Do I need to put sealant on the intake manifold gasket surfaces?
Also, just wondering, but why are some passages on the heads covered up by said gasket?
Thanks as always
Lots of ways to skin that "kat" (pardon the pun) --- it depends on which gaskets you're using, how often you think you might want to remove that manifold, etc. The main thing is to not have leaks, either in the intake ports, or the front water crossover where it meets the heads and the timing cover. Not all gaskets are created equal. Some have a raised round rubber/silicone seal around the ports and the front water passages in the heads, some don't. I agree with Dimitri in that you probably should use some sealant on the front rubber seal to the timing cover, both sides of the rubber, and make sure you've got the metal surfaces very clean and very flat. Corrosion especially on the intake side there is common and it can play havoc with getting it to seal.
Doesn't hurt to put a THIN bead around all the ports on both sides, just don't go nuts with it. Lately I've been using Permatex Blue Hylomar on everything instead of silicone, and I love the stuff --- it seals really well but it never hardens, so it tends to make parts removable without tearing up the gaskets and I like that ---- but it's kinda pricey.
Secure the gaskets to the heads first, then carefully set the intake on trying not to move the gaskets. Start the bolts but leave them loose. Tighten the front bolt that tugs the manifold up to the timing cover first (which is going to mess up your port alignment unless you've cut the crossover apart from the manifold, but them's the breaks) - otherwise you're likely to get a leak there. Once you've got that tight, then torque down the rest of the intake bolts a little at a time (do them all in increments of 5 or 10 lbs, repeating the sequence until you get them all tight). Start with one of the middle bolts on the head on one side, then do the "diagonally opposite" bolt on the other side, then back and forth side to side in an X pattern, working your way out to the end bolts, until you've done them all.
About the passages: If you'll notice, the intake manifold surface on both heads is identical, even the holes that go into the water jacket. On the "front" end they're open, on the "back" end on one head it'll be plugged but on the other head that hole has the heater hose nipple. None of those open passages and none of the bolt holes open into the water jacket. They all open into the oil drain-back passages, so none of them are subjected to any significant pressure, so it doesn't take much to seal them. I don't know for sure, but I suspect the reason they're left open has to do with the casting process. The factory has to have some way of removing the casting sand after they're made, right? The passage in the center of the head connects to the center two exhaust ports and is the source of heat for the intake manifold heat crossover, in the middle of the manifold. Those two are the ones that folks sometimes seal off to keep the manifold cooler for more power.