How to Replace a Generator Crankshaft Seal

What the crankshaft seal does

The crankshaft seal sits where the crankshaft exits the crankcase on the engine side. In a portable generator, the crankshaft is the central shaft that converts the up-and-down motion of the piston into rotation. The alternator that produces electricity is mounted on one end of the shaft, and the flywheel and recoil starter are on the other. The seal keeps engine oil inside the crankcase and keeps dirt, dust and moisture from being pulled into the engine along the shaft.

Over time, the lip of the seal wears, hardens or cracks from heat, age or oil contamination. When it fails, oil seeps or drips past the shaft. A small weep can become a steady leak that drops the oil level and leaves stains under the generator. Because the seal is inexpensive and the labor is the main effort, it is one of the more practical engine repairs on a portable generator.

Signs of a leaking crankshaft seal

Before you assume the crankshaft seal is the problem, check the common leak points first. A loose drain plug, a leaking oil filter or a damaged valve cover gasket can mimic the same symptoms. Wipe the engine clean, run it briefly outdoors, and look for the source of the oil with a flashlight. If the oil is coming from the seam around the crankshaft, the seal is the likely suspect. Related reading: how to change the generator oil filter and how to check the oil level.

• Oil spots or drips beneath the engine side of the generator, especially after it has sat overnight
• Oil mist or a grimy buildup around the crankshaft hub where the shaft enters the crankcase
• The engine oil level drops faster than normal, even with no visible leak elsewhere
• Oil appears on the flywheel or on the alternator side, which suggests the seal is letting oil move along the shaft

Tools and preparation

Work on a clean, level surface with the generator shut off and the spark plug wire disconnected. If the generator has electric start, also disconnect the battery. A workbench or sturdy table is better than the ground because you need to see the sealing area clearly. Keep a catch pan under the engine and have clean rags ready for oil that runs out during disassembly.

Do not run the generator to warm the engine before you start. Warm oil drains faster, but it also makes the job messier and increases the chance of fuel vapor igniting near a hot muffler. Work cold, drain the oil, and take your time.

• Socket or wrench set for the engine side cover
• Seal puller or a flat screwdriver for removing the old seal
• Clean, lint-free cloth for the seal bore
• A new crankshaft seal that matches the original in outer diameter, inner diameter and thickness
• Fresh engine oil of the type and viscosity specified for your engine
• A socket or seal driver for pressing the new seal in evenly

Removing the old seal

Drain the engine oil into the catch pan before you remove the housing, or oil will pour out when you open the crankcase. Then remove the engine side cover or the housing that holds the seal. On many portable generators, the seal is pressed into the crankcase or into a separate cover plate. Look closely at how the seal sits before you remove it, so you can install the new one the same way.

Use a seal puller or a flat screwdriver to pry the old seal out gently. Work around the circumference rather than prying hard in one spot, so you do not bend the metal case. Do not scratch or gouge the bore where the seal seats. Any damage there can let oil bypass the new seal even if the seal itself is perfect. If the bore is scored or pitted, the housing may need to be replaced rather than just the seal.

Installing the new seal

Clean the seal bore with a lint-free cloth and inspect it for burrs or debris. Compare the new seal with the old one to confirm the dimensions match. Apply a light coat of fresh engine oil to the lip of the new seal and to the crankshaft surface where the lip will ride. Do not use grease; engine oil lets the seal slip into place and break in correctly.

Press the seal in evenly and straight. The best tool is a socket or seal driver that is slightly smaller than the outer diameter of the seal, so it presses on the metal body of the seal rather than on the flexible lip. Tap it gently with a hammer until the seal is flush with the housing. Do not drive it in deeper than flush, because the lip needs to ride on the correct part of the crankshaft. A seal installed at an angle can leak immediately or damage the lip.

Reassembly and leak test

Reinstall the engine side cover and any spacers, washers or gaskets in the order you removed them. Torque the fasteners to the manual specification if you have a torque wrench. Refill the engine with the correct amount of oil and check the level on the dipstick. Reconnect the spark plug wire and the battery if you disconnected one.

Run the generator outdoors for a few minutes and inspect the sealing area with the engine running and after it has cooled. Residual oil from the old leak can make a new seal look faulty, so clean the area before the test run. If oil continues to weep, the seal may be seated incorrectly, the bore may be damaged, or the crankcase breather may be blocked, creating pressure that pushes oil past the seal. A blocked breather is a common cause of repeated seal leaks, so it is worth checking. Related: how to replace the crankcase breather. Follow your oil change interval after the repair so the fresh oil stays clean.

Never run the generator indoors or in a garage, basement or shed while testing after the repair. Carbon monoxide from a running generator is odorless and colorless. The U.S. Consumer Product Safety Commission warns that generator exhaust can incapacitate and kill within minutes. Run the generator outdoors only, more than 20 feet from windows, doors and vents, and use a working CO detector in the home. See generator carbon monoxide safety for the full guidance.

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Questions

How do I know if the crankshaft seal is leaking?

Look for oil drips or grime around the crankshaft hub on the engine side, a dropping oil level, or oil stains under the generator. Clean the area and watch it after a short run to confirm the leak is coming from the seal and not from a loose drain plug or filter.

Can I replace the crankshaft seal without removing the engine?

In most portable generators, yes. The seal sits in a bore on the engine side, and you usually only need to remove the side cover or flywheel housing to reach it. You do not need to split the crankcase for a seal that sits in a separate housing.

Do I need a special tool for this job?

A seal puller helps remove the old seal without damaging the bore, and a socket or seal driver sized to the seal helps press the new one in evenly. Basic hand tools are enough for the rest. A torque wrench is useful if you want to match the factory fastening spec.

What oil should I use after replacing the seal?

Use the oil type and viscosity listed in the generator manual. Most small air-cooled engines call for a standard small-engine oil. See generator oil types and viscosity for guidance.

Should I repair the seal or buy a new generator?

If the engine runs well, does not burn oil and holds compression, repairing the seal is worth the effort. If the engine is worn, smokes or consumes oil, or the generator is very old with other problems, compare the repair against a new model. Start with a good portable generator if you decide to replace it.

Can a bad crankshaft seal cause starting problems?

A leaking seal can change crankcase pressure and allow air into the crankcase, which in some cases affects how the engine runs or idles. The most common symptom is oil loss rather than a failure to start. If the generator will not start, check the usual causes first, like spark and fuel.