Well, it wasn't a broken ring...

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Stewart DIBBS - 23 Aug 2006 01:46 GMT

The head off and the pistons are out of the 4G63 in my Summit GTX. No broken
rings surprisingly. One piston is very carboned up, another less so, and the
other two are normal. Obviously the problem had been coming for a while, and
I had noticed the unexpected high oil consumption, esspec under boost.

Measuring with a simple protractor gauge shows none of the bores are
obviously oval. There are expected vertical scuff marks on the all
cylinders, but the cross hatching is still visible. I'll get a bore guage
into it tomorrow and see how much wear is present.

Piston-wall gap is the correct .020". The ring lands on the worst pistons
are not obviously non-parallel, but the #1 and #2 ring vertical clearances
seem not the same. The spec is approx .001 to .003, and .004" max: I'll
check with a feeler gauge tomorrow.

With any luck, the bores are OK and a light hone is only required. Hence the
problem is the old pistons. Given the amount of smoke it was blowing failure
was pretty severe, so ring clearance is more important on the turbo engine.

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Stewart DIBBS
www.pixcl.com/lancerproject.htm

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simpleton - 23 Aug 2006 11:04 GMT

"Piston-wall gap is the correct .020"."

Is this a typo Stewart?

> The head off and the pistons are out of the 4G63 in my Summit GTX. No
> broken rings surprisingly. One piston is very carboned up, another less
[quoted text clipped - 16 lines]
> failure was pretty severe, so ring clearance is more important on the
> turbo engine.

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Stewart DIBBS - 23 Aug 2006 11:48 GMT

> "Piston-wall gap is the correct .020"."
>
> Is this a typo Stewart?

Yes, yes, absolutely a Typo, and relying on memory (always a bad thing)
quoting from the manual. In fact the piston to cylinder clearance spec is
.0012 - .0020. I was visualizing the gap at the top of the piston to the
wall, which IS about 020.

After all, this is a 4G63. In my youth in Australia, we'd say the Holden
(GM) 308 V8s would themselves wear out into 309s ...

Stewart DIBBS
www.pixcl.com/lancerproject.htm

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Stewart DIBBS - 25 Aug 2006 00:40 GMT

The bore seems to be fine. The gauge STILL didn't arrive today, but indirect
measurements (ring gap and skirt clearance) have no indication of excessive
bore wear top or bottom. No head, valve or valve stem problems.

The current thinking (based on experiences with a Renault 5 Turbo) is

- rings were not sealing as well as they could be, allowing excessive oil
blow by. This was seen in the exhaust under boost (at night). Used about
1.5L / month for the last 4 months: irritating but accceptable.

- The excess unburned oil accumulated in the (original) rear muffler

- When the ECU fried itself it went into "limp" mode before it failed
entirely. This caused excess fuel into #2 cyl that washed off the carbon, as
seen. Unburned fuel in the muffer disolves the oil. Engine was running
rough.

- When the new ECU was installed, the engine runs smooth and clean for about
30 seconds, until the oil in the muffler heats up enough to start smoking.

So ... tomorrow we remove the rear muffer and see if there's copious
quantities of oil in it. If so, there's the smoking gun...It needs a
rerouted system anyway for the awd conversion.

I'll get a new set of pistons and rings, give the bores a light hone and put
it all back together.

Does anyone know of non-turbo rings are the same as turbo rings? The ones
we pulled out were supposed to be for a turbo.

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Stewart DIBBS
www.pixcl.com/lancerproject.htm

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