Why did this happen?

[gold911]

This happened in stop and go traffic. Why did this happen? Is this an indication of the overall condition of the motor? 150k SC, stock.

[GH85Carrera]

OUCH.

You should warn us before posting horrible pictures like that!

I would guess the damage was done previously and it just let go in traffic.

[Vin-barrett]

detonation

maybe too much timing advance, maybe using too low octane fuel, maybe both?

[Flat6pac]

From the oil control spring on the piston, it looks like it is a Deves rings, not the best replacement ring set.
Bruce

[Grady Clay]

Detonation

Maybe too much timing advance, maybe using too low octane fuel, maybe too hot, maybe too lean, ….
Probably some combination of all the above.

It appears that the top ring broke.
This is a common result of detonation.
The pieces of ring ‘rattled around’ for a while (weeks possibly).
The piston eventually failed.

How do the other pistons appear?
What does the rod bearing halves toward the pistons look like?

Sorry for the major distress.

Best,
Grady

[viejopatron]

Why? You were taking pictures in Stop n Go traffic maybe

sorry - I couldn't resist : )

Keep us posted

[euro911sc]

That really doesn't look like a stock SC piston... Perhaps just the dark photo, but its way too flat compared to mine:




OOOOppps Forgot to warn the feint of heart that the ugly pics were coming out! HAHAHAAAAaaaaaa

This kind of ring landing breaking has shown up a few times in SCs. See my thread here:
And the pieces came a tumblen' down...

-Michael

[euro911sc]

Oh yeah... 1 last great pic:



Most of the lost material was accounted for!

[db_cooper]

If the damage is on one piston only..the it is unlikely it was detonation with too much advance..at least the ignition timing being equal, as all six cylinders would be damaged.

It could of been detonation on one cylinder only with an air leak on the intake..on the offending piston. It would have taken some time and should of been obvious.

A rough running engine..some engine "ping" and then the detonation becomes preignition..and bang..a hole in the piston or busted rings and piston lands.

Yes they look like Deves rings..not the first choice for a 911. Being Deves rings..it could point to a sloppy build..ring end gaps not correct, etc. That would bust the rings and piston lands too.

Your top ring is in the best shape of all three rings. The top ring is the one that would typically break first if the engine is in detonation.

[gold911]

Now that I have looked at all the cylinders, the three cylinders on the left hand side all had broken rings. The right hand side had no broken rings.

[db_cooper]

Which rings were broken? Top, middle or oil?

[gold911]

Top rings. One was in about 10 small pieces. No bang or anything. Ran great with normal power. Just started to smoke alot.

[Steve@Rennsport]

Totally agree with Grady,.......it was caused by detonation. One needs to all the forensics to find the root cause.

One other trigger point besides the ones he mentioned is too much deck height. That leaves too much gas at the perimeters which can spontaneously ignite near TDC.

Remember, most US engines received cast pistons which do not tolerate detonation for long.

[db_cooper]

With the top rings being broken..and all the damage on one side.... detonation.

Check your air box to be sure it is not cracked on one side..inspect it carefully.

[Vin-barrett]

Originally Posted by euro911sc

That really doesn't look like a stock SC piston... Perhaps just the dark photo, but its way too flat compared to mine:

Thats the early 8.5 cr SC piston 78-79, they're totally flat on one side

[juanbenae]

Originally Posted by Vin-barrett

Thats the early 8.5 cr SC piston 78-79, they're totally flat on one side

dont the euro pistons eurosc may be refering to are domed more to get the higher compression ratio found in the euro motors? the spec 911 motors covet those higher compression pistons

[Vin-barrett]

Theres 3 shapes of SC pistons, you know the 9.8's from a cute little circle at the bottom...