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 you’re taking the time to fit an oil seal.
How to replace a mower oil seal? To fit a lawnmower 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
Oil Level – Just before replacing the crankshaft oil seal, it might be worth considering the oil level, is it too full?
Excessive oil will increase crankcase pressure, which causes seals to leak. Often removing the excess oil stops the leak.
A faulty carburetor float needle is a common cause of a persistently high oil level.
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 problems. Let’s 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
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 lawnmower engine oil leaks is too much oil. 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.
Oil level check is a simple check and an easy fix – 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).
- 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.
- 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.
- 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, a leak can add a few extra layers of complexity. The solution – add an oil dye to whichever system you suspect is leaking, clean, and inspect after the operation as before.
Leaks – A little detective work will confirm the source.
I use an 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 basic, selection of wrenches and sockets. An impact gun makes the whole job a ton easier, check out the Amazon link below.Amazon Impact Wrench
The tools and supplies you’ll need to fit a crankshaft seal include:
- Engine de-greaser
- Oil drain
- Adjustable wrench
- Wood screw
- Large socket or soft faced hammer
- Engine oil
An oil seal puller would be nice but not essential.
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 the 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 a 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 walk-behind mowers. An impact gun does make life easier when it comes to removing walk-behind mower blade bolts and is very useful on tractor mower clutch assemblies /PTOs.
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 To Remove Lawnmower Crank Seal:
Gain Access – Drain the engine oil to begin and pull your walk behind the mower spark plug wire (prevents mower starting). Always turn to 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. Need help, check out the “Blade fitting video”.
Tractor – Engine out.
Pic – Note how the seal seats, this will be valuable info to you later when driving home the new seal.
Drill – An oil seal puller is the right job, but you might not have, instead carefully drill into the center of the seal, it helps start the screw.
Fit screw – A rough thread screw is perfect for removing the seal, use two, one on each side if you prefer.
Vice Grips – What would the world look like without the trusty vice-grips.
Ease it out – Having a screw on either side and two vice-grips will give you greater control when removing.
Gently does it – Remove it with care, rough play will mark engine seal surfaces, and that will cause an oil leak that can’t be fixed easily.
Steps To Fit Mower 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, plastic plumbing pipe cut to size makes a useful driver, or use a rubber hammer. The seal must be driven home square, gently tapping uniformly all the way around the seal will seat it correctly.
Clean & lube – With the sealing surfaces clean, lube your seal if applicable.
Gently – Start the seal by tapping it gently, one side and then the other.
Drive – I’m using a deep socket to drive the seal. This is the preferred way as it drives the seal squarely.
Home – Use your reference marks or picture you took earlier to note where the seal seats are. 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.
Why is my lawnmower leaking oil? The most common cause of lawn mower oil leaks is an overfilled crankcase. Draining off the excess oil will often fix the leak.
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.