine

[DX20VT]

The small dints that appear around the edges on the top surface of the piston are from detonation, not from something in there.

I have a few 4AGZE pistons that have the same marks from running too much boost and advance.

Too many of those marks appear and you can break a chunk off the side like in the photo posted by hypo23.

[jeffro ra28]

Detonation leaves crators, not sharp indentations.....
Please tell me, how are the bearings? Tuning from the bearing is also an indicator that can help us determine whats happening inside the thing? An example is a sqaushed big end bearing, in some examples of top fuel, they tune the engine from the big end bearings, if they are sqaushed or not etc...
I take it they are full floating pistons, check to see that the small end bush hasnt been squashed at all... Just these simple things can tell us if its been detonating.


I wouldnt be worried about re using the pistons... I have welded up pistons to the point where the piston shrinks and having to bash the piston skirts out with a hammer to acheive correct bore size....After the engine completed its stask it was running with .080" piston to bore clearance. My point being, you would be seriously surprised with what you can get away with.
What i would do is linish the sharp lumps from the pistons and file away all sharp edges in the holes.... If they are still on size, they will be fine to run again with a new set of rings.

[MS-75]

Are your turbine blades intact?

[stradlater]

I think they are at this point, I have had a look and they look ok, but I haven't pulled it all down yet...

As for the bearings, they looked ok, but I'm not entirely sure what I'm looking at. All still square and stuff.

I'm pretty convinced that the marks aren't detonation, becuase they are way to sharp, not like I would imagine a force/explosion based impact would look like.

That and the bit of white stuff embedded in one of the pistons..

[jeffro ra28]

Is the white metal worn away from the bearings?

[stradlater]

Well, I don't know whether it's white Metal per say, I suspect it's infact white ceramic from the insulator on the spark plug.

I don't have any white bearings. They're all silver, and it appears that the bearings are in tact.

[jeffro ra28]

Well, im extremely confident in saying that the marks on your pistons are not from detonation. Thats not to say it wasnt detonating but thats not whats causing those marks.
Either spark plug remains or something small falling throught the intake, whether it be a pin or bolt head or somthing thats broken off.

With regards to your bearings, the silver you are seeing isnt exactly good. But its not extremely bad. It just means that under heavy load the oil film is being sqished away from the crankshaft journal and it contacts that bearing and also is a sign of light detonation. Idealy the crank journals should never contact the bearings. This is usally only ever acheived in a performance or heavily loaded engine by using a dry sump or extremely fine tuning. If you see any copper coloured metal on your bearings you have problems.
But if its only silver i wouldbt be worried. Yes your bearings are fucked. But its had a flogging and done alot of km's. If you replace them i recomend, federal mogul bearings, very hard to find but they leave acl, king and clevite for dead. Otherwise H series acl's are the way to go.

Have you decided whether to just replace the rings and re-use the pistons or not?

[thechuckster]

just a thought - any small bits of the gas supply system missing? and any marks (apart from oil stains) on the throttle body?

apart form the plugs, is it possible that you inhaled something post turbo?

[terryo]

those marks are most likely from detonation, but some of them maybe from damaged plugs at some time. the detonation cause tiny balls of molten alloy to be pulled out of the piston top, they go solid, then do that damage. the head is probabaly similarly marked. those are stock cast pistons . we see this everyday with Toyota engines.
changing pistons wont fix it. in fact, it will make it worse cause rebuilding it will raise compression slightly. you need to fix what is wrong, too much boost, too much compression, wrong computer, crappy fuel, wrong plugs etc