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Old 06-30-2007, 08:31 PM
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Intake manifold

OK found my problem: newly rebuilt 350 (just installed a week ago), irratic idle. I checked everything and finally found the problem: leaks at the intake manifold gaskets. Question: should I have the shop that rebuilt it remill the intake, or can it be fixed with new, thicker gaskets (Do they make such a thing?) Thanks!

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Old 06-30-2007, 09:30 PM
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felpro

May just be a fluke. It is not hard to damage a gasket. I think felpro makes thicker gaskets but if they don't use a felpro anyway. Make sure the head is clean and use a gasket quenck, it is like rubber cement. Use it on head side not intake. Tighten to 25 lb x pattern middle out.
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Old 06-30-2007, 09:33 PM
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If`s a aluminum intake do not use standard fel pro gaskets, they are made for cast iron intakes and won`t crush, therefore won`t seal. Fel Pro makes a gasket for aluminum intakes, but I usually just use the cheapy mr gasket paper jobs with the glue around the ports, then I use some RVT sealant lightly to be sure.
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Old 07-01-2007, 12:03 AM
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Its an original intake from an 80 Vette (I think its cast.) Could the manifold need to be shaved a bit? Or should I just try a new gasket? With sealant?
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Old 07-01-2007, 02:48 AM
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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. 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.... Measure the thickness of a new intake manifold gasket. 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. 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 and measure the thickness of the 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.
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Old 07-01-2007, 06:13 AM
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If the heads have been resurfaced they should have done the intake so go back to the rebuilder....doc
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Old 07-05-2007, 11:39 PM
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What about having the block '0' decked? This will take off +/- 0.025. How will this effect the intake sealing when using stock dimension heads?
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Old 07-06-2007, 06:40 AM
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intake

surfacing the heads or block will result in the same problem,,if that is the problem and you have had this done go back to the machine shop and have the intake surfaced to correct....doc
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Old 07-06-2007, 08:31 AM
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About 1980 Chevy started using an aluminum version of the standard Q-Jet intake on 305s and 350s, these were notorious for warping. If that's what you have pull it off, clean it and throw a straight edge on it to see of it is warped.
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Old 07-06-2007, 09:16 AM
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Quote:
Originally Posted by techinspector1
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. 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.... Measure the thickness of a new intake manifold gasket. 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. 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 and measure the thickness of the 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.

very informative nice write up
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