How to replace lawn mower oil seal

An oil leak can be annoying, but far more annoying is running low on oil and destroying an otherwise perfect mower.

Most people would likely just keep topping up the oil, so I’m genuinely impressed that your taking the time to fit an oil seal.

How to replace a mower oil seal? To fit a lawn mower crankshaft oil seal, begin by draining the engine oil, then:

  • Remove the engine
  • Using a drill, screw and pliers remove the old seal
  • Using a soft faced hammer, fit the new seal
  • Refit the engine
  • Fill with fresh oil

Just before replacing the crankshaft oil seal, it might be worth considering 

Mower engine oil level too full

too much oil

Overfull oil level will cause excessive crankcase pressure.

Common engine oil leaks

Some oil leaks can be tricky to diagnose. As a mechanic over the years I’ve learned not to jump to conclusions, it can lead to unnecessary expense, labor not to mention your pride.

It’s better to run a simple test to confirm your suspicions. Mower engines commonly leak oil from a few different locations, and as gravity causes the oil to run south, the lower crankshaft oil seal usually gets the blame.

The crankshaft has an upper/front crank seal also, but doesn’t usually cause a problems. Lets take a quick look at where the other common oil leaks are and how we can quickly diagnose the source.

  • Dipstick O-ring seal
  • Valve cover gasket
  • Crankcase breather
  • Cylinder head-gasket
  • Oil filter leak
  • Oil pan gasket
  • Oil drain bung
Mower engine dipstick

Seals

A bad dipstick seal or oil drain bung will cause engine oil leaks.

Too much engine oil

By far, the most common cause of lawn mower engine oil leaks is an overfull engine oil level.

When the oil level is too full it causes excessive back pressure inside the engine. The pressure forces the engine oil to escape from areas such as: the dipstick seal, lower crankshaft seal, oil pan gasket and can cause the head gasket to blow on push-rod ohv engines.

So if your engine is leaking oil, first check the oil level – too much oil means you likely found your problem. In most cases simply draining off the excess oil will cause the oil leak to stop.

Diagnosing an engine oil leak

You already know, any engine oil leak will migrate south, which can cause us to blame the crankshaft seal. While we may be right, crank seals cause lots of oil leaks. But it’s worth taking a few minutes to check a fact is a fact.

Here’s how I go about it.

  • Turn the mower over (carburetor side facing up – prevents oil spill). I clean the crankshaft seal and the engine pan thoroughly using a de-greaser or brake cleaner. You’ll likely have oil on the deck underside as the spinning blade distributes a coat, but don’t concern yourself, it won’t do any harm, it helps preserve the metal.
  • Turn the mower onto four wheels again and clean any oil trace from the top side of the engine.
  • I check and top up oil if necessary and simply go cut some grass.
  • After about ten minutes running, the engine oil gets hot which causes it to thin. This is usually when a leak occurs.
  • Now allow the engine to cool before turning it over (carburetor side up) and check for oil leak at or around the seal. No oil at the seal means the seal is good.
  • Now go ahead and check the upper side of the engine for an oil weep. Until you find the source, replacing seals isn’t advised.

More complex mowers may have more than one oil type in close proximity to the engine, such as hydraulic power assisted steering etc. In these cases identifying an oil leak can add a few extra layers of complexity.

The solution – add an oil dye to which ever system you suspect is leaking, clean and inspect after operation as before.

Mower Engine oil leak

Leaks

A little detective work will confirm the source.

Tools

I use a oil seal puller to remove the seal but you don’t necessarily need one. You’ll need to remove the engine from a tractor mower and just the blade and blade Boss from a walk behind mower.

The tools required for this are are a basic selection of wrenches and sockets.

An impact gun would be nice but not necessary.

The tools you’ll need to fit an crankshaft seal include:

  • Engine de-greaser
  • Oil drain
  • Adjustable wrench
  • Drill
  • Wood screw
  • Vice-grips
  • Large socket or soft faced hammer
  • Engine oil
Mower repair tools

Leaks

A little detective work will confirm the source.

Replacing Mower crank seal

Replacing a lawn mower crankshaft seal isn’t that difficult. However, a walk behind mower is easier than a tractor.

The walk behind mower will require the removal of blade, blade boss, plastic shroud and drive pulley (if fitted) from the crankshaft in order to gain access to the oil seal.

The tractor mower will require cutting deck, drive pulley and engine removal.

Crankshafts usually have a buildup of grass and corrosion, so cleaning them thoroughly with a wire brush and lots of wd40 will help when it comes to removing the drive pulley, especially on a walk-behind mowers.

An impact gun does make life easier when it comes to removing walk-behind mower blade bolts and very useful on tractor mower clutch assemblies /PTO’s.

Note of caution, when removing the old seal, you’ll need to be very careful not to damage the sealing surfaces of the crankshaft and oil pan. Using a screwdriver to gouge or pry will damage the metal and the new seal will leak.

Steps as follows:

Lawn mower on its side.

Gain access

Drain the engine oil to begin and pull your walk behind mower spark plug wire (prevents mower starting).

Always turn walk behind mower over with carburetor side facing up - prevents oil spill and starting issues.

Fitting a walk behind mower seal - remove blade and boss.

Fitting a tractor seal - engine out.

Tractor mower engine out.

Gain access

Tractor - engine out. Walk behind mower - remove blade and boss.
Tractor mower oil seal fitting

Take a picture

Note how the seal seats, this will be valuable info to you later when driving home the new seal.
Tractor mower oil seal fitting Tractor mower oil seal fitting

drill

An oil seal puller is the right job, but you might not have so, instead carefully drill into the center of the seal, it helps start the screw.
Tractor mower oil seal removal

Fit screw

A rough thread screw is perfect for removing the seal, use two, one each side if you prefer.
Tractor mower removing crank seal

Vice-grips

What would the world look like without the trusty vice-grips.
Tractor mower seal out

Ease it out

Having a screw either side and two vice-grips will give you greater control when removing.
Mower old oil seal

Gently does it

Remove it with care, rough play will cause marking to engine seal surfaces, and that will cause an oil leak that can't be fixed easily.

Fitting the crankshaft seal

Most seals are fitted using a wipe of engine oil to help fitting and sealing, however some seals need to be fitted dry, so consult with your part supplier first.

The mating surfaces need to be clean and free from dirt, but do not use anything metal to clean the surfaces, likewise do not use sandpaper. If you require an abrasive, use the scouring face of a household sponge.

I use a deep socket to drive the seal in this guide, you may not have one. Instead use a suitable size piece of timber and hammer or better use a rubber hammer to drive the seal. The seal must be driven home square, gently tapping uniformly all the way around the seal will seat it correctly.

Tractor mower seal

clean & lube

With the sealing surfaces clean, lube your seal if applicable.
Mower crank seal fitting

clean & lube

With the sealing surfaces clean, lube your seal if applicable.
Tractor mower seal

Gently

Start the seal by tapping it gently, one side and then the other.
Mower crank seal driving

Drive

I'm using a deep socket to drive the seal. This is the preferred way as it drives the seal squarely.
Mower crank seal driving

home

Use your reference marks or picture you took earlier to note where the seal seats. Over egging the seal will damage it.

That's it, you did it, Nice work!

Now refit the engine and not forgetting to refill the oil.

Related Questions

Why is my lawn mower leaking oil? The most common cause of lawn mower oil leaks is an over filled crankcase. Draining off the excess oil will often fix the leak.

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Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.