One of the most common and unfortunately expensive mistakes we see here in the shop is the misalignment of the intake manifold. Typically after the block has been decked and the cylinder heads have been surfaced one or twice, the intake manifold will no longer fit or seal properly.
Think of the letter V and imagine that represents the engine block (less intake manifold) as you would be looking at it from the front of the car. When the block is decked and/or the cylinder heads are cut, effectively the top of the two legs of the V get shortened. Whereas prior to the machine work the intake manifold fit nicely into the open part of the V, now it sits too high (because the top portion of the V is narrower) and the bolt holes don’t line up.
Most guys will typically try to get a couple of intake manifold bolts started and force the manifold into place. The result is that you distort the aluminum intake manifold and basically bend it to fit. The top side of the intake and water ports of the manifold may indeed look sealed. But the problem lies with what you can’t see â€“ the bottom side of the ports are not sealed to the cylinder heads.
You will then experience one or both of the following symptoms. If you’re lucky, the water passages will seal but the intake ports will be sucking oil from the intake valley. That will contaminate the intake charge with oil, foul the plugs, etc. but probably not cause a lot of damage. If you’re unlucky, the water ports won’t seal and then water will make its way into your oil supply. If you’ve ever had an engine with a blown head gasket, you’ve probably seen the oil on the dipstick exhibit a milkshake type of consistency. If left unabated, that will cause a host of catastrophic problems with your engine including wiped out crankshaft journals and bearings as well as camshaft and valve train damage.
The big block Chevy valve covers pictured below are an example of what can happen in an extreme case. When we removed the valve covers it looked like they were coated with tapioca pudding and unfortunately the rest of the engine was nearly as bad. It required a complete rebuild – everything needed to be replaced. Unfortunately, we see this exact situation once every few months in our shop. To give you an idea of the pervasiveness of this problem, we normally have to machine the intake manifold about 80% of the time when we are doing an engine rebuild. So be prepared to have this procedure done if you are having your engine rebuilt in the future and be sure to check the fit before you start torqueing your intake manifold into place.