Thread: Intake manifold sealing issues

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First post here so be gentle. I did a search over the last couple of days but didn't find a solution.
I just rebuilt my gm 350, new pistons, rings, block bored .040 over, new cam, lifters, heads rebuilt. The only aftermarket item installed is an Edelbrock intake. Engine has about 500 miles on it. My problem is cannot get the manifold to seal which results in oil burning from the drivers side. What I've done so far.
Replaced intake gaskets with 6 different style gaskets from OEM to Fel Pro.
Installed gaskets with sealer / without sealer.
Bought another Edelbrock manifold and installed.
Installed OEM cast iron manifold.
Pulled engine three times checked angles of all manifolds and heads, re ringed and de glazed cylinders. Everything checked out.
Checked block, heads and manifolds for warpage none found
Checked manifolds and heads for cracks none found.
Replaced valve stem seals three times.

Applied a smoke machine and get smoke coming up into the intake. The intake runners are oily even after doing all the above. It only smokes out of the driver's side, I have a dual exhaust with no cross over pipe. Does not smoke when cold just when it warmed up and only at idle. It goes away when throttle is applied. Does not smoke under hard acceleration. I'm not new to engine building I've done over a dozen SBC and have never had one do this.
Someone please tell me there is a solution to this issue before I douse the old truck in gas and burn it.

Sometimes when the block has been decked and heads shaved, the combination of the two changes width between the intake ports and doesn't allow the intake manifold to set down and seal on the underside of the ports. Check the imprint on your gaskets it will show if there is an offset. Your machine shop should be able to cut the intake to match if it's needed.

And Welcome aboard!

Hello baldy, this techinspector1. I used to be a member here, with over 12,000 posts. I don't post on here any more, but just happen to be visiting NTFDAY (Ken) in Springfield, Missouri. Ken asked me if I would field this question and so yeah, I'll give it a try.

I understand that you have had it apart and had it apart and had it apart, but did anyone record any dimensions? If the fellow at the machine shop was one of the people doing the examining, he might have no reason to find anything wrong, because he did the cutting and does not want to anty-up for the fix.

There is no doubt in my mind that there is one or more cylinders where the gap between the intake manifold and the cylinder head is thicker at the bottom than at the top, allowing oily air to be pulled into the head ports from the interior of the block.
Have you examined the intake gaskets which you have removed from the motor? Did you check the thickness of the "pinch" all around each and every port and record the numbers that you found?

Has the block been decked? How much in thousandths of an inch. Do you absolutely know that the block decks are separated at 90 degrees? Could the included angle be 89 degrees or 91 degrees?

Here's what you need to check what you have......
Intake Angle Gauge: BHJ Products, +1 (510)797-6780

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Thanks for the welcome and responses.
Heads and block have never been decked everything bone stock and in spec. The machine shop owner is my friend and did the work as a favor. I assisted with all the machining so no reason for him to be dishonest. All the intake gaskets when pulled off were squashed the sealing ridges were all flat. They appeared to have been squished properly. They were all covered in oil at the bottom.
The block is at 90°, I understand that some intakes need milling to fit correctly that is why I tried three. It's hard for me to believe all three were off. The OEM cast iron manifold was on the engine before the rebuild and didn't have this issue. The intake port angle of the heads are 80°.
I'll check the gasket pinch tomorrow off the last gasket change.

Welcome to CHR, lots of us old farts on here that have ran across almost all the weird little things an internal combustion engine can do! Just wondering what you are using for a breathing system on the engine, are you running a PCV valve or just breathers on the valve covers? Have you tried a leak down check on the cylinders on the left bank? Have you tried setting the intake in position with no gaskets to see what the gap is between it and the heads? Got to be something weird going on there, doesn't really make sense that it would only do it at idle!

Originally Posted by Baldy

Applied a smoke machine and get smoke coming up into the intake..

JMHO, the smoke is telling us the intake gaskets are not sealing at the lower port area..

Welcome, Baldy. I'm gonna look forward to hearing what fixes this one!

Like Tech & Dave said, I'm curious about the clearances between the intake you're using to the heads and front/rear valley machined surfaces. I think I'd set the intake in place without gaskets and see how it sits paying attention to any angled gap at the ports (like Tech mentioned) or gap at the "China walls" of the valley. I'm thinking that you may find that it's sitting on the valley walls, and standing proud from the heads a bit, maybe at a slight angle. Before you lift it off, put in four of the bolts to the head surfaces to see if that changes your gap at the valley ends. Your highest vacuum is at idle, so it's gotta be sucking oil that's running down into the valley area, seems to me.

Please share with us the tools and methods used to determine that the heads are cut at 80 degrees and that the block is cut at 90 degrees. Are you using the stock black rubber gaskets or silicone to seal at the China Walls? Somewhere in this motor is a pathway for the migration of oily air from the crankcase to the cylinders.......we just have to find it......... Be aware that time has elapsed between the removal of the intake gaskets and now, so there is a possibility that the gaskets have relaxed from their compressed thicknesses to what they measure now.

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Originally Posted by Dave Severson

doesn't really make sense that it would only do it at idle!

If it's oiling at idle, then it's oiling at all rpm's. The oil smoke would just be diluted at upper rpms with additional exhaust air, so that it wouldn't be so noticeable.

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Have you tried dry fitting the heads and intake? That is with no gaskets and then measure the clearances between the block heads and intake sealing faces? you may need some measuring wires and feeler gauges.

Woo Hoo I made 3 posts??? In one day?? Can it be real???

Ok finally found the issue and a fix. Took manifold off cleaned everything up then dry fitted the Edelbrock manifold. The manifold rested on the china walls with no gap this was without gaskets. With .060" gaskets the gap was about 1/16", my feeling was this was not right. I purchased .060" gaskets and .120" gaskets. With the thinner gaskets the bolt holes on the heads were at the bottom of the bolt holes on the manifold. The bolts would go in easily and the manifold would bolt down easily. With the thicker gaskets I noticed the bolt holes lined up right in the middle, as they should. I used the thicker gaskets and the china wall gap is now around 3 /16" so I bolted the manifold down. My problem is now fixed no more vacuum leaks or oil burning.
I'm gonna guess that sometime in this engines life the heads, block or both had been milled. I've dealt with milled heads and blocks but in my experience the manifold to head bolt alignment is so off the bolts won't thread into the heads when using the .060" gaskets so a thicker one aligns it up. I just presumed since the manifold bolted on so easily everything was normal. So don't assume anything always confirm, Hope this helps someone else.

Glad it all came together for you. Hope you can now enjoy the ride.

Very good! Thanks for coming back with the great explanation! Glad you got it sorted out.

Hi i once had similar problem pcv was sucking oil out of tappet cover as pcv valve sat above rocker with no baffel in tappet cover.