Wednesday, February 19, 2020

Rear-Main Oil Seal

(Posted on FB 2/18/20)

PROJECT 51: Rear-main oil seal. Just behind the rear main-bearing that supports the crankshaft on all older cars is a two-piece oil seal that keeps the oil in the crankcase and away from the flywheel and the clutch, if so equipped. Two-piece seals were never a great idea, but were necessary because where the seal needed to sit, right next to the bearing, was a far smaller diameter than the adjacent hub where the flywheel was bolted on.
Over the years, with the advent of precisely-molded high-temp rubber compounds, these seals have gotten quite good, but back in the 1930's, when the MG's engine was designed, many manufacturers were using what amounted to a piece of clothesline rope squeezed against the polished bearing journal to keep the oil from seeping out. The concept was usually good for 50K miles or so, after which you had the engine torn down to replace the seals, or you lived with an oil leak. Most owners opted for the latter.
But the designers at MG had a better idea! They apparently reasoned that if there was no rear-main seal, it could never wear out. So, instead, they used a "slinger" disk behind the rear main-bearing to spin the escaping oil off into a collection cavity that drained back into the sump. Then, if any oil that got past that first line of defense it was forced to squeeze through a close-fitting gap to escape farther. And inside that gap was a shallow groove cut in a helix (called a scroll) that would pump that wayward oil back whence it came.
On paper this idea must have looked brilliant. But just in case it wasn't, they also added a 1/4" diameter hole in the bottom of the oil pan to let the leaked oil drain out. (This made it easy for MG owners to know where to put the drip pan on the garage floor, and someone even made a bolt-on drip pan.)
Eventually (probably in the 70's), someone at Moss Motors, one of the two main suppliers of MG spare parts, came up with an aftermarket design to install a one-piece lip-seal around the flywheel flange at the end of the crankshaft. This design actually IS brilliant!
Since the engine was never designed for this add-on, it is often necessary to machine the back surface of the rear main-bearing cap to allow the seal holder to fit, but after that the installation was reasonably straightforward.
I will also replace the two-piece seal at the front of the crank with a one-piece lip-seal, (a much simpler task) and if Moss's claims are correct, I won't need a drip pan! And that's quite a claim for an old British sports car!

MG's no-seal- rear-main oil seal concept.

The just-in-case-this-doesn't-work hole.

Aftermarket bolt-on drip pan. Like a "Depends" for your MG.

Moss Motor's better idea.

Amount that needed to be machined from bearing cap.

Machining the .100" from the cast iron bearing cap.

New seal retainer is sealed against the block with  hi-temp silicone.

New one-piece seal will be installed just before
the flywheel is bolted in place.

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