The BMC A-Series engine fitted to the 948cc, 1098cc, and 1275cc Midget was designed with a scroll-type rear main oil seal rather than a modern lip seal. This arrangement relies on a fine machined thread to fling oil back into the sump, and while effective when new, it tends to weep oil as the engine wears, a long-standing and well-known A-Series characteristic. A rear oil seal conversion fits a modern lip seal to cure this persistent leak.
Why the Original Seal Leaks
The scroll seal works only when the crankshaft is turning and the clearances are correct. At rest, and as wear opens up the clearance, oil escapes past the scroll and emerges at the rear of the engine, often dripping onto the clutch housing and the ground. This is the source of the oil leak that affects so many A-Series engines, and no amount of careful assembly of the original design fully eliminates it over time.
The Conversion
The conversion kit replaces the scroll arrangement with a modern rubber lip seal that seals against the crankshaft whether stationary or turning. This provides a positive, lasting seal that cures the rear main leak. The conversion is carried out with the engine out and the crankshaft accessible, so it is most economically done during an engine rebuild or when the clutch is being renewed and the engine is already separated from the gearbox.
Application and Fitment
The conversion is specific to the A-Series engine and to the crankshaft and rear main bearing arrangement fitted, so the engine specification must be confirmed before ordering. Because access requires removal of the engine or at least the separation of engine and gearbox, planning the conversion to coincide with a rebuild or clutch change avoids duplicating a major job.
Ordering Considerations
Confirm the engine is the A-Series and identify the crankshaft type before ordering, as the conversion must match. The 1500cc Triumph engine uses a different rear sealing arrangement and is not covered by this A-Series conversion. Fitting the conversion during a rebuild or clutch job is the sensible approach, as the leak it cures is otherwise a recurring frustration on the A-Series Midget.