New Crankshaft Still Leaking Oil-Fixed

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If you’ve got a new crankshaft installed in your mower and it is still leaking oil,  either from the rear crankshaft seal or front crankshaft seal. 

There’s no cause for alarm, because, in this article, I reveal why your new crankshaft seal might still be leaking oil, possible fixes, as well as how to properly diagnose the source of oil leak; and preventive maintenance tips for optimal performance of your mower’s engine

What Causes a New Crankshaft to Leak Oil?


The most common reason for a new crankshaft to leak oil may include:

  • Gasket problem
  • Damaged seal during the rebuild 
  • Excess oil inside 
  • As a result of a collision 
  • Improper installation of the seal
  • Incorrect Tipping of Lawn Mower

New Crankshaft still Leaking Oil: Causes and Fixes

1.  Gasket Problem


One of the most common causes of oil leaks is gaskets that have gone bad. Gaskets are there to prevent oil leakage anywhere in the engine. 

But because they’re constantly heating up and cooling, they wear out over time. It doesn’t matter if the crankshaft is a new one, if the gasket is faulty, it will cause oil to leak from the crankshaft. 

Also, the sludge that has built up over time can increase pressure on the gasket,  which can cause breakdown and leaks in the gasket seal. 


  • Replace the oil seal if the leak is coming out too quickly.
  • Use an engine oil additive, only if the leak is minimal, because you may lose the additive if the leak is heavy before it gets a chance to do the sealing. But then, this is only a temporary solution till you are able to replace the seal.

Some of the recommended engine oil additives that promise the safety of the engine, with over 90% success rate include:

These additives restore the rubber gaskets to their original size and ability to seal oil

Blue Devil Oil Stop Leak promises positive results after approximately 1-2 hours of operation of your mower.

How to Replace Your Gasket Seal

  • Wipe off all the oil leaks around the lawn mower engine, identify the source of the oil leak
  • Put your mower to work, and run it long enough so the oil could heat up for a thinner texture which allows the oil to move more freely.
  • Drain the oil once the source of the leak has been identified. You can drain your oil through a drain plug (if your mower has it) or tip the mower (with the air filter and carburetor facing up, if possible) to drain from the reservoir directly. Don’t forget to remove the spark plug cover and cable before doing this. 
  • Dismantle the gasket casing to access the seal, and replace it with the new one.

2.  A Seal that is damaged during a rebuild


If in the process of removing and replacing the crankshaft, any parts in the engine gets damaged, there’s a possibility that it may result in an oil leak.


  • Find where the damage has occurred and replace or repair it, whichever is applicable. 

Note: always apply strict carefulness when trying to open up or replace the crankcase to avoid damage to other parts.

3. Excess Oil Inside the Engine

If you over gauge your mower engine with oil, it puts pressure on the engine and forces the engine oil to leak from some parts, such as: the lower crankshaft seal, oil pan gasket, and dipstick seal.


  • Remove the excess oil
  • Be sure to gauge your oil using the dipstick and follow the recommended standard in tripping oil.   

4. A Collision Occurred

A dented oil pan and altered seal may occur as a result of impact from a collision. This can cause oil to leak.


  • Check for dents and altered seals in the engine, then initiate repairs and replace appropriately.

5. Improper installation of the seal

Oil can leak from the crankshaft seal if it is improperly installed.


  • Check the crankshaft seal to ensure it is properly installed, you can always refer to the user manual for guidance.

6. Incorrect Tipping of Lawn Mower

Turning your lawn mower to lay on its side, while the air filter and carburetor are facing down is the wrong practice. 

This is because when the mower is tipped on the wrong side, oil is drawn through the engine and gets the air filter and spark plug soaked. This results in the dripping of excess oil.


  • Always tilt your mower the right way, with the air filter and carburetor facing up

How to Replace Crankshaft Oil Seal: Simple Steps to Follow

  • Drain the engine oil
  • Pull the walk-behind mower spark plug wire (this stops the mower from starting). 
  • Turn the walk-behind mower over so the carburetor side is facing up – this stops oil spills and hard starting issues.
  • Unmount the engine
  • Remove the old seal, with the help of a drill, pliers, and screw.
  • Clean the sealing surface and lube the seal, if necessary.
  • Replace the new seal, using a hammer with a soft face. 
  • Mount back the engine. 
  • Drive the seal with a deep socket. The deep socket is preferable as it drives the seal in a square manner.
  • Fill with fresh oil. Please avoid overfilling which can also cause oil leak.

How to Carry out Proper Engine Diagnosis to Identify Oil Leak Source 

Whenever there’s oil leakage, the crankshaft seal is always the first to be blamed, that is because oil migrates to the south when there’s oil leakage in the engine. 

The wrong diagnosis of the oil leakage source may lead to other serious problems, including spending and replacing engine parts unnecessarily.

Below are simple steps to follow for a better oil leakage diagnosis: 

  • Upturn the mower so that the carburetor side is facing up – this prevents oil from spilling).
  • Use a degreaser or brake cleaner to thoroughly clean the crankshaft seal, as well as the engine pan.
  • Return your mower on its four wheels to get rid of oil traces on the top part of the engine. 
  • Check if topping up oil is necessary, otherwise, don’t bother.  
  • Test run your mower by putting it to work for like 10 minutes. This duration is enough to warm the engine oil and thin it, which is when the leaks usually occur.
  • Return the mower to rest, and allow the engine to cool before upturning to have the carburetor side facing up. 
  • Take a close look at the seal or around it for any sign of oil leakage.
  • If there’s no noticeable oil at the seal, it means the seal has no fault.
  • Check the engine’s upper side if oil is weeping out. 
  • Replacing a new crankshaft is not advised until the source of the oil leak is discovered. 

Crankshaft Seal Preventive  Maintenance Checklist

  • Check and change your oil at a regular interval 
  • Proper gauging of oil (do not overfill the engine oil)


There are several factors that can cause a new crankshaft seal to leak oil, regardless of it being new. 

As stated earlier, the oil leak may be coming from other parts of the engine as a result of:

  • Overfilling the oil
  • Improper seal installation
  • Problem with the gasket, etc.

If you follow the recommended tips in this article, your new crankshaft oil leakage problem should be solved. 

If however, after trying all the tips and your crankshaft seal still leaks oil, quickly contact a certified technician to have a detailed check and possible repairs on your engine.