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Old 11-23-2009, 04:49 PM
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Oil on spark plug threads

Hi guys,

Just wondering if you can help. I have checked previous posts and couldn't find a specific answer to this one....

Recently had my 327 sbc rebuilt. I had to do 1000kms to run in, and then took it back to the mechanic to change oil, torque heads etc...Its now done about 2000kms

Car runs ok, doesn't seem to burn oil and there is no oil leaking down from the valve covers, however, every plug has oil on the threads.

They don't seem to have any oil on the tips, just around the thread.

It does blow a little smoke on startup, but when I asked the mechanic he said it was just condensation and was normal.

So the warrenty is to run out in a few months so I am trying to determine whether I need to do anything about it, or just leave it. I am sure if I mention it to the mechanic he will have a ready answer, so I feel I am going to need to go heavily armed with facts.

Is there any reason there should be oil on the plug threads? Its a dark dirty looking oil, whereas the oil on the dipstick is nice and clean.

I do have a compression tester, but will that tell me anything?

I look forward to hearing from you, and thanks in advance.

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Old 11-23-2009, 05:18 PM
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You say it's not using oil, are you sure, have you documented it? There is no way oil can get from inside the engine to the threads of the spark plugs. Perhaps they were oily when installed. Smoke on startup is a sign of valve guide wear, not condensation.

Vince
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Old 11-23-2009, 05:48 PM
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Oil on the spark plug threads is classic leaking valve covers, only it`s leaking so lightly you can`t tell. What`s the only oil source above the spark plugs? the heads, and where`s oil going to go once it`s leaked out? down due to gravity.
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Old 11-23-2009, 05:56 PM
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I noticed the same thing on my motor about a week ago and was worried I put the rings in wrong or something. Did a compression check and everything was good. I asked my machinist that rebuilt the heads and he said it was probably the valve covers leaking.
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Old 11-23-2009, 06:10 PM
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Wow I have the same problem with both of my 350's and both motors are fresh builds and the one was just done this last summer and I have checked my cylinder heads and they are not leaking a drop period. One engine is a bright purple color and the other is a red color. Both have brand new heads with all new seals etc. It could not be the valve covers leaking and just happen to just leak in the perfect spot and get on the plugs like that. If that where the case the outer part would be wet with oil as well as the threads and plug wires. Don't know why mine are like that but I have researched on this matter to not come up with a conclusion as to why. My plugs are clean on the outer part's and no oil foul and I don't burn oil or anything. The only thing I can think of if the pcv if it sucks up too much oil from valve covers then it's getting sent back into the intake to be burnt and that's what is left. My previous old worn out 350 with vortech heads had leaky valve covers but the spark plugs where dry as a bone on all threads and no oily residue.

Eric
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Old 11-23-2009, 06:13 PM
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While it may look like dirty oil, it isn't, this is unburnt fuel that all the lighter elements in the fuel have boiled off leaving the heavier hydrocarbon components behind. This is not unusual to find even on race engines. I wouldn't have known this myself if I hadn't run acrossed the explanation from a very respected pro engine builder on another site. This "fuel" is here because the seal(tapered seat or gasket) is above the threads but the threads themselves are shielded from the actual combustion process. Almost sounds crazy but it is true.
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Old 11-23-2009, 07:02 PM
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Quote:
Originally Posted by 350pano
Hi guys,

Just wondering if you can help. I have checked previous posts and couldn't find a specific answer to this one....

Recently had my 327 sbc rebuilt. I had to do 1000kms to run in, and then took it back to the mechanic to change oil, torque heads etc...Its now done about 2000kms

Car runs ok, doesn't seem to burn oil and there is no oil leaking down from the valve covers, however, every plug has oil on the threads.

They don't seem to have any oil on the tips, just around the thread.

It does blow a little smoke on startup, but when I asked the mechanic he said it was just condensation and was normal.

So the warrenty is to run out in a few months so I am trying to determine whether I need to do anything about it, or just leave it. I am sure if I mention it to the mechanic he will have a ready answer, so I feel I am going to need to go heavily armed with facts.

Is there any reason there should be oil on the plug threads? Its a dark dirty looking oil, whereas the oil on the dipstick is nice and clean.

I do have a compression tester, but will that tell me anything?

I look forward to hearing from you, and thanks in advance.
Keep in mind that the threads are not sealed, that happens above them. It is not uncommon to get combustion by products forced up the threads. This should be a problem, but just to check since you're running to the edge of the warranty, I'd do a cylinder leak down test. When the cylinder is pressurized for the test you can hear leakage past rings and valves as well as measure the pressure retention time, it's a good baseline to perform when monitoring an engine's health.

Hopefully except for residue on the threads the plugs are otherwise clean, that is no ash or wet deposits on the insulator, the insulator may be brown, red, orange, gray, or dingy white. Sooty black or wet are signs of too much fuel or oil getting in there, stark white or speckled white are signs of not enough fuel to air. Or all these from sooty to speckled white can be a sign of the wrong heat range plug with sooty or oily being too cold and fried white too hot.

I don't necessarily agree with your mechanic, smoke on start up is usually a sign that the valve seals aren't or the stem to guide clearance is excessive. On shut down oil creeps down the valve stems, either pooling into the cylinder if the valve is open or pooling on its backside if closed such that a spot of oil is available to burn on start up giving a brief blue puff. However, one has to separate the visual of this from extra start up fuel and a bit of moisture which could be the culprit behind the start up puff.

Bogie
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Old 11-23-2009, 07:07 PM
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IF its NOT smoking.........

and has good power...and doesn't use oil.....don't worry about it!! Jeez!! Replace the valve covr gaskets OR clean with laquer thinner...covers and head area were the gasket should go....RTV them onto the head! If they are hydraulic lifters you won't need to pull the valve covers anyway. Use a good thick bead but NOT so it oozes into the head area. Let it sit over night. End of leak!
6sally6
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Old 11-25-2009, 08:18 PM
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kinda related....

I just had a bad vacuum leak on the intake manifold gasket on one side of the motor. All the ports that were leaking had oil on the spark plug threads on that cylinder.
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Old 11-25-2009, 11:44 PM
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Hi guys thanks very much for all your answers and suggestions. I have had a real good look around the rocker covers, and can't see any leaks, fortunately the engine is real clean and newly painted so I am fairly certain its not that.
I will check to see if there are any vacume leaks, but it runs ok with fairly high vacume. The other suggestion is that it could be unburnt fuel that just looks like oil??! Might be that, not sure how I would tell...
I used the have a 400 chev, and it never seems to have oil around the threads, thats why i thought this might be some hidden problem, lurking, waiting for my warrenty to run out!
Anyway thanks again, I appreciate your thoughts
Rob
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