I have a '77 400M that I'm attempting to put a Offy Dual-Port intake manifold on. I have the Fel-Pro FPP-1240 intake manifold set, but I am not using the cork end seals.
My problem is end seal leaks. The front seems to seal, but the back will leak oil after the engine heats up and its shut down. It seeps out past the silicone (McKanica Max Gray). I have attempted to retorque all the fasteners but to no avail.

I've had this intake on two times now. The first time I stripped it back off, the silicone showed no sign of an impression. Its like the manifold hadn't touched it. The instructions stated to wait 10 minutes before assembly, and I also allowed over 12 hours for it to cure.

The second time I started assembly shortly after applying the silicone, and put two beads down the rail. This only served to make it leak for more areas than the first time. The silicone didn't squeeze out like I'm used to seeing. From the looks of the stuff I can see, it'll show no signs that the manifold touched it.

I'm thinking I either have a warped manifold (on the other hand, why wouldn't it seal with the thick layer of silicone?) or bad silicone.

Any thoughts? Can I use a steel valley pan with an aluminum manifold? What is the recommended way I seal this intake at the end seals? I've done a '77 302 and '65 289 with the silicone ends and Fel-Pro Printoseals and they did just fine.

 

It's possible that the manifold is warped, or that it was machined oddly at some point in the past.

I would pull the manifold off again and save the manifold-to-head gaskets (temporarily). Clean off all the silicone on both ends, then set the manifold back down on the head gaskets and measure the space between the manifold and block at both ends.

After measuring the spaces with the manifold sitting unfastened on the heads, put in the bolts and torque them all to spec, and then measure the end spaces again. Sometimes when you torque down the bolts, the manifold will distort slightly (especially if it has a hidden crack, which would be bad news anyway).

If the space is consistent from front to back and side to side, your measurements will tell you how thick the silicone bead needs to be.

If the space varies from front to back, or from side to side across the back, you may need to have the manifold machined to correct its mating surfaces, and I would also have the manifold pressure tested to make sure it's not cracked.

Before you lay down a bead of silicone, make sure the block and manifold surfaces are immaculately clean. Silicone will not seal to a surface with any trace of oil residue on it. I always clean the parts I'm sealing with naphtha first to remove the gross contaminants, then go over them again with acetone to make sure they are absolutely clean.

Normally, you can use a valley pan gasket with an aluminum manifold, but if the manifold has been machined at some point, you may need the slight extra thickness and compliance of the separate gaskets to get it to seal.

 

I have the same gasket set on my 400, the corks were installed. I had no problem with this until last weekend ( leak free for about a year and a half ). The back cork gasket glue down surface let loose and would slide back and forth between the valley flange and the intake. I bought a Victor steel valley pan gasket from Napa, just in case I needed it (a bit spendy at $26). Pulled the manifold, the Felpro intake to head gaskets looked great so I cleaned the valley rails and the area where the head meets with brake cleaner on a rag several times, this will eat paint and most everything else. BubbaF250’s cleaning method sounds like it would be a lot more pleasant and do at least as good of job, I just happened to have lots of brake cleaner on the shelf. The rubber front and back rail gaskets that came with the Victor pan have the nubs on them to hold them in place and the thickness on them was plenty thick enough to make up the gap so I used them without the pan. Applied a bead of silicon on each end of each gasket and so far it has not leaked a drop of oil.