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Bearing replacement

by Had Robinson

As many motorcycle and karting sites warn, bearings in aluminum cases must be removed carefully.  Do not use a torch to heat the case.  Only an oven should be used.  Using a press or a hammer and chisel to remove a bearing can ruin the bore that the bearing sits in.  This is because the bearing must be have a tight fit and pressing a hardened steel bearing that is tightly fitted into a soft aluminum bore will enlarge/damage the bore.  This procedure assume intermediate or better mechanical skills.  You must have the experience and proper tools to do this.  Do not attempt this project unless you have the special tools listed here.

Special tools needed

Make sure all tools are ready and in the right place/position.  The case cools quickly and you must be fast.


1. Heat the case

Place the aluminum case in an oven heated to 177ºC (350ºF) with the bearing bore facing down.  Rest the case on a cookie rack so that the bearing will not fall down into the burner/element.  The oven must be exactly this temperature and not hotter.  When things are heated to 205º C (400º F) you enter the danger zone for the metallurgical properties of the aluminum.  Use the infrared temperature gun to monitor the temperature of the case while it is being heated.  The bigger the case, the longer it will take to heat up to the proper temperature.  If the bore surface is 10ºC (20ºF) or more too cool, the bearing will probably not drop out.

2. Monitor the case temperature and remove bearing

When the case surfaces near the bearing reach 177ºC (350ºF), the bearing will likely fall out of the bore and go "clink".  If the bearing is still in the bore when the bore surface reaches the correct temperature, remove the case and quickly use the slide-hammer to remove it.  Practice with the slide hammer installation BEFORE putting the case in the oven.  The slide hammer will put minimum stress on the aluminum bore and case.  Be sure to have the slide hammer ready in case you need to use it.

The bearing in one case half of this engine required the slide hammer to remove.  It only took one light tap of the slide to remove the bearing.  The bearing in the other case half fell out when the correct temperature was reached.

Minari main bearing and seal removal replacement

3. Remove the seal

Before the case completely cools, remove the seal.  Be sure to determine which side of the bore that the seal comes out of before attempting to remove it!  The outer rim of seals is coated, the seals do not fit as tightly as the bearing, and can be pressed out using a drift.  A wrench socket slightly smaller in diameter than the smaller outside opening of the bore works well.  Be sure to examine the old seal carefully for any modifications needed that will allow oil to enter the closed space between the bearing and the seal.  If OEM seals are purchased, they will come with the proper modifications.

The main bearing seal below had been modified AFTER it was installed in the bore by placing the bit in the lubrication hole and cutting the notch visible in the photo here.

Minari main bearing and seal removal or replacement

The seal in this case half was modified BEFORE installation with a Dremel tool.  The lubrication hole was modified by the factory to allow oil to enter the closed space.

Minari main bearing and seal removal or replacement


Minari main bearing and seal removal or replacement


WARNING: The bores for main bearings have lubrication holes. BE CERTAIN THAT THE OIL SEAL DOES NOT COVER THESE HOLES!  Rehearse a way to hold the bearings so that you can align it exactly with the bore.  Fingers work with larger bearings but bearings with small inner bores may require a tapered dowel to hold and align them with the bore so that you do not burn yourself.

1. Freeze the bearing

Place the bearing in a freezer for an hour.

2. Prep the seal and case

For main bearing installation: You may need to make a special cut in the seal's rim so that it will not cover up the lubrication hole.  This can be done before the seal is installed with a Dremel type tool.  It is always better to do this before the seal is installed, if possible.  If the cut was made by the manufacturer or must be done after the seal is installed, you can skip this step.

Clean the casing(s) and especially the bore(s).  It is easy to mix up the interior and exterior of the case.  With a felt tip marker, make a big "E" on the exterior side of the case next to the bore.  This will help you be sure to place the exterior of the seal towards that side.

3. Install the seal

For main bearing and seal installation: Use the press to install the seal home against the rim in the case.  The rim always faces the exterior of the case.  A large socket tool can be used that is just narrower than the inside rim of the seal to press it in.  It is never a good idea to pound the seal in with a hammer as it is so easy to ruin the seal.

If the original seal was modified AFTER the installation, use a bit the size of lubrication hole to cut the seal.  BE EXTREMELY CAREFUL TO CUT THE SEAL'S RIM THE SAME AS THE OLD SEAL.  DO NOT LET THE BIT TOUCH THE INNER PARTS OF THE SEAL.  There will be some wearing away of the lubrication hole near the seal but this is not important.  THOROUGHLY CLEAN THE CASE OF DEBRIS USING COMPRESSED AIR BEFORE CONTINUING WITH THE NEXT STEP.

4. Heat the case

Heat the case in an oven until the area of the bore is 177ºC (350ºF).  Use must use an infrared gun to measure this temperature.  It must be within a few degrees of the correct temperature.

5. Remove case from oven.

6. Insert bearing

MOVE QUICKLY!  Note: Have the slide hammer ready to go in case you make a mistake.  Align the cold bearing with the bore and release it.  It will easily drop into place if the bore is the correct temperature.  Do NOT whack the bearing in with a tool.  This will jam the bearing and cause a disaster.  If the bearing does not nicely drop into the hole, you did not get the case the right temperature.  Repeat the process starting with step #4.


If the case is anything other than in this position (horizontal), it is possible for the bearing to slip out a fraction or fall completely out.  Things are ready when you can easily touch the case with your bare hands.

7.  Test bearing

To test the bearing clearances and determine if the might be any damage to the bearing, use compressed air (fine tip) on the balls of the bearing.  If they and the inner race move freely with air, you are good to go.  Be certain to test the lubrication holes before completely assembling the engine.  It is easy to dribble lubrication oil into the holes.  The oil must quickly disappear down the hole.  If it does not, stop, and disassemble everything to find out what you did wrong.

For more information on how to do this procedure, go to this excellent site by MicroBlue Bearings.  Much of the material posted here was taken from their site.  We thank them for their help.