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Crankcase Bearing Removal/Installation Anyone Can Do

Paul Olesen


Removing and installing crankcase bearings is an incredibly important part of the build process to get right. Today we’re going to cover the method I’ve found works best along with tips and tricks to make the job go easier.

The majority of crankcase bearings utilize an interference fit which means the bearings feature slightly larger diameters than the corresponding bores they fit in. The typical ways to install them are either to force them into place (think hydraulic or arbor press) or via shrink fitting where one part is heated, and the other cooled, so the parts change in size. The change in the size of the parts occurs due to a thermodynamic concept called thermal expansion. As temperature changes different materials expand and contract at different rates. To put it in more practical terms aluminum components will increase or decrease at roughly twice the rate of steel parts. This is the main reason interference fits diminish when the crankcases are heated up.

The method of bearing removal and installation I prefer and am going to cover is the shrink fitting option where the component temperatures are manipulated to make removal and installation as stress-free on the parts and builder as possible. The reasons I like this method are:

  • No special hydraulic presses required
  • Bearing bore marring is not a risk
  • Less stress is put on the bearings and crankcases

I’m going to gloss over the standard tools and cover the ones that are unique to this process.

  • Oven - this could be any heat source that uniformly heats and has temperature control.
  • Freezer - enough said.
  • Old cookie sheet lined with aluminum foil - cheap insurance to ensure the oven doesn’t get soaked in engine oil.
  • Blind bearing puller - necessary to remove any bearings that don’t drop out on their own after heating the crankcases.
  • Assorted punches and hammer - useful to confirm full seating and correct any bearings that go in cockeyed.

How Hot?
Different components within the engine undergo different heat treatment processes which are utilized to increase strength or hardness of the part. The crankcases are the part of concern and should not be heated in excess. In my books, more details surrounding heat treatment processes are covered, however, for our purposes, I recommend bringing the cases up to 300°F which is usually enough to get the bearings to release and yields easy installation.

crankcase bearing oven

Bearing Removal Preparation
The most important thing you can do before heating the cases is to ensure they are as free of oil and debris as possible. Minimizing the amount of oil left on the crankcases is key to minimizing ensuing smells. Also, double check that all bearing retainers have been removed.

Bearing Removal
The crankcases should be placed in the oven for around a half hour and positioned so that the majority of the bearings will fall out due to gravity. After a half hour remove one case at a time. The case should be set on a wooden work surface or supported by wooden blocks so that heat transfer from the cases is slowed. If the cases are set on a metal work surface, the amount of working time available will be far less.

Some bearings will likely have fallen out on their own while others will need to be tapped or pulled out. The general rule of thumb is to work from the smallest diameter bearing up to the largest. The smaller the bearing bore, the less it will expand which means the working window of time will be less than for larger bores.

To remove bearings a punch and hammer is an effective combination as are many other tools. The critical detail when tapping the bearings out is to ensure they are tapped out evenly so that they do not cock in the bore. Cocked bearings can quickly lead to marred and damaged bores.

bearing removal

Before reinstalling all the bearings be sure to inspect the bores for signs of distress. Bearings that have spun or have previously been installed cocked can make reinstallation problematic. Areas of concern should be addressed by carefully removing any high spots.

damaged bores

Once all the bearings have been removed a final cleaning should be performed. This will further reduce any burning oil smells but more importantly ensure that no debris is left in the engine. Cleaning is especially important if the engine suffered a catastrophic failure. Don’t forget to clean out any oil passages!

A successful installation is often a matter of careful preparation. Organize the new bearings so that the sets for the left and right crankcases are easily identifiable. Plan to install the bearings starting with the smallest and working up to the largest. With this in mind, appropriately arrange them (egg cartons work well). On some engines, it will be necessary to install a small bearing (think balance shaft) on the opposite side of the cases from the majority. It is usually best to install these bearings first, retain them, and then flip the cases to the other side.

Once the bearings are organized, they should be placed in a freezer for around an hour. This will give them ample time to cool and maximize the available clearance between them and the cases during installation.

At the appropriate time reheat the crankcases at 300°F for around a half hour. Once ready begin installation on the first case. The trick to trouble-free installation is ensuring the bearings are dropped in square to their respective bores. In all honesty, this takes a bit of technique and practice; however, isn’t too hard to do. I like to hold bearings from their inner diameters and guide them into place.

If a bearing is dropped in crooked a punch and hammer is an effective means of correction. Carefully tap on the bearing until it becomes square with the bore. In most cases, once it squares up, it will drop the rest of the way in. Be sure to tap on the outer race of the bearing as opposed to the inner race so that load isn’t being transferred through the balls. Once the bearings drop into place, give them a final tap to ensure they are fully seated. There will be an audible difference between bearings that have fully seated and ones that have not.

bearing installation

Wrap Up

I hope you’ve found this article informative on bearing installation and that the removal and installation processes are easily executable. Successful bearing removal/installation is dependent on proper preparation and attention to detail. If you found this post useful, I encourage you to check out both The Four-Stroke Dirt Bike Engine Building Handbook and The Two-Stroke Engine Building Handbook. These books teach you how to do a full engine rebuild yourself from tear down to inspection to reassembly. Where a service manual can leave you scratching your head, my 300+ page books teach you exactly how to tackle a full engine rebuild without the guesswork. To learn from a professional, fix your own bike, and get back to riding - click here.

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Paul Olesen

Paul is a Wisconsin native, industry Powertrain Engineer, Author, and avid motorcycle enthusiast & racer.


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Great write up and pikkies.

One thing - are the figures for the oven in Fahrenheit (F) or Celcius (C)?

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16 hours ago, Prof75 said:

Great write up and pikkies.

One thing - are the figures for the oven in Fahrenheit (F) or Celcius (C)?

Thanks, I'm glad you liked the article. The temperatures referenced are in degrees Fahrenheit. The article has been updated accordingly.

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Before grabbing the hammer and punch take the case halves and slap them down on a solid surface, most if not all the bearings will fall right out,

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When installing the new bearings, using a socket the size of the bearing instead of a punch, and gently tapping the socket drives the bearing in evenly.  Making it difficult to install crooked or cocked.

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I hate pounding on things. Might want to add, dont handle the case halves too much until they've cooled. A bearing that dropped into place, can fall right back out. If its a two stroke, just wait to put the seals and retainers in.

Also, if it's a KX500, or some other older, vintage style two stroke that requires the seals to press in from the inside and pack the bearing in second, typically the seals will handle 250F safely. Just freeze the bearings a little longer.

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