August 2010 Chevy High Performance Q & A - Performance Q & A

Kevin McClelland Jun 17, 2010 0 Comment(s)
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Vapor Trail
Q: I have been enjoying your Performance Q&A column for a few years now; you do a great job. Now I need your help. My son and I have a '69 Camaro we take to track days. Unfortunately, it has had a nagging coolant leak since we first dropped in the ZZ385 crate engine about a year ago. The first leak was at the back of the intake manifold on the driver side, right where the head, intake, and block come together. We fixed that and soon had another leak at the head gasket, just above the headers. It wasn't long before we noticed vapor coming out of the tailpipe. We will be changing the head gasket soon, PN 12557236, but I am not sure what may be causing this. Could it be the Dexcool-equivalent coolant we are using? Or the lack of a bypass hose between the water pump and intake manifold? The engine only has a few thousand miles on it. Track officials frown on leaking fluids, and any suggestions would be appreciated. Also, do you know the head bolt torque setting we should use on the aluminum heads?
Gene Welch
Daly City, CA

A: First, I've never liked the Vortec bolt pattern for the intake manifold. This, combined with the fact that your Fast Burn heads are dual-bolt pattern (both early and late Vortec manifold patterns), you have very little sealing area between the cylinder head, manifold gasket, and manifold. Finally, the production plastic/silicon sealing ring manifold gaskets have issues leaking on the production L-31 Vortec engines.

The ZZ4 short-block that your Fast Burn 385 is built on has an internal bypass at the front of the passenger-side deck that passes from the cylinder head and into the inlet side of the water pump. This is a very small bypass in relation to the volume that can be pumped. If the engine is run at high rpm before the thermostat has a chance to open, you can build very high pressure in the cooling system. This could compromise the gaskets. For many years on all big- and small-blocks we've been drilling a couple of 3/16-inch holes around the perimeter of the thermostat. This gives you additional bypass volume and gets the hot water up to the thermostat to open quicker.

When you remove your cylinder heads to replace the gaskets, replace the factory-style intake manifold gaskets with Fel-Pro PN 1255. These are composition-type gaskets that will compress and give you a better chance to seal the limited sealing area. When installing these gaskets we prefer to use a contact cement gasket sealer to hold the gaskets in place. Gasgacinch Gasket Sealer has been around forever, and you can't beat it. Paint the cylinder head side of the gasket and the intake flange of the head with sealer, and let it dry. Then use a high-quality silicone sealer like Permatex #598 Ultra Black Hi-Temp RTV around both the head and manifold side of the intake gaskets in the water passage region. It doesn't take much. A very thin layer of sealer around the water ports will do it. Once you have coated both sides of the gasket around the water ports with silicone, place the intake gaskets to the head flange. Be very careful when placing them on the heads. You will want to be in perfect alignment when you set them down. The Gasgacinch will hold them right in place where you place them. Now, use Ultra Black to seal the end seals of the block to manifold with a nice 1/4-inch bead.

Finally, we don't think your issues are due to your coolant. Your cylinder head bolts should be torqued to a value of 65 ft-lb. You'll want to apply a Teflon sealer to the threads of the head bolts to prevent water from migrating around the threads and up around the heads of the bolts. The head bolts come from the supplier coated with a sealer. Clean off the remainder of the sealer with a wire wheel and apply new Loctite PST #565 Thread Sealer. This will best replicate the factory sealer.


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