I do it two ways. One is to use a hyd press to push out the old mains and install the new mains. Another way I use is to heat the case half around the boss with propane and drop bearings in that were in the freezer for an hour or so. When I'm raedy to install the crank (it's in the freezed also for an hour or so) I insert an aluminum slug in the bearing, heat the slug and the crank drops right in. No forcing needed. SO, All you really need is a freezer and propane torch to get it together. I've never removed the old bearings w/o a press though. The slugs I make myself with a file and grinder. I don't have a lathe. I just cut away the meat of the slug with a hacksaw, grind, then file smooth, they almost look like they were turned on a lathe. The aluminum rod can be had at a local metal supply, they usually have scrap laying around. Question: How do ya'll get the old bearings out w/o a press? I'm hesitant about suggesting a slide puller although that's what I use for wheel bearings.