intake manifold / cylinder head match up

[briv71]

The heads on my '71 Rivera have been milled and consequently when a stock intake manifold gasket is used, it results in a vacuum leak and the car runs poorly. I have no idea how much has been milled off the heads, (it was done to smooth the heads because of a blown head gasket). Is there an intake manifold gasket available to compensate for this or do I have to have the manifold machined to mate up properly? How do I determine how much to machine off the manifold?

[NTX5467]

Typically, just surfacing the cylinder heads once should not cause any problems as described. IF they were warped that bad, they probably should have also been cracked somewhere, I suspect. Or, in the case of a blown head gasket, if there was a "fire slot" between two cylinders and they surfaced that out, probably figuring with a low compression motor to start with, it would help things, when those good intentions resulted in other issues.

If they were surfaced on a "rotating rock" surfacer, how much is taken off is open for discussion. I know that each pass is supposed to be only a certain amount of metal removed, but I've never seen an operator count the passes of put a level and feeler gauge on the head surface after each pass to keep up with how much has been removed. When using a lathe type of surfacer, each pass is a known amount of metal removed (from the depth setting of the cutting fixture), so it's a little more accurate to keep up with how much as been removed. But just "normal" surfacing should not result in any intake manifold sealing issues . . . unless they were "angle milled", which is a performance-type situation that WILL require intake manifold milling to get things right.

Problem is that unless you know how much metal was removed from the stock-production thickness head surface, you won't know how much to mill from the intake manifold mating surface--as in for, for example, every .010" removed from the cylinder head surface you remove .005" from the intake manifold sealing surface (on the manifold). One of the other Buick websites might have the milling information you desire--or perhaps somebody in here knows that information off the top of their head. KEY THING, once you cut the intake manifold, it MUST stay with that particular set of cylinder heads--period!

As for the intake manifold gasket, with the "bathtub" valley pan intake gasket, which is a one-piece stamped steel item, you are pretty much "locked-in" (a nice way to put it) with what's available. Only a certain amount of mismatch can be tolerated and compressibility of the gasket is limited. Still, normal surfacing should not cause a problem with this setup (Buick was not the only manufacturer to use that type of gasket and unless the heads were really cut upon, things worked ok).

I thought that it might be possible to find some Mr. Gasket intake gaskets (which would be just the intake ports that would be used on top of the valley pan gasket), but I found none in their online catalog. But I know they used to make them for Chrysler B/RB engines with the same type of intake gasket, so I was hoping to find them for the Buick 455. Nothing in Fel-Pro other than the valley pan gasket, either.

To retrace your steps and make sure where you're at . . .

Get a new valley pan gasket set. With the intake manifold off, place the new gasket in the valley and use some shorter bolts (same thread as the intake manifold bolts, but shorter) and some wide flat washers to, after placing the intake manifold pretty much "in place", then lightly snug down the bolts to simulate the intake manifold laying on top of it. What you'll be checking for is intake port mismatch between the cylinder head and the intake gasket. IF the heads have been cut as much as you suspect, you could well find some mismatch (top to bottom, especially) between the gasket and the intake ports. IF everything does match up well, with the simulated manifold snugged down, then things might not be as bad as might be suspected. If there is significant top-to-bottom mismatch, you might contact some of the Buick GS-type engine builders and see what their suggestions might be to fix the situation.

IF there is no significant mismatch, then my desired method of doing the valley pan gasket is to get some high-heat silicone sealer and place a bead in the indented areas of the metal gasket, around the intake port holes. Then, with a gasket scraper, scrape off the excess and use that to put a thin layer (just enough to tell it's there, spreading with your finger--appropriately protected, if necessary--on the rest of the areas between the ports on the gasket). Do the same thing on the top side of the valley pan gasket, too. Let it dry overnight.

Also, carefully inspect the existing gasket to see if the indentations around the intake ports are uniformly compressed, top to bottom, or if either the top or bottom is compressed more than the other. Another indicator of too much being cut from the cylinder heads in the surfacing process. In effect, the ports location relative to the block deck surface has decreased from production-stock. Therefore, the intake manifold is not sitting down far enough between the cylinder heads to fully seal the intake ports. This would also make it hard to get ALL of the intake manifold bolts started (i.e., one side in, the other side will not start into the hole), I suspect.

What this does, in effect, is to make a "print-to-seal" gasket of the basic metal gasket. It adds some extra sealing capabilities to the basic metal gasket as it compresses as the intake manifold is torqued down (in proper sequence, from the inner bolts, circularly, outward to the end bolts). You can also add a thin layer on the end rail areas of the gasket where it seals to the valley area, with a little dab in the corners for good measure--which you can smooth out later when the valley pan in fully installed.

At the time of manifold re-installation, with the end rail areas bolted down, you can place the intake manifold onto the valley pan gasket and make sure it's fully seated and squared-up. At this time, you can visually see if all of the bolt holes are lining up. Put the center bolts in and snug them down . . . not fully tight just yet. Then the next one on each side, first the front one, then the back one. Repeat until all bolts are installed and gently snugged down. Then recheck the "snugged-down" situation, starting in the middle and working outward.

You'll need to find the torque values for the intake manifold bolts in a Buick sevice manual. From the snugged-down torque, with an accurate torque wrench (and related swivels and extensions), work in 5-10lb/ft increments from where you're at until you reach the specified torque level. Again, starting in the center and working outward, circularly. I also hope you're using the stock intake manifold bolts (hopefully with a flange head on them). In some cases, some Permatex #2 should be on the intake bolts to seal them if the intake manifold bolt holes are open on the other end (inside the cylinder head).

Then reinstall the carb (using an accurate OEM-style base gasket) and other related items. When it's all put together, you can fire it off and see what happens. Run the engine for a while so everything can get up to operating temperature and stabilize there. During this time, you can check for other vacuum leaks, too, possibly. Then, shut it off and let it fully cool back down. Recheck for fluid leaks, too, plus rechecking the intake manifold bolt torques.

Hopefully, this will take care of your intake vacuum leaks and you won't need to go farther with the deal. Recheck the intake manifold bolt torques after a few more run cycles and everything stays tight.

Remember that a telling situation will be if there is a significant port/gasket mismatch when you do the intial check (as described above). In a worst case scenario, you might need to get a set of reman cylinder heads from a reputable engine rebuilder (possibly via NAPA or similar?), but you're still getting something of an unknown quantity with them . . . except that if they don't work, you have some recourse to get them to get some that do work (with proper installation procedures). But also remember that if you guess-cut the intake manifold in the hopes to compensate for something that's been done in an unknown quantity, it "ruins" the manifold for ever being used with another set of cylinder heads--your judgment call. Just make sure that if you do cut the manifold, it's done on a lathe rather than a rotating rock surfacer.

Hope this helps . . .

NTX5467

[briv71]

NTX5467,

I really appreciate your concise reply regarding my situation. You have broadened my knowledge base and given me some new angles with which I can confront this issue. I'll let you know what happens.

Thank you very much!

briv71