SBC oil leak puzzle - Chevelle Tech
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post #1 of 32 (permalink) Old Sep 6th, 19, 9:49 AM Thread Starter
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SBC oil leak puzzle

I've got a very challenging (infuriating) oil leak on a fully rebuilt 350. Basic specs: '69 iron block; hot tanked, magnafluxed, bored to +0.060" (last go-round for this block), various other machining operations but both decks were machined and confirmed flat. Edelbrock E-street heads on Cometic 0.036" gaskets. Edelbrock intake manifold on Felpro intake gaskets with ultra-black sealed ends. One brass fitting on rear shelf for oil sender; threads sealed with permatex thread sealant. Distributor gasket is felpro with a light coating of permatex gasket sealer on the block side only.



This engine has 40 minutes of cam break-in time and the oil has been changed once. Break-in went smooth, engine runs beautifully, no evidence of any failures, abnormalities or other issues with engine. It has since seen only several sub 5-minute runs for moving around the shop and loading/unloading on the trailer (engine is part of a chevelle restoration that isn't finished yet).


So I notice a few drops of oil under bellhousing dust cover. Traced to outer rear corners of head gasket between head and block, trickling down various routes along rear of block, starter, etc.; eventually down the outside of the dustcover and dripping onto the floor. I added an ounce of dye to the oil and with the UV light cannot detect any evidence of oil running down to the rear outer corners of the head gasket area from anywhere higher than that location. This is happening on both banks. A small ribbon of oil, from the rearmost corner of the head gasket, extending maybe 2" forward.


So the first thing anyone; including me, would think is that it's definitely the rear corners of the valve cover leaking and running down the back corner of the head. I started the build using the silicone molded over steel core valve cover gaskets under cast aluminum valve covers with the gasket surfaces free from casting defects, flashing etc. (I've been down the leaking valve cover gasket route before). The valve covers are retained with ARP studs, nuts and washers torqued to 3lb-ft. There wasn't any evidence of oil running down the head from the rear corner of the valve cover gaskets, but since "it had to be coming from there", I swapped the gaskets for 7/32" cork gaskets, sealed to the underside of the valve covers with permatex gasket sealer.


Oil leak persisted, still no evidence of oil running down back of the head gasket from the rear end seal of the intake or from the oil sender or distributor. Again, no evidence of oil originating from rear corners of the valve cover, either on intake or exhaust side.


Oh, between each solution attempt, all the places oil had traveled were cleaned thoroughly with rags/solvent.


Another puzzling aspect of this is that no oil is actively observable anywhere while the engine is running; I've tested this a few times. After shut down, I'll check with the UV light and various little mirrors and over hours/day or two, oil will collect at the rear outer corners of the head gasket seam and eventually start to make it's way south. The maximum amount of oil observed on the floor was a quarter sized puddle after about a week sitting perfectly still after a ~5 minute engine run.



So now I'm stumped and could use some of your thoughts. Conventional wisdom suggests that the oil "can't" be leaking from the actual rear corner of the head gasket on this engine and certainly not when the engine isn't running, but I'll be damned if I can find the answer to this.


Oh, the head bolts are still at 65lb-ft torque with ARP bolts and ARP thread lube on the ones that get it and permatex thread sealant on the ones in the water jackets.


Thanks, Ben
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post #2 of 32 (permalink) Old Sep 6th, 19, 9:57 AM Thread Starter
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Re: SBC oil leak puzzle

One more detail; there is no evidence of coolant in the engine oil or oil in the coolant. No bubbling in the radiator while the engine is running or other symptoms of a blown head gasket. Engine starts, runs and sounds healthy. I can compression/leakdown test it but I'm not seeing any signs of head gasket failure.

I don't want to pull the engine out of this car for something like this, but if I'm going to pull it to solve this leak, now is the time.

Thanks!
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post #3 of 32 (permalink) Old Sep 6th, 19, 10:44 AM
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Re: SBC oil leak puzzle

Ben,
I have seen some MLS gaskets that do not go all the way "up" to the edge of the head where the intake gasket meets them. The ones I have seen are for Big Blocks but Small block could be the same. The Big Block ones are almost 1/4" short and you have to put RTV on them when you put the heads on. Also there is a 1/4" pipe plug on the drivers side of the deck right in the area I am talking about. That could leak too but that would only be on one side

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post #4 of 32 (permalink) Old Sep 6th, 19, 10:46 AM
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Re: SBC oil leak puzzle

By the way, the die is not going to show up in 5 minutes of running. You probably need to get it good and hot and run it a half hour or so for it to make its way up there.

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post #5 of 32 (permalink) Old Sep 6th, 19, 11:02 AM
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Re: SBC oil leak puzzle

Ben,
Here is a picture of what I am talking about. I took an old Fel Pro gasket and trimmed it to make an example. The yellow spot is where you have to put RTV on the ones I have seen. This pic also shows the plug I was talking about.
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post #6 of 32 (permalink) Old Sep 6th, 19, 11:23 AM Thread Starter
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Re: SBC oil leak puzzle

I've been very suspicious of the rear edge of the intake manifold, I just can't get any evidence. The head gasket fit to the upper edge of the deck nicely on this engine and when I run a bead of RTV for the rear intake seal I squeeze it into any gap at the top edge of the head gasket. I can run a q-tip, pipe cleaner or paper towel all over the rear of the intake, head, valve cover gasket and into the rear head gasket mating surfaces and they're all bone dry with no neon glow under UV. But the puddle forms at the rear outer corner of both heads every time it runs and cools down.

Here's a pic of the head gasket during assembly, not the best view of the area in question but you can kind of see the fit.
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post #7 of 32 (permalink) Old Sep 6th, 19, 11:30 AM Thread Starter
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Re: SBC oil leak puzzle

I thought maybe the dipstick tube was a suspect, but that's only on the driver side as well and rarely does the oil show evidence that far forward.

I have run the engine maybe twice to full operating temperature, but the leak seems independent of run time. It's like as soon as the oil is distributed after startup, it's getting to wherever it sits to wait for the engine to cool down so it can run out to those rear corners. At this point the possibility that the engine is doing this deliberately to mess with me is not as impossible as it sounds

Thanks for giving it a think

-Ben
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post #8 of 32 (permalink) Old Sep 6th, 19, 11:48 AM
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Re: SBC oil leak puzzle

Do you have any friends that work at a modern auto repair shop ? If so see if they have a smoke machine for checking for emission system leaks. That would find it.

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post #9 of 32 (permalink) Old Sep 6th, 19, 12:01 PM Thread Starter
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Re: SBC oil leak puzzle

Good idea. I'll ask a few guys and see if I can get access to that. Thanks.
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post #10 of 32 (permalink) Old Sep 6th, 19, 12:09 PM
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Re: SBC oil leak puzzle

Take a look at the spot I have the arrow pointing to. It looks like there is not a sealing rib the entire length at that spot ? Dont know what the underside looks like. Just looks suspicious to me.
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post #11 of 32 (permalink) Old Sep 6th, 19, 12:16 PM Thread Starter
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Re: SBC oil leak puzzle

I've gone through all the build photos I have and I don't have a good look at the head gasket features beyond the one posted. One thing I've been thinking about is seeing if a set of similar gaskets exist on the shelf around here to study some of the features that could be an issue. Hard to believe a common Cometic head gasket has a design flaw or production defect, but I'm open to any possibility at this point.

I have an audible leak detector and I thought about connecting a low level compressed air line to the block and trying to find something hissing. May give that a shot before trying to smoke it (If i can get access to that even).
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post #12 of 32 (permalink) Old Sep 6th, 19, 12:26 PM
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Re: SBC oil leak puzzle

Just above the seam that Bill pointed out ,there appears to be a rivet or is that just a hole?
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post #13 of 32 (permalink) Old Sep 6th, 19, 1:09 PM Thread Starter
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Re: SBC oil leak puzzle

They're small rivets if I remember correctly; presumably to hold the gasket layers together prior to installation. When assembled on the engine, they overhang areas of the castings that won't interfere with gasket sealing against the head or block.
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post #14 of 32 (permalink) Old Sep 7th, 19, 7:46 PM
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Re: SBC oil leak puzzle

any way you can find to feed air into the engine will help. I've found that the regulator on my leakdown meter is handy for this type of thing. You can always resort to the old heater hose stethoscope to track it down if it's hissing a little.

I don't understand your dye test deal. When I've used the stuff it took awhile.

for reasonable builds I swear by the regular old Fel-Pro blue striped gasket. I don't see the point of the MLS gaskets when the old faithful works so well.

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post #15 of 32 (permalink) Old Sep 7th, 19, 7:52 PM
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Re: SBC oil leak puzzle

Quote:
Originally Posted by Tom Mobley View Post
I don't see the point of the MLS gaskets when the old faithful works so well.

I agree Anything up to 10.5 to one and the stock head gaskets work fine. Aluminum heads and higher compression take the Fel Pro performance ones. MLS is for Boost or Laughing gas. The traditional stuff has worked fine for 60 years, why mess with it ?
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