Thread: Intake manifold leak

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I purchased a crate engine 383 Chevy with Brodix IK series heads and a no name air gap manifold. It has a bad vaccum and oil leak around intake manifold. The manifold is tight and it appears the manifold is bottomed out and leaking around top edges. It looks like the angle of the manifold does not match the angle of the head. Is there a way to correct this with a gasket or what? Thanks in advance for any help.

Occasionally, you may experience a vacuum leak into the intake ports from the crankcase of the motor due to the intake manifold / cylinder head interface not being machined parallel. Here, I'm talking about looking at the manifold from an end view, from the front or rear of the car for instance. Unless you're using all brand new parts, you have no way of knowing if the heads or manifold have been machined by the previous owner, so the interface at the manifold/head could be out of parallel. In other words, the gasket might be pinched tightly at the top of the port and sealing fine, but might be wide open to the crankcase at the bottom of the port. The manifold may also be warped a little. No amount of propane or carburetor cleaner will find such a leak. The best way is to insure the elimination of such a problem while building the motor.

Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports....Begin with the heads properly torqued to the block for the final time and ready to go. Measure the thickness of your new intake manifold gaskets. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on the washers/shims and stick them on each corner bolt hole on the cylinder heads. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. (Use oil-based modeling clay from a craft store, not Play-Doh). De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port of the manifold, squishing them down well so they stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the heads to keep it from sticking to the heads. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully. Cut half the clay away at each position with your pocket knife. Measure the thickness of the remaining clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing up the motor.

Pay particular attention to interference at the bottom of the manifold at the block rails front and rear. If you cut the manifold, you'll be dropping it down a little into the valley, so PAY ATTENTION. Also, with the manifold down a little in relation to the head ports, the ports may not line up exactly. On a street motor, I wouldn't be overly concerned about it because the laws of fluid flow dictate that the majority of the flow is right down the middle of the port and the surfaces of the port wall will be relatively slow moving. If you want to be dead-nuts on your build though, you should port match to the gasket. Also, using two gaskets on each side may line things up better without any need to grind on the ports.

A crate engine should have matched parts????

You could find some real thick gaskets. I had a 318 that the heads were planed, they milled the manifold surfaces at the wrong angle. I even tried double intake gaskets. Worked for a summer, threw the heads away.

Originally Posted by BigTruckDriver

A crate engine should have matched parts????

It should, but that doesn't mean the intake manifold will match.

Have you checked to see if your heads have been angle milled? Or perhaps the heads are stock and the manifold has been cut for angle milled heads? How about trying to set the manifold in place with no gaskets at all,and just a popsicle stick or tongue depresser stuck in either side by the exhaust heat risers. now look in from the end,do the angles match? Hope this helps,Hank

Great post, Tech. But after having read the post (twice), I would probably re-crate the engine, post it on eBay, then use the proceeds of the sale to buy a new crate engine complete carb to pan.

Hope I never run across this problem, I probably waste half my time looking for the clay.

The heads and intake are brand new. I would suspect that the problem is with the intake since it has no name on it, May purchase new Edelbrock intake. Thanks again.

Originally Posted by NTFDAY

It should, but that doesn't mean the intake manifold will match.

It really depends on who built the engine.
I have never seen any GM engine with port matched heads and intake.
Any more that is an option from most builders.

Originally Posted by erik erikson

It really depends on who built the engine.
I have never seen any GM engine with port matched heads and intake.
Any more that is an option from most builders.

That was an ambiguous statement I made. Just meant with an off brand intake you don't know what you're getting.

Contacted builder and he air freighted a new intake & gaskets + 100.00 bucks for labor. Problem solved! There are a few honest people left in this world.