I purchased this old (1979) trailer. I need to get new bearings in, but have no idea the size. How do I define what size/type bearings would be used? The tires are 12 inch.
It depends on the size of the spindle and whether it is a straight spindle or not (more than likely it is straight). There are two common sizes for small trailers (I believe). It should be the A14 bearing set for the 1" spindle, or A6 set for the 1 1/4" spindle. If in the US, you should be able to pick these up from any of the major parts stores (AutoZone, Checker, Advance Auto, Etc). They have them in sets with the inner and out bearings (and races), seal, and cotter pin. Looking at your hub, I would guess it is the 1" set that you need. And obviously you'd need two sets, one for each side of your trailer. Double check the part number, as different store may use different part numbers.