Front Crankshaft Seal 121850-01800

Function and technical context

The front crankshaft seal is the sealing element responsible for preventing engine oil from leaking through the front end of the crankshaft, where the shaft passes through the gear housing. It works within the lubrication system, maintaining oil pressure inside the block and protecting the distribution area from oil contamination. Its deterioration causes visible lubricant losses in the front area of ​​the engine which, if not addressed, can lead to an insufficient oil level, accelerated wear of internal components and damage to the timing belt or chain if the oil reaches that area. The official Yanmar reference for this seal is 121850-01800, located in section 2 of the exploded view (Gear Housing, callout 6).

When to replace this part?

The most common symptom of a front crankshaft seal in poor condition is the appearance of oil stains or streaks on the front of the engine, sometimes accompanied by the smell of burnt oil if the lubricant contacts hot surfaces. It is recommended to inspect the seal during each seasonal maintenance and replace it whenever work is done on the gear housing or the crankshaft is dismantled; In engines with many hours of use, preventive replacement avoids major breakdowns and unwanted stops in navigation.