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I have a 69 Cougar, which the valves needed replacement. The heads were replaced on engine, then 2v manifold and carb. were assembled. The car ran fine for a short time, then ran poorly. It was determined that There was a vacuum leak, which caused the engine to run from 1 plane of the manifold, and would not fire on the other plane ,effecting cylinders 2 ,3, 5 & 8. Since the gaskets looked to be ok, it was assumed that the manifold was cracked internally. (no external cracks visible) No surfaces were machined on heads or intake, and looked flat when a straight-edge was placed across them.
I was able to locate a 351W 4V manifold. When assembling to engine, it matched, and fit, but the 4V manifold used 12 bolts, where the 2V used 16! Now, I still have a massive vacuum leak at edges where intake , gaskets and heads meet. (bolts were all tightened to spec @ 23 ' #s) Any ideas? And why would a 2V manifold use less bolts than a 4V? You would think it might be the other way around! Thanx for any help. DOK
 

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Ford found that 12 bolts were sufficient, so the 16-bolt thing was only for the first year or so.

I seriously doubt that the intake cracked internally. Most likely a carburetor problem. - either a vacuum hose came off or the carb isn't tightened down. Or one of the intake valves is burned. Did you verify the cylinders with the alleged problem by pulling spark plug wires or checking exhaust temp?

You must follow the proper torquing sequence when bolting down the intake manifold - starting at the inner bolts and working to the outermost. Do this several times! As the gasket compresses, the other bolts will need to be re-tightened. Start with one inner bolt on the driver's side, then one on the passenger side, then the other inner on the driver's side and so on. Otherwise, the intake will want to kick out to the other side and cause leaks...

Also, did you use the end seals on the intake gasket? If so, throw them away and use a bead of Permatex Ultra-Blue or Ultra-Black, not regular RTV silicone. Belive me, that Ultra-stuff seals good! You'll probably have to cut it with a razor blade to remove the intake later. Anyway, the reason you must not use the cork- or rubber end seals is that it makes sealing the side intake gaskets that much tougher - very hard to get the proper torque on the bolts since that end seal keeps compressing, thereby reducing the clamping force on the intake bolts and possibly causing a water or vacuum leak later on.
 

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All the factory 351W intake manifolds, including the C9OX and D1JE 4V intakes, had 16 bolts until '77 or so, when Ford switched to using 302-style heads.
The only FoMoCo produced 12-bolt 4V intake for the 351W was used on trucks during the Eighties. Flow wise, they're comparable to the original C9OE 4V, but have integral EGR passages cast in. Unless blocked off, these will create a vacuum leak.
Carb gaskets are also important, especially if using a factory Ford spacer between the carb and intake. There's a protrusion on the back of the carb pad on the '69 and '71 4V intakes that matches the vacuum-port "bump" on the back of the spacer. Universal 4V gaskets don't have enough material to seal up this area, which will cause a leak. Likewise, the universal gaskets typically have two holes for each carb stud. The extra holes will prevent the carb spacer from sealing to the intake.
As far as sealing the intake to the head, with a 12-bolt intake, use '77-up Windsor gaskets rather than the '69 - '76 style. Just make sure the openings for the ports are big enough that they aren't a restriction. The pre- '77 heads have larger intake ports, and some gasket brands have small port openings that are only large enough for the later heads.
Along with the RTV tip from above, that ought to get your intake sealed up.
 
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