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Discussion Starter · #1 ·
Rebuild included Block Machine Shop Valve Job & Heads Resurfaced. After assembly it ran pretty good but had an enormous amount of vacuum suction through the Oil Fill. Break in Oil has been changed and upon restart an upper motor knock developed. I have torn it down to remove the new plastic plenum and aluminum lower intake. It appears that the suction was so strong that oil was pulled up enough to fill the valleys of the intake. It looks like the breach was at the gaskets between the Heads and the Aluminum lower intake. Apparently when the intake valve opened it was sucking as much air & oil, from lifter valley as it was from the throttle body. And depositing oil into the top of the Aluminum Intake. It also appears that it was robbing lubrication from the counterbalance, that I can now feel a wiggle when I grab the counterbalance and reproduce the knock sound.

Now for related Parts used, rebuild steps taken or not, and my questions.

Gaskets used
Lower Intake to heads. Fel Pro Metal Gaskets , no Permatex. Torqued to specs. (What looks like the leak)
Intake Ends to Block. Rubber strips with Permatex in the 4 corners only.
I sprayed a flammable around the outside while running and it appeared that no outside air was being pulled in.

Rebuild steps or Corner cut
Shop furnished .30 over cylinders and polished the main crank.
Nothing was done with the counterbalance or its bearings.
Now with the motor back in the vehicle It looks like my counterbalance options are limited.

Questions
Did the head resurfacing change the fit of the intake?
Any tips on best gaskets or use of sealer?
Can I replace, or cut, and remove the counterbalance without pulling the motor?
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The balance shaft isn't much of an issue, and has a little slop in the bearings that it rides on. Many of us remove the gear and let it sit when running a double chain, because the best way to plug its oil passage requires extensive engine disassembly to drill/tap the hole. I'd get the real problem with the oiling/crankcase system fixed before worrying about this.

If they overbored and didn't fit pistons/rings properly, the amount of blowby pressure could be pushing a bunch through the PCV draw into the upper intake.

Diagram someone on another forum annotated:

Green: intake air drawn all the way to ports
Yellow: pressure built up in crankcase under pistons
Red: venting thru head to PCV valve under MAP into upper intake (typically drawing fumes with vacuum)
Blue: fresh air draw thru other head to permit Red pathway without compromising gaskets/seals (passage inside TB, opening between blade and MAF)


You can do a compression check to see if it's making much pressure (I just posted general guide in another thread tonight, if you hit the New button... Nevermind, linked below).

Stinks, and I hope you get to the bottom of it.
 

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Discussion Starter · #3 ·
Thanks for that diagram; as I write this I am probably swayed by wishful thinking, and not looking forward to pulling this motor again. I get the oil being pushed up through the PVC into the plenum but I am still trying to wrap me head around how blow by would create such enormous suction at the oil fill at the same time?? Again, it ran good and had no tailpipe smoke. A compression test will tell me a lot. The tester is on the way. I was the assembler, of the new .30 over pistons, rings. The machine shop was slammed and it was something I wanted to do. This is a Papa / Grandson project and not my daily driver.
A review of possible paths of oil finding its way into the plenum has your diagnosis looking like the most plausible. Ouch!
I gapped every ring per spec, and positioned them with gaps staggered as recommended.
Taking a break from it to avoid getting overwhelmed
I have always been a fan of this era 3800s and had my share of them. This is another Florida rust free solid car. And at this time quite a challenge.
I’ll see what the compression results say when I get back to it in a few days.

Thanks, John C.
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Understood. Take your time, that's the only way I've been able to keep my sanity (and some sort if budget) while dealing with my projects recently.

The Red pathway is thru the front head, so it's part of the vacuum path. A restriction on the Green fresh air draw path would also make an issue. Just take your time, diagnose, and scrutinize each part as you dig thru it all.
 
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