"Is it possible to upgrade FE-FC front brakes to EJ-EH or HD" is a question which comes up fairly often. The accepted method is to change over to EJ-HD steering knuckles, supports, steering arms, backing plates and brakes as an assembly. But I got to wondering -
Is it possible to bolt EJ-HD front brakes directly onto FE-FC steering knuckles without changing to EJ-HD steering knuckles and supports? There is no firm consensus. So I decided to find out, by doing.
Here is an HD kingpin front brake assembly happily bolted up to an FC steering knuckle.
(https://i.imgur.com/ZIDU4Cyl.jpg)
From this angle it looks no different from the original HD steering knuckle and support. From the back side, though -
(https://i.imgur.com/T59cQJ5l.jpg)
A couple of things of note -
The upper holes in the FC steering knuckle support are threaded, so I had to change to short 3/8" UNF bolts.
The steering knuckle forging has the number 3415/6, which identifies it as later FE and FC (7413415/6).
And here is a photo of an FE-EK steering arm (left and bottom) bolted up to an EJ-HD steering arm (right and top), showing that the bolt hole spacing is the same between both types of steering arm:
(https://i.imgur.com/9QTF5nJl.jpg)
Conclusion: Yes, you can bolt EJ-HD front brakes onto (late) FE and FC steering knuckles. You need to change the top two backing plate to steering knuckle attaching bolts to short 3/8" UNF bolts to suit the threaded holes in the FE-FC steering knuckle. You also need to use the EJ-HD steering arms. And if you are using HD kingpin front brakes (probably rocking-horse-poo rare these days), you will need to use HD or HR wheels.
Rob