Changing crankshaft bearings: pulling off, reading the bearing code, shrink fitting

Workshop guideUpdated 2026-09-13ELDI / Racing Planet editorial team

A rattle from the crankcase, an engine that draws false air once it is warm, or a crankshaft with noticeable play: then the main bearings are due. The change is the supreme discipline on a two-stroke, but with the right puller, a little heat and the correct bearing clearance it is well within reach.

This guide shows you how to pull the bearing off at the inner race without damaging the shaft, how to decode the bearing designation, why engine bearings need C3 bearing clearance and how to fit the new bearing with 80 to 100 degrees of heat. You renew the oil seals while you are at it.

When the crankshaft bearings are due

Crankshaft bearings are deep groove ball bearings, and in a two-stroke they are lubricated only by the oil in the fuel mixture. They wear from mileage, from corrosion after a long lay-up and from impacts, for instance when the gudgeon pin is driven out with a hammer. The symptoms:

• Metallic rattling or howling from the crankcase that rises with engine speed.
• Radial play: the crankshaft can be moved back and forth noticeably at the flywheel or at the clutch stub.
• A rough feel when turning the engine over with the cylinder removed.
• Leaking oil seals, because the shaft wobbles in the worn bearing. The result: false air, a lean mixture, a white plug reading.

The engine has to be split to change them. The clutch, flywheel, cylinder and gearbox come out first. The tool finder shows which pullers your engine needs for this.

Decoding the bearing designation

Every ball bearing carries a designation, for example 6203 C3 or 6204 2RS C3. The digits and letters follow a fixed system:

Part of the designation	Meaning	Example 6203 C3
First digit	Bearing type, 6 = single row deep groove ball bearing	6
Second digit	Dimension series, sets the width and the outside diameter	2
Last two digits	Bore code, from 04 upwards times 5 = bore in mm; 00 = 10, 01 = 12, 02 = 15, 03 = 17	03 = 17 mm bore
Suffix 2RS, 2Z	Seal (RS) or shield (Z) on both sides	none = open
Suffix C3	Bearing clearance greater than normal	C3

The bore codes 00 to 03 stand for 10, 12, 15 and 17 mm. From 04 upwards you multiply the code by 5. A 6205 therefore has a 25 mm bore. Outside diameter and width depend on the dimension series and are listed in the ball bearing table. If you have a designation in front of you and want the dimensions, use the bearing decoder. It breaks the number down into type, dimensions, sealing and bearing clearance.

If an old bearing carries nothing but a maker's mark, measure the bore, the outside diameter and the width with a vernier caliper. With those three dimensions you will find the standard designation in the table.

Why engine bearings need C3 bearing clearance

Bearing clearance is the play between the balls and the raceways in an unloaded bearing. Normal bearing clearance, with no suffix, is intended for applications in which the inner and outer race reach roughly the same temperature. In a two-stroke engine that is not the case.

The crankshaft takes up heat from the piston and from friction in the conrod. The inner race becomes considerably hotter than the outer race in the crankcase, which is cooled from outside. The inner race expands more, and the play in the bearing shrinks. On top of that, the bearing is fitted with an interference fit onto the shaft and into the case. Both reduce the bearing clearance further.

Under these conditions a bearing with normal clearance runs at operating temperature with no play at all, or even under preload. It gets hot, the lubricating film breaks down and the bearing seizes. A C3 bearing has more play from the outset and runs exactly right when hot. That is why engine makers almost always specify C3 for crankshaft bearings. A slight, perceptible play in a new, cold C3 bearing is normal and not a fault.

Pulling the bearing off: step by step

1. Split the engine

   Once the cylinder, piston, flywheel, clutch and gearbox are out, undo the case bolts crosswise. Separate the case halves with gentle blows of a rubber mallet on the crankshaft stubs, never with a screwdriver in the joint face. Some engines need a separating tool as specified in the workshop manual.

2. Establish where the bearing sits

   When the case is split, the bearing stays either on the shaft or in the case. A bearing in the case is driven out with a suitable drift after the case half has been warmed. For a bearing on the shaft you need the puller.

3. Fit the puller to the inner race

   Set the crankshaft bearing puller so that the jaws reach behind the inner race. An ordinary two-leg puller grips the outer race and transmits the force through the balls. That destroys the bearing and, worse, it can bind the inner race on the shaft and damage the web.

4. Centre the pressure piece

   The spindle of the puller has to sit centrally on the shaft stub. A centring piece or the centre hole in the shaft helps. If you protect the thread on the shaft stub with the old nut, the spindle cannot deform anything.

5. Pull evenly

   Turn the spindle slowly until the bearing is free. If it is stuck, warm the inner race with a hot air gun, not the shaft. No hammer, no jerking.

6. Check the shaft stub

   The bearing seat on the shaft has to be smooth and to size. Scoring, or an inner race that has turned on the shaft, is a job for a specialist. Check the seal running surfaces for wear marks too.

Shrink fitting the new bearing: step by step

A ball bearing sits on the shaft with an interference fit. Fitting it with heat is gentler than pressing it on: the inner race expands, the bearing slides onto the shaft without force and shrinks tight as it cools.

1. Heat the bearing

   Warm the new bearing to 80 to 100 degrees. A hotplate, the oven or an oil bath with a thermometer will do it. No hotter: from about 120 degrees the grease in open bearings suffers and rubber seals are damaged. No naked flame, no blowlamp.

2. Prepare the shaft

   Clean the bearing seat and oil it wafer thin. The shaft stays cold, otherwise it expands as well and the advantage is gone. Pre-chilled in the fridge it goes on more easily still.

3. Slide the bearing on

   Pick the bearing up with gloves or a pair of pliers and push it briskly and squarely onto the shaft up to the stop. The bearing must not tilt. You have a few seconds before it grips.

4. Hold it in place until it cools

   Hold the inner race lightly against the stop with a tube of the right size until the bearing has cooled. Otherwise it can lift fractionally off the web as it shrinks.

5. Warm the case

   For the outer race, warm the case half to a similar temperature. Aluminium expands more than bearing steel, and the shaft with the fitted bearing drops into its seat almost on its own.

Renew the oil seals at the same time

The oil seals sit outboard of the crankshaft bearings and separate the crankcase from the ignition side and the clutch chamber. They hold the vacuum and the pressure that a two-stroke needs in the case to draw in and transfer the charge. A leaking seal draws air on the ignition side and gearbox oil on the clutch side. Either one makes the mixture lean or oily.

Once the engine is open, new seals go in. The size is marked on the seal as inside diameter by outside diameter by width, for example 20x30x7. If in doubt, measure the old seal.

When fitting, make sure the sealing lip points towards the side under pressure, which on the crankcase means inwards towards the crankshaft. Grease the lip lightly and press the seal squarely into its seat with a suitable sleeve. Slide it over threads and keyways on the shaft with tape as protection, otherwise the lip is damaged before the engine has even run.

Reassembly and checks

Before you put the case halves back together, check that the crankshaft turns freely in the bearing and that the conrod moves without catching. Clean the joint face thoroughly and coat it thinly with sealant, or fit the gasket specified. Bring the case halves together squarely and tighten the bolts crosswise in several stages. The tightening torque for the case bolts is given in the workshop manual for your machine.

Then turn the crankshaft at the stub. It has to rotate freely and without binding. Set the axial play with shims as specified in the workshop manual. The rebuild continues with the piston change, described in the guide changing the piston on a two-stroke. The figures for case and cylinder bolts by thread size are in the tightening torque table.