There’s more to sealing an intake manifold than just using the right gasket and using a little RTV. We’ll show you how to check for proper intake seal so you can add this to your engine-builder repertoire.

Words And Photos: Jeff Smith

That new small-block Chevy you just built runs great and it cranks gobs of power. But it has an annoying oil control problem. It doesn’t leak and doesn’t smoke, yet it consumes a quart in 600 miles. The aluminum heads have high quality Viton rubber valve guide seals so you know that’s not the issue. It doesn’t appear to be related to ring seal. After some simple diagnostics, it was time to look at the intake manifold.

Just because the manifold bolts in place doesn’t mean it’s truly sealing. We’ve run across this issue with several engines including small- and big-block Chevys and small-block Fords, but this issue is applicable to any engine where the bottom of the intake ports are exposed to the lifter valley. The problem originates where the bottom of the intake port to the intake manifold creates a diverging angle near the bottom of the port. This reduces the clamp load on the gasket and after multiple heat cycles the gasket eventually allows engine vacuum to pull oil directly into the intake port. Think of this problem as the manifold creating a tiny triangle between the intake manifold and the cylinder head. With the wide part of the triangle at the bottom between the intake manifold and the cylinder head, this will allow engine vacuum to pull oil into the port. While you might think that completely parallel surfaces are what you want, the ideal situation is to have the intake manifold angle converge at the bottom of the intake ports to lightly pinch the gasket to enhance the seal.

Common causes of improper intake manifold seal can originate from blocks or heads that have been milled more than 0.010-inch. Lowering the block deck height or milling the heads changes the position of the heads and may require machine work on the intake to reproduce the stock 80-degree included angle.

It might seem difficult to measure these angles with any type of accuracy but the engineers at Fel-Pro came up with a slick backyard method that will produce reliable data. We learned this little trick from long-time Fel-Pro engineer Greg West. His trick involves placing lead shot pellets inside small holes placed in the intake gasket near the top and bottom of the intake port.

Fel-Pro has unfortunately discontinued this tool but BHJ makes a similar one that verifies the manifold and head sealing angles. We tested our intake by using feeler gauges on both sides to determine the gap. On our intake, the bottom of the manifold face created an 0.008-inch gap per side. We decided to verify this with the pellet test.

Placed on the engine, our Fel-Pro tool noted that our cylinder heads with the gaskets in place were parallel since no light leaked from behind the tool. This reinforced the fact that the manifold needed attention.

To get started, we used a hole punch from Harbor Freight to cut a total of eight 3/8-inch holes in the intake gasket where we wanted to measure gasket crush. Greg recommends using Number 6 lead shot pellets that measure 0.110-inch in diameter. The shot we found was larger at 0.150-inch (Number 2 shot). Instead of searching for the proper shot, we mounted our pellets on adhesive tape to make them easier to handle and flattened them slightly with a hammer to a caliper-verified 0.110-inch. This was a bit of a hassle to flatten eight pellets with a hammer and to make matters worse, we realized after our first test that we were using steel shot – which literally dimpled our heads and intake manifold. So if you are going to use shotgun pellets, make sure they are lead and not steel.

We’ve since learned that lead shot is banned in certain applications and therefore might be more difficult to locate. So in looking for an alternative, we found a roll of plumbing solder that measured 0.125-inch that would serve our purpose and is much softer so it won’t dimple your intake or heads.

We peeled open this shotgun shell to use the pellets. What we didn’t notice until later was the shell was clearly marked “STEEL”. Lead shot is much softer and works far better!

The best way to measure vertical intake taper is to place pellets near the top and bottom of the gasket surrounding the intake port. We tested our manifold using eight balls with two at each of the four corners. Hughes Engines offers an excellent web page outline for this test and suggests using adhesive tape to make sure the pellets don’t fall inside the engine. If one does escape, you have to find it right away before you move on with testing. Lead shot jammed inside the oil pump would not be a good way to start your day.

With the shot or solder in place, torque the intake evenly from the inside out – this will take several passes over the bolts to make sure everything is snug before removing the intake. With it fully torqued, remove the manifold and record the thickness of all eight pellets in their respective positions. This is the only way to calculate the taper. Top to bottom is the critical issue here, but front-to-rear can also be evaluated although usually not an issue.

Even though our test engine was a near-new small-block Chevy crate engine with new Edelbrock heads and an Edelbrock Performer RPM intake, we found an average of 0.010-inch taper top to bottom at all four corners of the intake. This does not mean there was a vacuum leak or an oil leak into the ports, but clearly there was less crush imposed on the intake gasket at the bottom than at the top. We called our friends at Jim Grubbs Motorsports in Valencia, California and owner Jim Grubbs told us they prefer a 0.005-inch reverse taper with the bottom tighter than the top.

Adding that 0.005-inch reverse taper to the manifold’s existing angle meant that JGM had to remove 0.015-inch from the top of the ports relative to the bottom. We took our manifold to JGM and machinist Ryan Peart set it up in the horizontal mill and used our numbers. He removed a total of 0.015-inch on each face to create the desired angle. Before the machine work, we also measured the clearance between the front and rear of the manifold at the china wall to ensure the manifold did not bottom out. This came out to a front and rear clearance of 0.055-inch. Peart suggested removing another 0.010-inch from the end rails to ensure plenty of clearance. A 0.050-inch clearance should be considered a minimum. With the cut, our manifold clearance came out to 0.090-inch.

JGM placed our intake on their horizontal mill and machinist Ryan Peart removed 0.015-inch from the top of the manifold ports at a slight taper to increase the gasket clamp load at the bottom of the port. JGM prefers 0.004- to 0.005-inch tighter clearance at the bottom of the intake ports to create a wedge seal.

Over the years, we’ve heard complaints that Fel-Pro’s Printoseal gaskets seemed “too slippery” to prevent oil from entering the intake ports on street-driven engines. This is despite the Fel-Pro gasket’s additional sealing bead around the intake port. But when we talked with several professional machine shops and engine builders, they preferred these gaskets. An improper intake manifold seal angle would explain why replacing the intake gasket only resulted in a temporary repair of the problem.

After JGM machined our manifold, we re-measured the angles using solder and the results confirmed that the manifold now had an average of 0.003-inch of taper with the tight portion now at the bottom instead of at the top. We can be assured that this manifold will not allow any oil into the intake ports. Now that this issue is solved, we can move on to thinking about how to make another 20 horsepower!

Milling Chart

This chart lists the specs for milling the intake manifold face for every 0.010-inch removed from the cylinder head or cylinder block deck surface. The second column lists the amount of material to remove from the intake manifold end rails for every 0.010-inch removed from the head or block.

Parts List

Sources

BHJ Products
bhjproducts.com

Federal-Mogul (Fel-Pro)
federal-mogul.com

Jim Grubbs Motorsports (JGM)
plus.google.com/jgm

Harbor Freight
harborfreight.com

Hughes Engines
hughesengines.com