Tech Stuff

Engine sealing tips: How to avoid problems from annoying leaks to devastating damage

Cylinder head studs

Although not common on most production engines, you may encounter an engine wherein the cylinder heads are secured with studs and nuts instead of bolts. Studs should never be torqued to the block with any amount of force that is anywhere in the neighborhood of the published clamping force. In the case of cylinder head studs,for example, the studs need to only be finger-tight or with a very small preload of perhaps 5 ft.-lbs. to 10 ft.-lbs. (adhere to the specs in the service manual or the stud maker’s instructions).

Tightening the nuts onto the studs creates the necessary clamping load — the studs only need to be fully engaged into the block deck. Over-tightening the studs into the block will only create a splaying condition (where the installed angle of the studs can easily deviate from 90 degrees), making cylinder head installation difficult and adversely affecting the required clamping load when the nuts are torqued.

One issue to bear in mind deals with cylinder head bolt or stud thread sealing. If the thread holes in the block are blind, there is no need for any type of sealant. Simply lubricate the stud or bolt threads. If any of the threaded holes are open to either oil or coolant, only then is a thread sealant needed.

Note that some LS crankshaft rear flanges feature flywheel bolt holes that have been drilled all the way through the flange. Always apply a thread sealant to the flywheel bolts.
<p>Note that some LS crankshaft rear flanges feature flywheel bolt holes that have been drilled all the way through the flange. Always apply a thread sealant to the flywheel bolts.</p>

As I mentioned earlier, there is no need to apply a bunch of torque to install the studs into the block. Some builders may prefer to “anchor” the studs in place to prevent stud loosening during future engine service. With regard to cylinder head studs, this really isn’t necessary unless you plan to routinely remove and reinstall the heads. But if you do opt to chemically hold the studs in place, avoid the use of an anaerobic compound (a thread locking compound). A thread locking compound may expand as it cures, and if the cylinder walls are relatively thin (possibly due to a recent overborek, for example), and if the thread holes are in close proximity to the cylinder walls, this mechanical expansion can result in excessive pressure, with the potential for cracking of the cylinder wall. If you are bound and determined to “glue” the studs in place, a better alternative is a two-part epoxy such as JB Weld. This will harden without expanding and can easily be broken-down for stud removal by applying torch heat. Another alternative is a high-temperature RTV. Again, there is really no need to try to “anchor” studs to the block via excessive torque or a thread locking compound. Simply obtain full thread engagement, and let the tightening of the nut apply the required clamping force torque.

Rocker bolt holes

In some engine designs, again citing the GM LS series as an example, the intake rocker arm bolt holes in the heads are open to cylinder head intake ports. Because of this, a thread sealer should always be applied to the intake rocker arm bolts in order to avoid potential vacuum leaks. An adequate sealer is Teflon thread sealing paste. Do not use Teflon tape, as this presents the potential for a stringy piece of tape to become dislodged and present a possible contamination issue.

Each engine design has its own quirks, so unless you’re already very familiar with a specific engine, take the time to research and pay attention to the factory service manual for any special notes regarding engine assembly and/or sealing concerns before blindly beginning assembly.

Whenever the cylinder heads have been removed, always inspect the deck of each head and the block decks for straightness using a precision machinist’s straightedge and a feeler gauge. Carefully check the deck surface front-to-rear as shown here.
<p>Whenever the cylinder heads have been removed, always inspect the deck of each head and the block decks for straightness using a precision machinist&rsquo;s straightedge and a feeler gauge. Carefully check the deck surface front-to-rear as shown here.</p>

Whenever you deal with fuel system plumbing that involves a threaded connection, chances are that you’ll be dealing with a straight thread connection that features either a tapered seat or a sealing aluminum or copper “crush” washer. In either case, no additional sealing material will be needed. If the connection features a tapered seat, make sure that both male and female tapered surfaces are clean and free of burrs. If a crush washer is involved, always use a new crush washer.

A note regarding copper or aluminum crush washers: You may encounter an initial installation (tightening the fitting only once) that results in a minor leak. If that’s the case, loosen the fitting and re-tighten. In some instances, the soft-metal crush washer may need to be compressed more than once to achieve a positive seal, to “find a home.”

If a tapered thread is involved (as opposed to straight threads), a sealing material is always required. A good choice is a Teflon paste. DO NOT use Teflon tape. If you’re not extremely careful, excess tape material that is not fully captive inside the threads can break off and enter the fuel stream, potentially clogging small ports in a carburetor (such as the needle and seat) or critically small passages inside fuel injectors. If a sealing material is required, use a minimal amount of Teflon paste.

Also perform a straightness check diagonally between opposing corners. Refer to the service manual for acceptable tolerance range.
<p>Also perform a straightness check diagonally between opposing corners. Refer to the service manual for acceptable tolerance range.</p>

Cylinder head gaskets

Cylinder head gaskets are all too commonly blamed when a leak occurs between the block deck and head. The over-used term “a blown head gasket” is uttered whenever this happens. A head gasket can be viewed somewhat as a “fuse,” where the gasket itself isn’t the root of the problem, but where something has taken place to cause the gasket to fail. The cause of the problem might be a warped cylinder head, improperly surfaced block deck and/or head deck surfaces, improper tightening of the cylinder head fasteners, or excessive overheating of the engine that has caused the head to distort.

Before simply replacing a “bad” head gasket, you need to closely examine the mating decks of both the block and head. These surfaces must be checked for both flatness and for surface finish. Clean the block decks and cylinder head decks to remove all foreign deposits. DO NOT use an abrasive pad on a power tool, as this can easily result in creating waviness on the deck surfaces. It might look nice, but it won’t be flat.


The surface finish of both head and block decks must conform to the factory specifications in terms of Ra (roughness average) surface finish. A finish that is too rough may prevent the head gasket from moving during thermal expansion and contraction, and may present potential leakage paths for both combustion pressure and fluids.
<p>&nbsp;</p><p>The surface finish of both head and block decks must conform to the factory specifications in terms of Ra (roughness average) surface finish. A finish that is too rough may prevent the head gasket from moving during thermal expansion and contraction, and may present potential leakage paths for both combustion pressure and fluids.</p>

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