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

[jeffro ra28]

As all pistons are well now are cam ground or turned the "stained" area is were the piston has most of its clearance (normally .020"), and the bore having so much clearace it wont scrap off, normal for a full skirt piston. The suffing (top ring land) can also be from combustion being to lean or engine running hot exactlly as bored said.
Forged pistons would be the go. But in saying that everthing will go bang if it detonates

stu.

The carbon is from blowby... yes the remainder isnt scapped off.

Everything goes bang? cast pistons go bang. forgies are more resistant to detonation

[crostek]

Foreign Object Damage (FOD)

Abbreviation I still use from my days at Qantas.

"FOD" it sure was,
Be it the plugs that have died/ destructed from over boost and or the motor running lean and the combustion chamber temp sky rocketing, (this will also melt the tops off pistions)
Or from something coming in though your intake, turbo blades or rocks?

I had some arsehole at the drag's drop a hand full of steel ball bearings down the throat of my unfiltered turbo and the piston tops look much the same after one run.

Detonation is more likely to cause the pistons to fail by cracking, I have a set to prove it from a 1G that broke the ring lands.

That motor needed rebore and new pistons from the deep scratches in the bore from the broken pistons

hope you find the cause

[twentyEight]

I had some arsehole at the drag's drop a hand full of steel ball bearings down the throat of my unfiltered turbo and the piston tops look much the same after one run.

I hope you shoved a handful of steel ball bearings down his throat...

People have died for less you know!

[stradlater]

That's fucked, why on earth would someone do that?

[DX20VT]

Detonation is more likely to cause the pistons to fail by cracking, I have a set to prove it from a 1G that broke the ring lands.

Ring land breaking is just too much cylinder pressure not detonation.
Our 1G Turbo broke the ring lands on 3 of the six, and with a set of forgie's and the same boost fuel timing etc etc it ran for years.

Forged pistons will stand detonation better,
but enough will still destroy them.

[stradlater]

Well, I didn't destory pistons, other than my FOD.. I just blew my head gasket to pieces.

[TheToyman75]

Ring land breaking is just too much cylinder pressure not detonation.
Our 1G Turbo broke the ring lands on 3 of the six, and with a set of forgie's and the same boost fuel timing etc etc it ran for years.

Forged pistons will stand detonation better,
but enough will still destroy them.

What do you think causes this excess Cylinder pressure ?????? DETONATION

Because I am Lazy here is a link for some light reading.
http://www.contactmagazine.com/Issue...ineBasics.html

[MS112]

I don't know how much use this post will be...

But my question is, on the first photo the damage to the piston was restricted to those two areas, how come the rest of the surface is relatively in tact? An object inside wouldn't limit itself to those two areas, it would have had to of bounced across to the other side, right?

Of course, I haven't asked how that piston is orientated...Do the three circles mean that the two indentations on the left are the intake valves? If they are, that could explain the localised damage, as two particles may have entered from each intake, bounced around and left via the two exhaust ports...

As for the damage being due to detonation, I was of the impression that detonation caused more horrible things to happen like conrods bending, crankshaft shear and twist etc, right up to blowing a hole straight through a piston or cylinder literally exploding.

[stradlater]

Yeah, I don't think it was detonating...

But it's a valid point about the damage only being around two areas, the same goes for all the pistons. And I think the answer is because the pressure buildup is in the middle of the piston, which under compression would push the foreign object to the outsides of the cylinder as it tries to 'escape' under the pressure.

[oldcorollas]

I don't know how much use this post will be...
But my question is, on the first photo the damage to the piston was restricted to those two areas, how come the rest of the surface is relatively in tact? An object inside wouldn't limit itself to those two areas, it would have had to of bounced across to the other side, right?
.

the areas of damage are from where the foreign object was squashed between the cylinder head and the piston. if you look at the head, the squish areas are at the sides, and just bouncing around won't make such bad indents.. even tho the thing would have been bouncing around like crazy

i had a K engine do the same thing, and the marks were over 1/3rd of the piston.. corresponding exactly with the squish area...

squish area is only 1mm or a bit less, so any chunk of spark plug or whatever that is bigger than that, will be quashed into the piston and head.

[Hen]

I'll add my support to the FOD idea.

Something got in there (possibly including the sparkplug electrode) and got smashed between piston and head. It was only around the edge that it actually damaged the piston because that's where the squish areas are.

Any photos of the head? Corresponding marks there?

A while back my 4A started WRXing. Plug 2 had lost its electrode and insulator completely. Maybe I'll have nice chomp marks on that piston too.

Hen

[MS112]

the areas of damage are from where the foreign object was squashed between the cylinder head and the piston. if you look at the head, the squish areas are at the sides, and just bouncing around won't make such bad indents.. even tho the thing would have been bouncing around like crazy

i had a K engine do the same thing, and the marks were over 1/3rd of the piston.. corresponding exactly with the squish area...

squish area is only 1mm or a bit less, so any chunk of spark plug or whatever that is bigger than that, will be quashed into the piston and head.

And I guess that is why you have 2000+ reputation

I was mulling over stradlater's reply trying to verify his explanation, and I simply couldn't. If that were indeed the case, the pressure still wouldn't have limited it to those areas, just the outer edges, which it hadn't. I think the centre of the piston would have to be raised in order to displace the air to the outer edges, and from photo #2, the 1G piston seems to have a depression instead.

That actually makes a lot more sense; I didn't consider if the impact from the foreign object alone would actually damage the piston. [a 'questionable' calculation shows to cause any damage to a cast iron piston may take nearly a kilonewton of impact force (assuming 100MPa yield stress), and for something of little mass, would take a massive acceleration of the piston.]

[jeffro ra28]

cast iron piston aye

[hypo23]

go for the forgies $1400 landed from the states including rings

[stradlater]

Don't think I'll be going for forgies, but thanks for the tip. :-)