My Hemi Is Pinching Plugs Closed...Help Needed!

Recently, the ol' 330 has been pinching spark plugs closed. Always on the driver side in cylinder number 4. Detonation? Carbon build-up? The timing has been set the same for the last 2 years with no issues. All of a sudden it wouldn't start, so I moved the timing around and it fired right up. I don't have a timing light, so I just turned it by hand to see if it would catch. I am also wondering if possibly because of my intake set up, and only the center two carbs running, that there is a lot of carbon build up in the center two cylinders. Anyway, I am planning on just pulling the heads tomorrow and all will be revealed. My biggest fear is that pistons might be wasted. Any thoughts, similar stories, or advice would be great... Thanks guys.

 

no idea about Hemi's here...couldnt you put a "shim" in to compensate for the length???

 

Has someone heli coiled that plug hole to deep?

 

No. Its been running fine as it was for at least two years now. This is a recent development.

 

If it is closing the gap on the spark plug it means something is touching the electrode causing it to move. The position of timing normally does not affect it - the other option is that it has slipped its timing and the valves are opening/closing at a different time however Mopar engines are not prone to losing their timing - the timing chains are a stout item and hold up very well under yearrs of use - they may stretch slightly but not slip.
The build up of carbon maybe a reason however unless you look in there no one is going to know

 

detonation can close plugs. it has to be fairly severe to do it. look closely at the intake side of the piston. if it detonates it will show up here first. you also need to check the upper bearing shells to see if you hurt them. measure the width at the center of the shell. if you run a roller cam check all the rollers for smooth action. while you are at it also check the timing chain and get a timing light. 28 degrees pump gas 32 race gas. if its not torn down yet a leak down test is recommended.

 

Could a foreign object have found its way into the cylinder? A Hemi spark plug is a long way from the piston or valves. I don't see how they could hit the plug.

 

remember a early hemi cannot take the timing of a 426. as far as carbon build up it would have to be severe. does your engine come up to temp and do the other plugs look good and have good color. it seems strange to be in only one cylinder. is the idle rpm higher than normal? when you take it apart look closely at the intake gasket you may have a leak there causing that cylinder to go lean. compare all the combustion chambers for equal color to. my hunch here is the piston is coming apart. i have seen this in my own hemi. i thought there was contact piston to plug to. it was not. the piston was coming apart. let us know what you find.

 

weird......

 

Yes it is.

Didn't you rebuild that engine recently? Can you post some photos of the offending sparkplug along with the rest of them? That usually tells you the most about what's going on in the cylinders (in my opinion) I run a 4X2 on a 392 Chrysler (no issues) so I'd like to follow along here.

 

possibly #4 rod bearing is worn,causing the piston to move up further,thus hitting the plug

 

I have had the same probs recently.Ive got an ol FE that had been running great,then all of a sudden,2 plug gaps closed on either side.I pulled the plugs ragapped them & took it for a drive.Went great but only did again.Looked in through the plug hole & you could see something had been hitting the piston.Pulled the heads as I knew that a piston cant hit a plug unless it screwed right down into the combustion chamber.Found nothing in the cylinders,but you could see where it had been smacking the pistons.Pulled the intake & resting on the gasket was a small piece of a spring washer that must have fallen in when I was rebuilding one of my strombergs.
As Boyd has said that If your engine has had bad detonation then maybe your big end bearing is "hammered"? I just dont understand how a small peice of washer can go through the intake valve when it "would be against the flow".Good luck,keep us posted!

 

Maybe try another plug with a extra gasket seal on the bottom to raise the plug higher in the head,just a redneck brain thought....................YG

 

Shouldn't have ANY gasket seals on an installed hemi plug. The plug tube serves as the gasket.

 

Motor was rebuilt and assembled in mid 2009. Probably doesn't even have 10k on it.

 

Hey dude I found this...

DETONATION INDICATORS
The best indication of detonation is the pinging sound that cars, particularly old models, make at low speeds and under load. It is very difficult to hear the sound in well insulated luxury interiors of today's cars. An unmuffled engine running straight pipes or a propeller turning can easily mask the characteristic ping. The point is that you honestly don't know that detonation is going on. In some cases, the engine may smoke but not as a rule. Broken piston ring lands are the most typical result of detonation but are usually not spotted. If the engine has detonated visual signs like broken spark plug porcelains or broken ground electrodes are dead giveaways and call for further examination or engine disassembly.

 

For gawds sake buy/borrow a timing light, what are they $20 to save a couple thousand dollar engine rebuild.

 

Interested to hear what you find.

 

Before tearing it apart, try one of these:

http://www.google.com/products?q=borescope&hl=en

Maybe a mechanic near you has one. All you need to do is stick it down the spark plug hole and you can see what the piston looks like, and if there is any foreign debris in there. You can put it down the intake manifold to check that too.

 

Hmmm... Interesting. Weird wild stuff...

Watching as well. Fingers crossed for something minor!

 

I had the 360 in my Duster close off a couple of plugs when the fuel pump was dying and it leaned out pretty badly at high RPM - probably saved the pistons in those cylinders from being destroyed.

 

I second the bore scope, to have a look into the cylinder. All your plugs are the same length, right? Is there any evidence on the electrode of something hitting it? You might also be able to just look down the plug hole with a bright light and see a mark on the piston where it is contacting the electrode. It's nearly impossible for a valve in a hemi to hit a spark plug. If it's a bearing worn badly enough to let the piston come up that much further in the cylinder, or a wallowed out piston pin hole, you'd be hearing a death knock. A chunk of carbon could do it, but I'd think that after a bit it would get busted up and blown out the exhaust.

If it is as you suspect, from one of your carbs, then it would be going with the flow (along with all the air and fuel), not against it. Or maybe I don't understand what you meant......

 

Unless he has high domed pistons, contact would seem unlikely.

 

I had a used & abused SBC apart that had the broken ring lands as well as the top edge of the piston dome on something like 3 cylinders. Other had broken up rings that fell out of the grooves when the pistons were taken out. The chances of something floating around or gradually breaking apart are good.

Bob

 

Bent or broken wrist pin or maybe one side of the "ears" on the piston that the pin goes thru has broken? Just thinking out loud. Seems like that would make all kinds of unhealthy sounds though.

 

did you change the plug? or just reset it? maybe the electrode is week.

 

Sure is an odd problem, we should get a poll going with the possibilities. You'd figure something that smashes an electrode would be making some racket. I still think the top edge of the piston is coming apart and what doesn't go out the exhaust gets 'et up by the plug.

Bob

 

You can buy a really cheap little inspection light at HF. It is an LED on a flexable stalk. It is small enough to go thru the sparkplug hole. I have done it. Get a timing light too.

 

Butterfly screws

 

Everytime I've seen a plug gap close, something floating around inside the cylinder closed it. I'd just take the head off and check it out...