Hi, I would like to install oilite bushings in an aluminium casting for a small steam engine which presently has poor bushings. Would the proper procedure be to drill the casting undersize and then ream the bore to the correct size for a snug fit? For a 1/4 in O.D. bushing would I ream the hole to exactly 1/4 in or should it be slightly undersize? Can you buy a reamer that is slightly undersize for this purpose? What about adjustable reamers?
A lot depends on the shaft size. Typically, Oilite bushings are made oversized, both ID and OD, but when you press them in the properly sized hole, they are the desired finished size. Without a micrometer to determine the exact size you need, it's a crap shoot. The shaft would determine that, and it may or may not be nominal. Assuming you want an undersized reamer, yes, they are readily available, particularly if you shop for a chucking reamer. These are intended to be run in a spindle and don't work worth a damn by hand, so if you're restricted to hand work, don't buy one.
I'm not convinced you'd find an adjustable 1/4" reamer, but I'm not sure you'd want to do this job without the use of at least a drill press, if not a mill.
In order to insure that the holes are properly aligned, and not bell mouthed because of hand misalignment, it would be in your best interest to not attempt this by hand. If bushing are to be installed that should be aligned, boring the holes would be desirable, and for that you don't need a reamer.
Hope this helps, Rodney. I realize it's not the greatest news, but it will keep you out of trouble.
I've gotten 1/4" adjustable reamers, the ones that are readily available are the sort that are one-piece with longitudinal slits and an end tapped for the adjustment set-screw. These are usually what's available in the smaller sizes, when you can find them. I also have the multi-blade sort, these took a bunch of working over to get all the blades to cut and to get a decent finish. The one-piece adjustable types don't necessarily return a straight hole for your efforts, they're kind of barrel shaped to start with. The best sort of reamer for doing multiple in-line bushings is one intended for electric motor rebuilding, this has a long pilot so you ream one bushing with the far end supported by the other bushing. A fellow could probably make up something like this with a lathe, making a D-bit in the center for the reamer part.
The rule of thumb I've read and have used is .001" interference fit per inch of bushing diameter, then ream the shaft hole to desired clearance after pressing in. For under 1", I've always used .001" interference between housing hole and bushing. A loose fit could be compensated with one of the anaerobic adhesives they have these days, or just use the stuff to start with.
If you freeze the bushings and heat the aluminum to 250 degrees, the bushings will drop in, but you only got a couple of seconds to get them in the right place so I put a mechanical stop on the end so it is placed right.