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

by Had Robinson

As many motorcycle and kart racing sites warn, bearings in aluminum cases must be removed carefully.  Do not use a torch to heat the case.  It is best to use an 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.

Special tools needed

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

You must first completely disassemble the engine and split the case.  It requires a special tool to split the case.  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 MOST LIKELY RUIN IT.

Sometimes when the case is split, one or both of the bearings will remain on the shaft rather than stay in the bore(s).  The bearing cannot be pulled off the shaft because there is no clearance between the bearing and the flat face of the crankshaft to fit a puller.  At this point, the bearing must be sacrificed.  The best (and cleanest) way to remove the bearing is to heat the inner race with a fine point torch (propane will work) 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 gently pried off with a pair of thin chisels or screwdrivers.  Care must be taken not damage the crankshaft and the lower rod bearing.

Bearing removal 

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 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 surface near the bearing reach 177ºC (350ºF), the bearing will likely fall out of the bore and you will hear it 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 installing the slide hammer 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 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 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. Remove the seal

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 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.  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 cold 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 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. 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 can be used that is just narrower than the inside rim of the seal to press it in.  Do not use a hammer pound the seal in 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 an infrared gun to measure this temperature.  It should 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 hit 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.

Once the bearing is in place, carefully 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 this position (horizontal), 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.

7. Test bearing

To test the bearing clearances and determine if there might be any damage to the bearing, use compressed air (fine tip) on the balls of the bearing.  If the balls 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.  Oil must quickly disappear down the holes.  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.  Information from their site was used here.  We thank them for their help.

8. Assembling the case halves

Once the bearings and seals in a crankcase or redrive have been replaced, the case halves must be re-assembled.  This is straightforward if the torque specifications for the bolts that hold the case halves together are known.  Generally, the shafts will slide right into the bearings on each half easily.

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 very 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.