It was a 45mm square axle with Slimline bearings, 10" disks and cable calipers. I've just replaced it with a new upgraded 50mm square axle with parallel bearings, 12" disks and new brake calipers and brackets.
When we cut the damaged end off the old axle and shortened it to suit its new role supporting a water tank at the farm, we bought a new caliper bracket for the new stub.
And that's when the fun started.
I now know that there are UNC caliper bolts and M10 7/16" UNF caliper bolts. The industry seems to have completely converted to M10 (metric) 7/16" UNF calipers and brackets in the last five years or so; imperial (UNC) calipers and brackets for UNC bolts and matching bushes are no longer available.
The UNC bolts are visibly much coarser thread, and indeed if you look at the bolts holding your calipers on and think, gee, you wouldn't go any coarser than that!, then you're probably looking at UNC bolts. The metric UNF bolts are a finer thread and "look right".
The bolt hole spacing is unchanged, since the caliper casting is unchanged, but the machining of the caliper casting is different to suit different bushes.
If you change your axle:
- Imperial (UNC) bolts won't fit the metric UNF brackets now available.
- Metric 7/16" UNF bolts are a sloppy fit in imperial (UNC) bushes. You can't just put 7/16" UNF bolts through your imperial (UNC) bushes.
- Metric 7/16" UNF bushes are slightly larger diameter; you can't just put 7/16" UNF bolts and bushes through your imperial (UNC) calipers.
- You *might* drill out the holes in the castings to suit the metric 7/16" UNF bushes.
- Or you could rescue the caliper brackets off the old axle. (Which is what we ended up doing on the new stub on the old axle for the farm trailer.)
- Or just replace your calipers. (My new axle got new calipers.)
Similarly, if you get new calipers,
- Metric 7/16" UNF bolts won't fit your imperial (UNC) brackets.
- Imperial (UNC) bolts won't fit through metric 7/16" UNF bushes.
- Imperial (UNC) bushes are a sloppy fit in metric 7/16" UNF calipers; you can't just put your old bolts and bushes through your new metric 7/16" UNF calipers.
- You might drill out the imperial (UNC) bushes to suit the metric 7/16" UNF bolts. But they're stainless steel, so it won't be easy...
- You could cut off the old imperial (UNC) brackets and weld new metric 7/16" UNF caliper brackets to the axle.
- Or replace your entire axle.
The cost of a dozen special bushes is likely to be much the same as two, so if anyone finds themselves stuck with an imperial (UNC) axle and needing to fit metric 7/16" UNF calipers, I could look into getting some made and selling the excess through ebay, etc.
- There are metric 7/16" UNF and imperial (UNC) cable calipers and associated parts in circulation "in the field".
- Metric 7/16" UNF parts are quite recent and seem to have appeared within the last 5 years.
- Imperial (UNC) parts are no longer available.
- You can't mix and match calipers, bolts, bushes and brackets.
- The same caliper casting is still used, so pads fit either type of caliper.