Sunday, May 01, 2005


Old-Tech Problems With High-Tech Motors

It's nice to know that, even after spending about the equivalent to the sticker price on a Chevy Aveo to build the 396 LT4 engine in my Impala, I'm still at the mercy of 50-year-old sealing and gasket designs. If there's an example of something that money just can't buy, it's a better way to seal the intake manifold on a small-block Chevy.

Whereas the junction between the intake manifold and heads can be sealed relatively effectively with a standard flat gasket, the 1/8" gap between the intake and the block is more troublesome. Previous attempts at sealing this with a rubber gasket usually resulted in the gasket "squeezing out" as the manifold was tightened, or a leak developed at the junction between the block, heads, and manifold. In later years of the SBC, it was sealed with just a bead of RTV from the factory. But RTV isn't well-suited to filling large gaps, and a combination of heat and crankcase pressure (the LT1 can't draw a vacuum on the PCV system at WOT) means that oil leaks at the front and rear of the intake are a matter of "when", not "if". Ideally, the new intake design of the LT1 would have brought "spaghetti"-type press-in-place molded gaskets, or at least some cast-in channels to help lock the bead of form-in-place gasket. But that didn't happen, and one of the few flaws with Ed Cole's brilliant original design remained with the second generation of GM's small V8.

Having gotten quite tired of my car's recent propensity to mark its territory and a oil "consumption" rate of about 1 quart every 1200 miles, I finally pulled the intake manifold. Sure enough - a few bolts came out with oily threads and heads (most of the intake bolt holes are open on the underside to the lifter valley), and the beads of RTV at the front and rear of the manifold showed obvious signs of leakage across 1/2 to 2/3 of their lengths. Whew - finding problems is never good, but it sure beats not finding the problem! The port gaskets showed some wetness, but the intake ports themselves looked clean (and I had forgotten just how big those 200 cc ports are!), and examination of the spark plugs showed only the slightest signs of actual oil consumption (thankfully).

Instead of making another attempt to seal the front and rear of the manifold with standard RTV, I'm going to give Permatex's new Right Stuff sealant a shot, even though it cost me nearly $20 for a 7 oz. "EZ Cheeze"-style pressurized can. Supposedly, it's far more robust than normal RTV, and it cures much more quickly, but I'll still give it a good 24 hours to fully cure before attempting to start the engine. I'd previously ran a series of indents down the block and manifold with a center punch, so I don't need to do that again. To seal the intake bolt threads, I'm going with Permatex High Performance Thread Sealant, which hopefully will not only prevent future oil leaks around the bolts, but will also help prevent the loose intake bolts I've been experiencing with this motor (a well-known problem with LT1s, regardless of head material, but something that I hadn't encountered with the stock iron heads). The intake port gaskets will likely get a smear of Caterpillar's anaerobic sealer, which is a fiercely effective sealers for joints having gaps of less than 0.015" or so.

And while in the sealant aisle at the parts store, I discovered that Permatex now makes a RTV remover, which should make cleaning the gasket surfaces much easier than by using the traditional mechanical method (which is difficult to do on aluminum surfaces without damaging the component). If this stuff actually works, it'll be worthy of a Nobel prize.

I should be able to determine the success of this operation in a few thousand miles, so at my current rate of usage of this car, I should be able to report back in, um, two or three years .

More on gasket sealing here, courtesy of ASA.

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