Tech Stuff

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

When dealing with a two-piece rear main seal, the seal ends must protrude slightly above the parting line in order to obtain proper seal “crush,” allowing the ends of the upper and lower seal halves to mate.
<p>When dealing with a two-piece rear main seal, the seal ends must protrude slightly above the parting line in order to obtain proper seal &ldquo;crush,&rdquo; allowing the ends of the upper and lower seal halves to mate.</p>

Sealing an engine involves several concerns, including the containment of combustion pressure, preventing coolant and oil from entering the combustion process, and preventing oil, coolant and vacuum from leaking out of the engine. Theoretically this should be a straightforward process, but if performed incorrectly without attention to detail, problems can range from annoying leaks to devastating internal engine damage.

Sealing the rear main

Rear main leaks can be one of the most frustrating issues that a tech can face. While a minor leak may be livable (although annoying), a substantial oil leak that passes through the rear main sealing area can easily contaminate a clutch or present a serious-enough external leak to require repair, which translates into separating the transmission from the engine block all the way to requiring engine disassembly in order to access the rear seal. It’s not a comeback that any shop wants to see.

While early generation engines often used a two-piece rear main seal, late model rear seals commonly feature a one-piece design that establishes an oil seal between the crankshaft’s rear flange and the block’s rear engine cover. GM’s LS series of engines is a perfect example of this design. The seal is installed, not onto the block itself, but on the engine’s removable rear cover, much in the same way that an engine’s front cover (often called the timing cover) features a one-piece seal that rides on the crank’s front seal journal.

A one-piece rear main seal, citing the GM LS engine as an example, accepts a temporary plastic guide. This guide prevents the inner seal lips from folding rearward as the seal is engaged onto the crankshaft flange.
<p>A one-piece rear main seal, citing the GM LS engine as an example, accepts a temporary plastic guide. This guide prevents the inner seal lips from folding rearward as the seal is engaged onto the crankshaft flange.</p>

Citing the GM LS engine as an example, the rear main seal is installed to the engine’s rear cover, interference-fit in the same manner as you would install a timing cover seal. When installing the seal to the rear engine cover, make sure that the lips of the seal aim forward. Install a new rear cover gasket to the block (this gasket has a metal core design with an imprinted silicone seal, so no additional sealant is required).

The General Motors LS engine one-piece rear seal is installed to the engine’s rear cover, with the nylon guide in pace.
<p>The General Motors LS engine one-piece rear seal is installed to the engine&rsquo;s rear cover, with the nylon guide in pace.</p>

With the seal installed to the rear cover, the rear cover is then installed to the rear of the engine block, with the seal engaging the crank’s rear flange.

Do not apply any lubricant to either the crank flange or the seal lips. The seal must be installed dry to the crank. If the seal lips are lubed prior to installation, there’s a very good chance that the seal will leak. An installation-aid white nylon guide is inserted onto the seal (this guide is usually included with a new rear seal). With the rear cover held “square” to the block, push the rear cover while carefully centering the rear seal to the crank flange. Once the seal engages to the crank flange, the white nylon guide will pop off. This guide helps to ensure that the seal lips are not pushed rearward. After installing the rear cover, inspect the seal closely to make sure that the seal lips have not been pushed rearward. Install the rear cover bolts finger-tight. The rear cover features two studs or bolts that secure the rear of the oil pan to the bottom of the rear cover. Snug the two lower vertical bolts or stud nuts first, which draws the bottom of the rear cover toward the oil pan. Then torque all rear cover bolts to 18 ft.-lbs.

NOTE: The GM LS crankshaft rear flange may feature flywheel bolt holes that are open to the crankcase. Be sure to apply a thread sealant to the flywheel bolts to avoid a leak that could be caused by oil traveling past the flywheel bolts.

Naturally, follow the specific rear main seal installation procedure for the make, model and year of the engine at hand. I’ve referred to the LS engine merely as one example.

As the rear cover is placed into position and the rear seal begins to engage onto the crank’s rear flange, the nylon guide pops off. Closely inspect the seal to make sure that the inner seal lips have not folded rearward. If they have, remove the cover, reinstall the guide and reinstall the rear cover.
<p>As the rear cover is placed into position and the rear seal begins to engage onto the crank&rsquo;s rear flange, the nylon guide pops off. Closely inspect the seal to make sure that the inner seal lips have not folded rearward. If they have, remove the cover, reinstall the guide and reinstall the rear cover.</p>

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