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Discussion Starter · #1 ·
408 SBC built by me about 5-6 years and 30k miles ago. Has Edelbrock Performer RPM heads that were new when the engine was built. Has been driven fairly hard for most of those miles. Engine is currently in an 80 Malibu that runs mid-high 12's.

It recently started running rough and found that #5 spark plug was oil fouled. Looking to diagnose the reason. Pulled the carb and used my inspection camera to look down the intake into that cylinder. I didn't see any oil in the intake tract as I was looking to make sure it wasn't burning oil due to an intake leak. I did notice that all of the intake valves seem to have the same amount of carbon on them.

Next step was a compression test. All cylinders were between 130-135psi on a cold motor.

I'm thinking the next step should probably be a leak down test?

I'm hoping it might just be valve seals, but I realize it's probably wishful thinking. Hoping it's not guides or a bad ring.

Any ideas?
 

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To go with your suspicion of a broken ring...

What piston was used??

What's the compression ratio??

I've seen that problem with broken ring lands or outer edge on hyper and plain cast pistons, even with the rings still intact. Broken land above and below the second ring....which really isn't a compression ring, it is the final oil prep ring for the oil the top ring rides on and seals with.
Second ring gets unsupported and it turns into an oil pumper in that cylinder.

Sometimes you will find it on a brand new build when someone makes a mistake and gets the top and second ring mixed up, or gets the second ring upside down in the groove....but your run time eliminates those suspects.

Leakdown will not always find that since it is a static test.

Can you get your camera into the spark plug hole, see if you can take a look at the top of the piston all around the perimeter??

Valve doesn't look bad considering the mileage on the build and out-of-the-box Edelbrock heads. Guides may be worn but how much depends on how the geometry was set up and the parts used. Only burning oil in one cylinder leads away from a guide wear issue since you would expect them to wear fairly evenly, not just one random guide.

But I would still pull both intake and exhaust valvesprings springs on that cylinder and see what the seals and guides look like, check for excessive guide wear.
 

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First inspect the socket ends of that cylinders plug wire for looseness or corrosion. If in good shape then pop the valve cover, fire up the motor look to see if valve movement on that cylinder is lost indicating a lobe and or lifter is wiped. This only costs a valve cover gasket before going off the deep end of your wallet.

Bogie
 

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Discussion Starter · #6 ·
To go with your suspicion of a broken ring...

What piston was used??

What's the compression ratio??

I've seen that problem with broken ring lands or outer edge on hyper and plain cast pistons, even with the rings still intact. Broken land above and below the second ring....which really isn't a compression ring, it is the final oil prep ring for the oil the top ring rides on and seals with.
Second ring gets unsupported and it turns into an oil pumper in that cylinder.

Sometimes you will find it on a brand new build when someone makes a mistake and gets the top and second ring mixed up, or gets the second ring upside down in the groove....but your run time eliminates those suspects.

Leakdown will not always find that since it is a static test.

Can you get your camera into the spark plug hole, see if you can take a look at the top of the piston all around the perimeter??

Valve doesn't look bad considering the mileage on the build and out-of-the-box Edelbrock heads. Guides may be worn but how much depends on how the geometry was set up and the parts used. Only burning oil in one cylinder leads away from a guide wear issue since you would expect them to wear fairly evenly, not just one random guide.

But I would still pull both intake and exhaust valvesprings springs on that cylinder and see what the seals and guides look like, check for excessive guide wear.

Pistons are hypers from speed pro, I believe these are it: CHEVROLET Speed Pro H615CP 30 Speed-Pro Hypereutectic Pistons | Summit Racing. Compression is 10.7:1, block was zero decked and torque plate honed. Not sure which rings, by they were speed pro moly. Cam is a Lunati Voodoo flat tappet with ~.230/.240 @ .050 duration.

I could see about half of the cylinder wall and piston with the camera. Everything looked pretty good. Top of piston looked fine, better than I would have expected considering the cylinder is oil fouling a plug. I'll pull the valve springs tonight and see what's going on with the valve seals and guides. I have had the valve seals off this engine several times without replacing them due to changing spring shims. I have new seals I'm going to install since I'm this deep into it just to eliminate that.
 

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In the past I have had an oil ring fail, in my case it was the number 6 cylinder on a 23* small block. I was probably in the 12-13k range on the rebuild. I never came up with a reason for it, but a quick wipe with 600, new set of rings and it went for years after that problem.
 

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Spark plug looks like bad rings. Valve guides usually build up on one side of the electrode.

Nevertheless, check the valve guides - then put new Intake gaskets and valve stem seals on it. If you still have problems with that hole you'll have to fix the bottom end.
 

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When a valve is in service , it soon develops a burr were the keeper presses against the stem , when you pull a valve seal off , that burr damages the seals , if you install seals without the plastic protector sleeve , you damage the seal ..
 

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Discussion Starter · #10 ·
Tonight I replaced the valve seals on #5 that was fouling. Guides seemed tight and didn't see anything wrong with the seals I removed. Had very slight air coming back out of the pushrod holes so I'm thinking air is going past the rings. Seems like I'm going to need a ring job this winter.
 

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Tonight I replaced the valve seals on #5 that was fouling. Guides seemed tight and didn't see anything wrong with the seals I removed. Had very slight air coming back out of the pushrod holes so I'm thinking air is going past the rings. Seems like I'm going to need a ring job this winter.
Or maybe pistons and rings, depending on what you find when you get it apart.

Be careful running it hard with a cylinder using oil....small amounts of oil dilutes the fuel octane quickly and can push that cylinder into detonation and hurt it even worse.....especially with 10.7:1 compression on pump gas and hyper pistons.

Been there with Keith Black hypers, got the T-shirt....
 

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Discussion Starter · #12 ·
Or maybe pistons and rings, depending on what you find when you get it apart.

Be careful running it hard with a cylinder using oil....small amounts of oil dilutes the fuel octane quickly and can push that cylinder into detonation and hurt it even worse.....especially with 10.7:1 compression on pump gas and hyper pistons.

Been there with Keith Black hypers, got the T-shirt....
Thanks for the heads up. I'm hoping that that miraculously the valve seals help. Hoping it's just a re-ring because it's already .040 over and I don't really want to go more.
 

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Is it using any oil between changes? Is it exhausting any black smoke? I'd just replace the seal and the spark plug and drive it a few hundred more miles and check the plug again. Could just be an oil ring that has gone from being butted to overlapping. Do all the other plugs look good?
 

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Discussion Starter · #14 ·
It is using oil. No smoke that I can tell at any time, but I probably should have someone follow me as my exhaust exits to the rear so a small puff would be hard to see in the mirror. All the other plugs look good. I changed the valve seals on 5 and the guides and seals seemed fine. Went for a 30 min ride and the plug is starting to get wet again.
 

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Might try a little hotter plug and see what happens. You could check your distributor cap and plug wire to see if maybe its getting a weak spark. Firing but not real good combustion. Probably not the cause, but at least worth checking. If the hotter plug works, it may get you through to winter and you can remove the head and offending piston for a look see.
 

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Discussion Starter · #16 ·
Might try a little hotter plug and see what happens. You could check your distributor cap and plug wire to see if maybe its getting a weak spark. Firing but not real good combustion. Probably not the cause, but at least worth checking. If the hotter plug works, it may get you through to winter and you can remove the head and offending piston for a look see.
Good suggestions. I tried the hotter spark plug right away hoping to milk it to winter, but when that failed I started diagnosing further. That cylinder seems to have good spark.
 

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Hotter plug may burn itself a little cleaner and last longer, but it is making any ring condition worse at a faster rate, since oil is diluting the fuel charge it significantly lowers the octane and the engine gets more prone to detonation.
The whole thing can quickly turn into a runaway train of disaster. in the blink of an eye.

Been there and done that with a 406 in my younger days. Watched others do it with 350's and 383's.
If oil is coming up into the cylinder from below then that cylinder's hyper piston's days are numbered.
 

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@ericnova72 Going to pull the 408 soon. Thinking about switching to forged pistons when it goes back together, but I'd probably need to get it re-balanced.
Depends on the piston weight....get forged within plus 20 grams or minus 40 grams of the current piston, hone cylinders for fit, and run it no need to rebalance unless this is a constant high rpm race engine.
You'll never feel it and the crank will never know the difference....windage oil is making that much difference in balance every day it's run anyway
 
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