Temperature...required to install/remove bearings

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Questions about what is required to install or remove bearings (whether in engine cases, front wheel hubs, or otherwise) are often asked in various forums.

Here's how it works:

First, it's good to know what coefficient of expansion is: It is the amount that a material grows (or shrinks) for a given amount of temperature. This is usually given in Celsius, but I'll convert the numbers to Fahrenheit here for simplicity.

Aluminum  ~12 x 10-6 per 1o F
Magnesium  ~15 x 10-6 per 1o F (this # is for cast magnesium, which should be close for most of our applications)
Steel      ~6 x 10-6 per 1o F
(These numbers are not dead on, however they're plenty close enough for what we're doing.)

To put this in English: aluminum expands around 12 millionths of an inch... per inch... per degree, and steel expands around half that. If we had a cube of aluminum that measured 1" in all 3 directions, it would expand equally with temperature, and would measure about .000012" (twelve millionths of an inch) oversize in all directions if we heated the piece 1 degree F.

Example #1:
We want to install a main bearing in a Yamaha KT100 case half, which has a bearing bore size just over 2" (about 2.0460" to be more precise). That bearing bore would increase in size about 24 millionths of an inch for each degree that you heat the case (remember it's 12 millionths per inch, so we have to multiply by 2). If we are installing a main bearing, we don't need to heat the case nearly as much as when removing a main bearing, since the main bearing stays at room temperature. If you do the math, you'll see that increasing the temperature of a Yamaha case half about 42F will increase the bearing bore size .001. Since a typical main bearing fit might be in the .0014 range, that means we'd need around a 60F increase (over ambient) to make the bearing bore the same size as the main bearing. Obviously this would not be enough to allow us to install the main bearing (yet), so we'd actually need to go a bit hotter yet in order to be able to drop in the bearing without the chance of it "sticking" part way in. Around 200F is an adequate temperature for installing main bearings... that yields around .001-.0015 clearance.

NOTE: -- This is what the case temperature needs to be... not the oven temperature!

Example #2:
Let's say we want to remove the bearings from some front wheel hubs. The OD of the bearing is around 1.375"... so the aluminum hub (i.e. the bearing bore in the hub) will expand around .000016" (sixteen millionths of an inch) for each degree that the hub is heated. However... the steel bearing will also expand... but at about half the rate of the aluminum hub. For simplicity, we can safely assume that the difference in expansion is about .000008" (eight millionths). The aluminum expands .000016", and the bearing we are trying to remove also expands, but only about .000008", so we end up with a net gain of about .000008" for every degree that the hub and bearing are heated. If the bearing has .001 interference, the hub and bearing will need to be heated around 125F over ambient temperature in order to get to "net" (where the bearing OD and the hub ID are the same size). Since there is always dirt and grease involved in the equation, going around 175F or so over ambient (for a total temperature of around 250F) should cause the bearing to drop out (though a slight tap on the hub or bearing might still be required to knock things loose).

It's always best to use a lower oven temperature, and allow the parts to sit in there longer... rather than use a higher oven temperature in an attempt to do a "quick" job. In something like a front hub, this can be important since the bearings have rubber seals, and there is always grease involved. If you set your oven temperature at no more than 275F or 300F, you can safely leave the hubs in there for a long time. If, however, you were to set the oven to 400F, the rubber seals might burn quickly (since they have very low "mass"), and chances are you'd have a smoking mess in the oven because the surface of the hub (and the grease that might be on the surface) would overheat very quickly. Also... at anything over 350F, the chances of permanently softening aluminum increase quite a bit... so if you happen to set the oven temperature high (in a hurry?), and you forget the parts in the oven... you could end up with some aluminum hubs that are no longer 6061-T6... they might now be 6061-T-butter!

Never set parts directly on the lower wire rack in an oven without having something underneath them. This area is close to the flame or heating element, and it's easy to overheat parts even if your oven temperature is only set to 300F. It's a good idea to use an aluminum plate under your parts. This not only prevents direct heat/flame from overheating your parts... but also prevents a bearing you're removing from dropping into the bottom of the oven when it falls out.

With the coefficient of expansion numbers given at the top of this page, it's simple to calculate what temperature is required to remove or install bearings. Remember that if you are removing bearings, you only gain about half of what the aluminum expands since the steel bearing expands as well, though at only half the rate.