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Posted by: ripper911 Feb 10 2016, 08:05 PM

I am here to draw upon the wealth of mechanical knowledge on this forum to help diagnose my brothers chevelle. I don't know much about that type of engine.

I believe the engine is a 400ci that has been destroked to 377ci.
It recently started flattening the spark plug gaps. He doesn't think the rods have stretched because they are supposedly high quality. It doesn't sound like the crankshaft is moving around or anything.

He's a mechanic, neither he, his boss, nor anybody else he's asked has been able to figure it out.

Posted by: messix Feb 10 2016, 08:22 PM

carbon on the pistons contacting the plug ?

Posted by: r_towle Feb 10 2016, 08:26 PM

He may be using the wrong plug or he may have forgotten the spacer to that goes on each plug

Posted by: walterolin Feb 10 2016, 08:26 PM

Wrong plug?

Posted by: ripper911 Feb 10 2016, 08:30 PM

I'll mention those things. It started doing it on one cylinder, but now it's happening on six of the eight.

Posted by: Bruce Hinds Feb 10 2016, 08:32 PM

There long and short reach plugs.

Did he change plug recently??????

Posted by: rick 918-S Feb 10 2016, 08:41 PM

Who built the engine? Are the heads shaved? I agree with the others. If the heads were shaved and he is using plugs with a long electrode and domed pistons that would do it. Leaving the crush ring off could do it. and Carbon built up on the piston would do it. But if he has that much carbon he has other issues and it's time for a teardown anyway. I bought a 2.0 914 engine that I thought had a failed seat. Couldn't rotate a complete revolution. When I pulled it apart I found carbon buildup that was stopping the piston on one cylinder.

Posted by: ripper911 Feb 10 2016, 08:49 PM

He says that he's running the same plugs that have always worked for the engine, but he has noticed some buildup on the plugs.

It was running fine, he pulled the tranny, it set for a few months, put in a different tranny, and it started fouling the plugs.

Posted by: zipedadoo Feb 10 2016, 09:24 PM

Is the transmission an automatic?

On some of the older 3 speed automatics there is a vacuum line connected to the transmission. It is possible to pull transmission fluid into the engine through this line if the modulator fails.

Posted by: ripper911 Feb 10 2016, 09:38 PM

I believe he changed from a 3 speed to a 4 speed, automatic.

Posted by: zipedadoo Feb 10 2016, 09:49 PM

I don't think any of the 4 speeds used a vacuum line....

A lean condition or detonation can damage a spark plug but not usually in the way you are describing.

If he changes the plugs, does it run ok? and for how long before the plugs are damaged again?

Posted by: Steve Pratel Feb 10 2016, 10:37 PM

Plugs are likely too long.

Even if they are the stock plug, a modified setup may require a shorter plug. Things may have been fine for a while but carbon buildup on the piston may raise the height just enough to now make contact. I would measure the threaded portion of the head, then the threaded area of the plug. If the plugs have been in for a while and the plug is too long, even long enough to protrude in the combustion chamber the lower threads will be discolored. That is an easy check.

One other thing to think about, the plugs for most SBC engines DIDDNT come with a crush ring or jacket. Back in the day we would add a copper crush ring to the plug when we were worried about this. I'm sure they are still available.

I'd add a gasket then run some serious cleaner at high concentration, then change oil and see how things go.

Posted by: Tom_T Feb 11 2016, 12:49 AM

I think the above are all good suggestions to look at, but it may be in the build itself where the TDC is higher than original spec & it's just taking some time banging away to flatten them - but it's actually happening from the start, & not all of a sudden now.

So he probably needs to check the deck height of those pistons at TDC, & if they're domed that could be a problem because IIRC the OEs were flat topped & wouldn't have a clearance problem either.

However, I had the Pontiac 400ci with 4 spd. AT in my old 69 Ventura & worked on it back in the day (my car #2 of 5 ... ever) - a great powertrain, but for the life of me I can't understand why anyone would want to destroke it down to 377!!??

Good Luck!
Tom
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Posted by: jmill Feb 11 2016, 07:03 PM

QUOTE(Tom_T @ Feb 11 2016, 12:49 AM)

a great powertrain, but for the life of me I can't understand why anyone would want to destroke it down to 377!!??

I've stroked a 350 to 383 with a 400 crank. No idea why you would put a 350 crank in a 400 for 377? Only thing I can come up with is the 400 crank was bad and he had a good 350 crank around. It would reduce piston speed but the valve train if usually what limits your RPM.

Posted by: Bruce Hinds Feb 11 2016, 07:31 PM

QUOTE(jmill @ Feb 11 2016, 06:03 PM)