I'm a bit desperate here.
Is there any way you can hammer out a wheel bearing and replace a new one without a press or puller.
I am talking raw hammer to knock out and press back in with hammer and block?
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I've replaced bearings without either a puller or a press, it's perfectly doable, although a bearing insertion set does make things much easier. These are cheap and you can get them off the internet. Failing that you can do without.
Bearing race removal:
First you need a hammer, punch, gloves, safety glasses, a torch and a piece of scrap wood to go between the hub and whatever surface you're hammering the hub on. After removing wheel from the hub you should be able to get the seals and bearings out with basic tools (Keep the seals for the time being). Once you have those out you need to get the inner bearing races off, which is the more challenging part. To do that you heat the hub it using a torch until the bearing grease starts to drip out, then hammer the races out using the punch. There will be grooves machined out of the hub to do this, you just hammer one side, then the other until it drops. Flip and repeat for the other race (assuming you have 2). Keep the old races.
Next clean the hub up, remember it's still hot! Get all the grease, grime and dirt off or it's going to get into the new bearings. Inspect it for cracks while it's clean.
Bearing race insertion: Before you try to get the new races in, you need to take the old races and cut each across using a rotary tool, saw, angle grinder or the like. Stick the new races in the freezer, this will cause them to shrink (30 minutes at least in the freezer). Heat the hub again, this will cause it to expand. Get one of the frozen races from the freezer and quickly drop it into the hole. If you're lucky the combination of the expansion of the hub and contraction of the ring may let the ring just drop straight down.
If you're me that won't happen as it's never that easy, this is where the old bearing races come in. You could use the hammer and punch on the new races directly, and I've heard of people doing this successfully without damaging them, however it's risky. Put the old race on top of the new one and hammer on that instead, the old race will spread the force out and prevent damage. The slit cut through it will keep it from getting stuck along with the new race. Work around, hammering as you go until it's seated. Note that this method can be annoying and time consuming as the old race has a habit of jumping out. Repeat for the other side, you will probably want to reheat the hub.
There's other options than reusing the old races for getting the new ones in that work better, providing you can find materials of the right diameter. A length of steel pipe just smaller than the race works great, as does a big socket.
Once you have the races in and the bearings greased and inserted you need to get the seals on, most of the time these are synthetic and can be tapped in with a rubber mallet, but sometimes using the old seals to protect the new ones is a good idea as well.
I would recommend taking the hub to a local garage and for a very small fee of around £10 they will press out the old bearing and fit the new one. I have done this several times and found the garages very accommodating. All you then need to do is fit the hub assy. Failing that you could knock them out but I recommend you take the easy option as you may damage the bearing trying to fefit it. They are very tight.