Bearing removal, replacement, and case assembly in 2 stroke paramotors

by Had Robinson
updated December 20, 2022

As many motorcycle and kart racing sites warn, bearings in aluminum cases must be removed carefully.  Small cases may be heated with a torch but it is not nearly as precise as heating the case in an ordinary oven.  Using a press or a hammer and chisel to remove a bearing from an aluminum case can ruin the bore that the bearing sits in.  This is because the bearing must have a tight fit and pressing a bearing that is tightly fitted into a soft aluminum bore will enlarge/damage the bore.  This procedure assumes that you have intermediate mechanical skills.  Do not attempt this project unless you have the special tools listed here.

Dan's Motorcycle Repair Web Page has great advice for newbies on this.  You'll love his photo.  If you are not that experienced in tearing an engine apart, has these tips to help prevent serious problems.

Special tools needed for bearing removal and installation

 When things are badly frozen/corroded on shafts, more complex (and expensive) tools are required.

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

Splitting the case

See splitting the crankcase on the "Rebuilding a paramotor" page for instructions.  It requires a special tool.  DO NOT ATTEMPT THIS PROCEDURE UNLESS YOU HAVE THE REQUIRED MECHANICAL SKILLS.  PRYING THE CASE APART WITH A KNIFE, SCREWDRIVER, OR A FLAT BLADE WILL LIKELY RUIN IT.

Sometimes when the case is split, a bearing may remain on a shaft rather than stay in its bore.  The moment you see the bearing start to move out of the bore, *STOP* then follow these instructions.

The bearing often cannot be pulled off the shaft because there is no clearance between it and the flat face of a gear or a crankshaft counterweight face.  This will not allow the jaws of an ordinary a puller to fit under the bearing.  In this case, a crank bearing/gear puller must be used.

Sometimes the bearing is frozen on shaft so tenaciously that it must be sacrificed.  The inner race must be heated with a fine point torch (propane will work but acetylene is much better) while holding the shaft with a wet rag.  It takes about 5 minutes to get things hot enough.  Almost always, the inner race will expand slightly and will either drop off or can be removed with crank bearing/gear puller.  Care must be taken not to damage the crankshaft and the lower rod bearing.

preheat the oven to 190ºC (380ºF).

Bearing removal

1. Heat the case

Some small cases can be heated with a hot air gun to a lower temperature in order to install a bearing.

Place the aluminum case in the oven with the bearing bore facing down.  Make sure there is some flat surface between the heating element/burner and the case, like a cookie sheet or a piece of ceramic flatware.  Rest the case on a cookie rack so that the bearing will not fall down into the burner/element.  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.  I takes about 30-40 minutes for the correct temperature to be reached.  If the bore surface is 10ºC (20ºF) or more too cool, the bearing will probably not drop out.

2. Remove the bearings

When the surface next to the bearing, the bore, reaches 177ºC (350ºF), the bearing will likely fall out of the bore and you will hear it go "clink".  177ºC (350ºF) is the magic temperature.   If the bearing is still in the bore when the bore surface reaches the correct temperature, heat the bore another 10 degrees F.  Then remove the case and let it fall a few inches on a semi-hard surface.  If it still does not drop out, quickly use the appropriate bearing removal tool to remove it but this should be rare.  It is better to heat the case to 188ºC (370ºF) rather than risk injuring the case bore.  Practice installing the bearing removal tool on the bearing before putting the case in the oven.  The slide hammer should just barely be "tapped" so that minimum stress will be put on the aluminum bore and case.  Be sure to have the bearing removal tool ready in case you need to use it.

The bearing in one case half of this engine required the bearing removal tool to remove.  It only took one very 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. Evaluate the bearings

To test the bearing clearances and determine if there might be any damage, use compressed air (fine tip) on the balls of the bearing (video here).  If the balls and the inner race move freely with air, you are good to go.  In the video, the bearing on the left freely rotates but not the bearing on the right, even after it was cleaned.

If the bearing came with grease/heavy oil on it, clean it thoroughly or this test will not work.  Rarely, you can clean the bearing yet it still may not turn freely, as in the above video.  This bearing was defective and cannot be used.  These bearings have tremendous loads on them, get hot, and spin up to or exceed 8,000 RPM.  If the bearing is already binding in some way, it will bind all the more once it heats up and may even freeze, ruining the engine.

Note: This test will not work for sealed bearings e.g. the ones in the clutch on some engines.  These bearings are not subjected to the kinds of temperatures and loads that the main bearings are.  However, I have noticed that the Italian manufacturers have not used quality bearings in some of their assemblies.  If your clutch drags no matter how you adjust it, it is likely that the clutch bearings are worn out or contaminated with dirt (the usual problem).  Disassemble the clutch and examine the bearings to see if they turn freely without any roughness.

4. Remove the seal

The seals are removed after the bearings because it is much easier.  Before the case completely cools, remove the seal.  The seals do not fit as tightly as the bearing and can be pressed out using a drift.  A large socket can be used that is just narrower than the inside rim of the seal to press it in.  For example, the inside dimension of a Minari main seal is about 43mm.  The outside diameter of a 1 1/4" socket is just under 42mm.  The socket will push on the bottom of the seal.  A press should be used to push against the socket and then against the seal.  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.  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.

preheat the oven to 190ºC (380ºF).

Examine the new bearings to see if they have any grease/heavy oil on them.  If they do, you must thoroughly clean them.  Use brake cleaner to remove every trace of oil/grease and compressed air to dry/remove residue.  It is easier to do this before installing them.  DO NOT INSTALL BEARINGS THAT ARE NOT PERFECTLY CLEAN.  This does NOT apply to bearings that are sealed (low speed, low load bearings).

1. Freeze the bearings

Place the bearings in a cold freezer for a few hours.  However, overnight, is best.  Insert the correct sized collet from the bearing removal tool into the bearing and tighten BY HAND.  Having the collet inserted in the bearing helps align it when dropping it into the bore. 

2. Prep the seals 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 become confused which is the interior and exterior of the case.  With a felt tip marker, make an "E" on the exterior side of the case next to the bore.  This will help to make sure that you place the exterior of the seal towards that side.

3. Heat the case

Heat the case in an oven until the area of the bore is 177ºC (350ºF).  Use an infrared gun to measure this temperature.  It should be within 3 degrees C (5F) of the correct temperature.

4. Remove case from the oven

Remember to work QUICKLY.

5. Insert the bearings

If the new seals need to be modified, install them at this time rather than after the bearings are installed.  Use a bit the size of the 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.

The next step is always the most difficult.  Make sure you understand exactly what needs to done, have the right tools, follow the steps below exactly.  Remember that the case halves must be the right temperature.

Remove the bearing from the freezer and install the collet in the bearing (without the rod).  While it does not functionally matter, it is a good idea to have the identification markings on the bearing facing up when placing the bearing in the bore.  Have the rest of bearing removal tool ready to go in case of a mistake.  The collet will help insure that the bearing goes in correctly.  Remove the case half from the oven, align the bearing with the bore, and let the bearing drop in.  It should go in easily.  Make sure the bearing in fully into the bore.  If the bearing gets crooked while inserting it into the bore, the slide must be quickly attached to the collet and the bearing removed ASAP.  It may be necessary to remove the partially inserted bearing per the removal section above.  The aluminum bore cools very quickly and grabs the bearing.

Wait a few minutes and THEN loosen the collet and carefully pull it out.  It is unlikely that the bearing will move in the bore.  Do NOT hit the bearing in with a tool.  This can jam the bearing and ruin the bore.  If the bearing does not nicely drop into the hole, the case is not at the right temperature.  If necessary, repeat the process starting with step #3.  Put the bearing back in the freezer.

Once the bearing is in place, move the case with the bore opening facing up to a safe place where it can cool slowly to room temperature.

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

6. Install the seals

Use the press to install the new seals flush with the rim in the case.  The rim always faces the exterior of the case.  If you use a dead-blow hammer to pound the seal in, make certain the seal goes in square as it is so easy to ruin the seal.  Never use an ordinary hammer to pound on anything on a an engine.

Always put copious amounts of anti-seize or Vaseline on the parts of the seal that contact the shaft, including the shaft.  If the shaft rotates with a dry seal, it will quickly ruin the seal.

7. Assembling the case halves

Be certain to test the lubrication holes before completely assembling the case halves.  Oil must quickly disappear down the holes.  If it does not, stop, and find out why.

Before sliding the crankshaft into the case halves, put petroleum jelly (Vaseline) around the inside surface of each seal.  This will insure that the seals are not "dry".  If they are, they will quickly be destroyed when the engine is first started.

It is uncommon that the shafts will slide right into the bearings on each case half easily.  Make sure any dowel pins are in place.  These pins make sure the case halves are exactly centered.  The most difficult task when re-assembling case halves is NOT to use too much RTV sealant.  ONLY apply the thinnest amount of RTV to one side of the case sealing surface.

Too much RTV sealant will form beads on the inside of the casing joint, will eventually break off and, unfortunately, find their way into the holes in the case which provide oil to lubricate the bearings and seals.  The clogged holes prevent oil from reaching the bearings and, more importantly, the seals.  The net result is that the seals fail and the lives of the bearings are shortened.

Here are photos of a Top 80 crankcase where the pilot overdid the RTV on the halves.  Already, pieces of sealant are loose in the crankcase and will eventually find their way into the small holes that provide lubricating oil for the bearings and seals.

Excessive RTV sealant used when reassemblinb engine case halves

Excessive RTV sealant used when reassemblinb engine case halves

It is difficult to fix this mistake  The cylinder must be removed.  A piece of cloth will have to be jammed into the area just below the lower connecting rod bearing and the crankshaft turned back and forth to loosen and remove the excessive sealant which lies in the joint surrounding the crankshaft.  For other parts of the joint which are exposed, it is easy to remove the excessive sealant.  Compressed air will have to used to blow out and pieces that remain.  Hopefully, you got them all.  This engine was not run so it is very unlikely that any bits of sealant are already in the bearing lubrication holes.  On the other hand, this engine could be put into service and the pilot can just hope for the best.

Make sure you identify which end of the crankshaft goes in which half.  Use a felt tipped marker to write on the shaft.  It is embarrassing to put the crankshaft in backwards.

The drive pulley side of the case half should have the crankshaft inserted into the bearing first.  It helps to put the crankshaft in the freezer for a few hours beforehand.  If the drive pulley case half (with the bearing installed) is heated to the boiling point of water, the case might just slide home on the crankshaft.  Otherwise, gently use a press to push it all the way in.  In this step, we are not pressing the bearing into its bore but pressing the crankshaft into the bearing.  Once the crankshaft is in place, put the whole thing back in the freezer so it is ready for the next step.

Make sure all of the bolts that hold the case together have been cleaned and are handy.  Apply RTV to the other half.  Use a popsicle stick with the rounded end cut off to apply a very thin coating of RTV on the case half.

Slide the other case half onto the flywheel side of the crankshaft. The bolts must be tightened evenly as you pull the halves together.  It requires some skill to pull the shaft fully through the bearing.  If it's not far enough, the bearings will be pre-loaded (bad).  The bolts will bring the case halves together but there is enough flexing of the aluminum in the halves that they may push against the bearing.  You will know you have it about right if you can easily turn the crankshaft.  Finally, take a plastic headed hammer and whack each end of the crankshaft 6-10 times.  This will help seat the crankshaft on the bearings. 

*Check to be sure that the crankshaft turns freely by hand.*

If you see a tiny bead of RTV forming at the joint, you used the right amount of RTV.

DO NOT FORGET to put copious amount of engine oil on the bearings.  Make sure some goes down the lubrication holes in the crankcase.

For more information on how to do this procedure, go to this excellent site by MicroBlue Bearings.  Information from their site was used here.  We thank them for their help.