Minis are notorious oil leakers due to the number of joints in the engine and gearbox that are exposed to oil. The secret is in the preparation. When assembling an engine or gearbox ensure that the mating surfaces of the joints are flat and smooth with no nicks or scores. Warped casings are a big cause of leaks. Next thoroughly clean all surfaces with solvent, PrepSol is ideal for this. Now apply lots of gasket cement. Coat both of the mating surfaces as well as both sides of the gasket. Non hardening gasket cement is the best one to use because it is easier to get apart and withstands vibration better. Let it all stand for a couple of minutes before assembling until the cement becomes tacky. Finally assemble the joint, being careful not to dislodge or damage the gasket, and tighten up the nuts and bolts evenly.
All gaskets and seals will deteriorate with age and will gradually start to leak. Some are easy to replace but most involve disassembling the engine. The most common seals and gaskets to start leaking include the rod type gear change selector shaft seal (easy to replace), the side cover gaskets (easy to change but you have to remove manifold) and the half moon seal behind the timing chain cover (requires engine and gearbox separation to replace).
One factor that is often overlooked and will contribute to oil leakage is insufficient crankcase ventilation. Without sufficient ventilation, especially with worn engines, crankcase pressures will rise significantly above atmospheric pressure and effectively force the oil out.
Don't be tempted to use these oil additives that claim to stop oil leaks. They don't work for long and neither will your engine.
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