replacing only one piston

[Ebo100]

PeterBurgess said:

I cannot see why over revving would do that to a piston? Overrevving usually seems to show up as shagged ring lands (worn out so too wide) and barrel pattern wear in the bores.

Nice pics of your broken lands, from your pics it still looks like an inverted V as the cracks do not appear 90 degree, but that may be the pic angle?. As I said earlier we were asked about the pics of the Mini piston and it cries out excess pressure from above. If the material is brittle it will fail very easily as shown, including the cracked gudgeon pin boss.

On a few of occasions we have seen wrecked ring lands/nipped rings when a rocker arm has broken on an ex valve and allowed excess pressure in the cylinder. On one occasion the customer had low compression on the cylinder after repairing the rocker assembly. He knew the top ring at least must have nipped, amazingly after 50 miles of use the ring must have made its own clearance again and compression restored...lucky bloke, if it was my engine it would have required a complete rebuild!

Peter

If we could only be that lucky. Does this excess pressure from the top suggest boost pressures then? If so then my plan to have it checked on the rolling road seems to be the way forward then after having the other pistons crack detected.

[Ebo100]

227bhp said:

Does this engine have port or direct injection into the head?

Direct injection, it's the R56 turbo JCW

[227bhp]

Ebo100 said:

Direct injection, it's the R56 turbo JCW

That might explain it.

[Ebo100]

Explain what, excess pressure due to overfueling? Injector fault running lean?

[GavinPearson]

How modified from standard is the engine please?

[227bhp]

PeterBurgess said:

I cannot see why over revving would do that to a piston? Overrevving usually seems to show up as shagged ring lands (worn out so too wide) and barrel pattern wear in the bores.

Nice pics of your broken lands, from your pics it still looks like an inverted V as the cracks do not appear 90 degree, but that may be the pic angle?. As I said earlier we were asked about the pics of the Mini piston and it cries out excess pressure from above. If the material is brittle it will fail very easily as shown, including the cracked gudgeon pin boss.



Peter

I was just confused over the area where it failed, without seeing the actual engine it's difficult.
They're just pics from the 'net I found after Googling 'det broken ringlands', although i've seen it myself too.

I was assuming your data was collected (by AE) in the 70s with 8v heads, my pics were from typical 90s engines with 16v heads, maybe we are both out of date.... I'm pondering if direct injection is putting the detonation in this 16v engine where it wouldn't be if it were port injected?

I have in my piccy collection these pics of pistons which were from two engines of the same type and similar spec, I believe the first set were cracked by over-rev and the second by det, but am open to opinions of course.

[227bhp]

Ebo100 said:

Explain what, excess pressure due to overfueling? Injector fault running lean?

No not that interesting i'm afraid, just pondering why the piston broke where it did, see my last post.

[Ebo100]

GavinPearson said:

How modified from standard is the engine please?

No idea, it's JCW so running approx 210bhp but there is a forge dump valve but no other mods (standard air filter; exhaust etc)

[PeterBurgess]

Ebo....get it on the rollers, I haven't a clue why it had excess pressure from above. Just know the pressure has killed the piston.

bhp227.....I have had the bulletin for donkeys years. My experience is based on loads of different pistons over 30 years, from 2v, 3v, 4v and 5v motors and 2 stroke . I think you have to remember the pressure is probably highest at the exact spot it detonates but the weakest part of the piston should go first, maybe where the oil control ring area is reduced for gudgeon pin?Just a thought. I still don't see how overrevving will break the ring land, the rings would have to be depleted uranium or some such heavy material for inertia to snap the ringland? We usually see big end bearings fail from mechanical over rev...change into lower gear instead of higher gear for example.
All your pics still look inverted v to me.

Peter

[Boosted LS1]

Or, pressure (perhaps force would be a better word) will probably be at the hottest point in the cylinder if you have abnormal combustion which leads to detonation. On pistons I've seen that exhibit detonation this is often on the exhaust side of the piston.

[227bhp]

I don't see any inverted Vs myself.
I'm thinking the crack propagates from the gudgeon pin area up to the ringlands.

[PeterBurgess]

You would have needed to take the pistons out before the crack was top to bottom to see where it started. I have only seen cracks starting in the gudgeon pin boss area when the bores are way too big and masses of slap or knackered pistons in a worn bore and slap induced cracking, soft skirt yields and the bending point is around the thickening of the gudgeon pin boss, keep repeating this and it eventually cracks. It is a Hertzian failure.

Peter

[Boosted LS1]

227bhp said:

I don't see any inverted Vs myself.
I'm thinking the crack propagates from the gudgeon pin area up to the ringlands.

And your diagnosis is?

[227bhp]

Boosted LS1 said:

And your diagnosis is?

Of what?

[Pumaracing]

227bhp said:

I don't know why we have two offerings of pics of the same engine in this thread, is this the same person or two people?
Why was the cylinder head repaired? More to this story than initially meets the eye I think.

Note the cracks around the gudgeon pin:

There are no cracks around the gudgeon pin. You've highlighted a couple of handling / casting marks or tricks of the light.

The cracks through the ringlands are textbook top propagated cracks as per my first post. Through each land the crack is wider at the bottom than the top and the initiating "beachmark" pattern is just under the top land.

[Pumaracing]

227bhp said:

I have in my piccy collection these pics of pistons which were from two engines of the same type and similar spec, I believe the first set were cracked by over-rev and the second by det, but am open to opinions of course.

The first piston has cracked from long term flexure of the piston skirt. The second from a top propagated pressure loading.

[Pumaracing]

In response to some previous posts I've never seen a piston fail catastrophically from just what could be attributed to "over-revving" whether long or short term. They just don't break that way. Only extreme cylinder pressure or very long term fatigue break the aluminium up. Obviously high revs shorten the fatigue life but something else in the engine will nearly always break first.

Back in the early 90s a friend of a friend took his highly modified VW Golf 16v 2 litre to the dragstrip. K jet fuel injection and distributor ignition. He changed from 2nd into 1st rather than 3rd and dropped the race clutch in hard. The engine died instantly and had to be towed back. Much doom and gloom. Rough calculation indicated about 11,000 rpm. Before ripping it apart to see which bits had grenaded someone thought to check the basics. No spark. The rotor arm had flown apart. Everything else was fine.

[Boosted LS1]

227bhp said:

Of what?

You seem to suggest the crack originates from the gudgeon pin when the evidence shows pressure from above. Hence, if you believe this to be the case, what do you think is the actual cause of the piston failure?

Me, I think it's a combination of either eutectic or hypereutectic pistons being unable to deal with a high pressure event in the combustion chamber but I'm open to other possibilities, always.

[227bhp]

Boosted LS1 said:

227bhp said:

Of what?

You seem to suggest the crack originates from the gudgeon pin when the evidence shows pressure from above. Hence, if you believe this to be the case, what do you think is the actual cause of the piston failure?

Me, I think it's a combination of either eutectic or hypereutectic pistons being unable to deal with a high pressure event in the combustion chamber but I'm open to other possibilities, always.

After reading what the others have said and giving it some thought i'm in agreement. As you may have noted earlier it was the position of the breakages which threw me initially, whether my ideas on that are correct or not I don't know.
I'm not the kind of person who takes a reply on something like this without knowing the whys and wherefores as it interests me to know the background. It's been an interesting debate with lots of useful info patiently imparted by various people, I don't think any abuse was thrown or anyone fell out which is a bonus and a rarity!

[227bhp]

Pumaracing said:

227bhp said:

I don't know why we have two offerings of pics of the same engine in this thread, is this the same person or two people?
Why was the cylinder head repaired? More to this story than initially meets the eye I think.

Note the cracks around the gudgeon pin:

There are no cracks around the gudgeon pin. You've highlighted a couple of handling / casting marks or tricks of the light.

The cracks through the ringlands are textbook top propagated cracks as per my first post. Through each land the crack is wider at the bottom than the top and the initiating "beachmark" pattern is just under the top land.

I would appreciate some confirmation from the OP as to whether those are cracks around the pin, the one on the right looks very much like one to me, but maybe it isn't.
Thanks for taking time to post, now I understand PBs earlier 'inverted Vs' - I was taking that to mean the edges of the whole group of damaged 'lands being in a V formation, not just the angle on each individual one so now it makes sense.

Pumaracing said:

The first piston has cracked from long term flexure of the piston skirt. The second from a top propagated pressure loading.

So i'm afraid the obvious question is: What causes the piston skirt to flex so much it eventually breaks?