73-87 Chevy _ GMC Trucks > Engine/Drivetrain

Oil leak


I had my 400 built about 7 years ago.  It leaked from the rear up around the intake manifold.  I chased it so far as to remove the intake and reseal it.  When I reinstalled the intake, it was extremely difficult for the bolt holes to align.  It needs a tiny amount milled for a better fit.  It will bolt on, but it's VERY difficult.

After all that the leak persisted.

I had an issue with something else and took the engine out.  The original builder put it back together.  Didn't mill the intake.  And it still leaks oil. 

I put dye in the oil but because the block is blue and where the leak is, there's not really a way to see a source of the leak.

I had someone else take a look and they seem to think it's from the intake.  Of course you can't really "look" at it because of the location. 

My question is:  Would the poorly fitting intake possibly be causing an oil leak?

The "china wall" has been a constant source of irritation since 1955.  I've never has one leak on me, but I started with a brand-new (aluminum aftermarket) every time.  I've always used Permatex black rtv in place of those squigely rubber strips as well.   Don't discount the little brass extender for the oil pressure sender, I've seen them with a barely visible crack. 

I had the same situation on a 350 4 bolt I had spent a lot of cash on Machined and heads built by a professional race engine building shop (blueprinted) some time ago for a 77 Monte Carlo. Ended up an awesome car and would still have it,, had it not been stolen.

In any case, the back of the aluminum intake manifold did not seat right and was squashing out the seals when torqued it down. I went to the machine shop and they helped by determining the back of the intake needed to be machined (milled) down. I believe they also milled the front as well. It was not fitting properly and would force out the seal and even cause a problem with it not sealing in other areas correctly. After the excess material was removed, it fight great and it was an awesome engine, wish I still had it.

You should always dry fit the intake manifold to check fitment & alignment. I also check my port match by using tape and a template. I use the tape to flip over the template to see where my ports align when I pull the manifold back off. You should pin punch the the China wall area to create peaks and valleys to help with sealing when you lay down the RTV. Check with an inspection mirror, it isn't that hard to see back there.

I have the same issue, thx for the tips!


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