Archimedes-Type Rear Crankshaft Oil Seals

Perhaps the most annoying source of oil leaks in older British sports car engines is the mechanical oil slinger/seal arrangement designed as the primary rear crankshaft seal. The theory of this archimedes-principle seal, is that the spiral grooves at the back of the crank fit with only a slight clearance to the stationary upper and lower oil control surfaces. As the crankshaft rotates, the action of the reverse oil control threads against the flat sealing surfaces draws the excess oil back into the hollow cavity next to the rear bearing where the oil drains harmlessly back to the oil pan.

In reality, this seal arrangement works well as long as minimum clearances are maintained. A slight amount of leakage is inevitable when the engine is shut down. This residual oil drains through a small hole in the bell housing and should not amount to more than a few drops escaping on to your driveway!

Since the oil control threads and the sealing surface are not supposed to wear, few workshop manuals offer detailed instructions on the fitting of these seals nor quote specific clearances. In studying engineering drawings for the T-series crankshaft and rear main upper seal, factory clearances can be calculated to .0053″ minimum to .0088″ maximum. The MGA Workshop Manual specifies a total clearance of.003″ to .006″. Although the T-series spec. seems overly generous, the MGA spec. appears more reasonable and can be considered a guide for all applications.

The sealing surfaces (and, to a lessor extent, the oil control threads) will wear if the rear main bearings become excessively worn or if the block alignment becomes distorted. Leakage problems can also result from inaccurately refitting the replaceable sealing surfaces used on many British sports cars. Jaguar and TR2-4A used replaceable upper and lower seals, Austin-Healey 100-4 and MG TC-TD-TF used only a replaceable upper seal while the MGA and 6 cylinder Healey incorporated sealing surfaces cast integrally with both the block and rear main cap.

In renewing this critical seal, a number of alternative techniques can be employed. Perhaps the most foolproof and effective method is to have your block and the sealing surfaces line-bored by a competent machine shop. By carefully measuring bearing saddle dimensions and/or the diameter of the oil control threads, correct oil seal diameters can be determined. This is a fairly expensive operation but is the only way to restore non-replaceable sealing surfaces. (MGA owners take note, as this is a common problem!)
Replaceable seals can be hand-fitted with care and considerable patience. The crankshaft and main bearings should first be installed and checked for proper clearance using “Plastigauge”, available from any automotive machine shop. Also, check to be certain the crank rotates freely and has proper end float, then remove crank and prepare to trial fit bolt-on sealing plates. On MG TC-TD-TF blocks, remove the two small dowel pins, as these would effectively prevent any adjustment in the location of the seal. Install seal loosely, using appropriate gasket and gasket cement. Apply a thin film of engineer’s bluing (the thicker type sold in squeeze tubes is easier to work with) to the sealing face. With the seal fixing bolts slightly loose it should be possible to snug and center the seal against the crank. After tightening up the seal, torque the lubricated crank assembly to full spec. Now, carefully rotate the crank once or twice before removing the crank once again to inspect the contact pattern on the seal. The ideal situation is to adjust the seal so that you are left with a very thin film of engineer’s bluing on the sealing surfaces. Particular attention should be paid to the upper sealing surface on the block as these are most subject to wear and are consequently most critical.

In some cases, it may be necessary to remove some material from the parting face of one or both seals. This must be done carefully; lay sandpaper on a dead flat surface or pane of glass to help insure accuracy . It may be necessary to remove and refit the crank five to six times to insure that you have achieved a correct fit. A certain degree of light contact is not generally objectionable, particularly with the replaceable aluminum seals, as these will bed-in as soon as the engine is started. Heavy contact that makes the crank difficult to rotate could, however, cause serious problems. The small dowel pins originally used with the T-series seals are not really required and their reinstallation can cause distortion and/or a shift in location of the plate. If these pins are reinstalled, recheck your work once again.
Once the seals have been installed, checked and rechecked, assembly can continue but may require that the crank be removed once again in order to install connecting rods and pistons. (Remember folks, patience is the ultimate virtue!) Last, but not least, particular care should be exercised in installing the oil pan together with all appropriate gaskets and seals. While assembling these components apply silicone gasket cement to clean dry surfaces.

Having hand fitted and determined with all certainty that you have achieved a correct fit, your rear main seal should be nearly 100% drip free. A last word of advice: be sure that your crankcase breathers are clean and free of obstruction and that your gearbox first motion shaft seal is in good order. A problem in either can otherwise mask over a job well done.

Chris Nowlan