As for your initial question, the easiest forks to mount are 35mm Honda forks from the late 70s/early 80s because a lot of the critical dimensions are the same. You will most likely have to swap stems or use a spacer - the stem swap is better because other weird shit happens with a spacer (ask me I know because that's what I was planning to do first).
CB450/500t stems are probably the same length as yours but might not be. Later model forks including various CBR/GSXR, etc. are cool but the length, offset, front wheel design and various other things make them a much harder swap. Plus they don't look vintage, if that's the look you're going for.
The funky swinging brake link disc brakes on 70s Hondas suck. I don't care what anyone else says, it's a Mickey-mouse setup that was designed to avoid having a floating caliper mount directly to the fork. I've never been able to figure out why they didn't just design a floating caliper but whatever.
To avoid said sucky brakes, you want the forks (or at least the fork tubes and legs) from a later CB750F, CB650 or likewise. They will be too long but you can either slide them up or have them shortened. Or use a spacer under the top out spring like I did (which is not the best way to do it because you lose travel but it's cheap). CB450 and 500/550/650 forks are close to the same length as CB400f, but they have the aforementioned sucky brakes. Use the longer forks but just watch the travel so they don't bottom on the triple clamp.
Anyway, the other problem is offset. If your bike uses the same or similar forks to a CB400f, it has the same offset as CB450/CB500T, CB650 Custom and early CB750k models so you want those triple clamps to maintain stock offset. CB750f, CB500/550, CB650 Custom and CBX have different (less) fork offset that will give you more trail and heavier steering. CB450/500t have rubber-mounted handlebars which is cool or you can ditch the mounting and have a cleaner top clamp with clip-ons. CB750k lower triples are almost the same as CB450/500t but the stem is longer and the top triple has solid bar mounts. CBX triples have less offset and are aluminum with no upper bar mounts which is even cooler, but they have a weird fork stop which means using them is hard.
Still with me?
For some reason Honda uses a million different gauge mounting patterns and brackets, so the chances of finding one that lets you use your stock gauges with the new fork are minimal to none. You will most likely have to fabricate it.
All of these forks are simple damping rod forks which means they're either harsh in compression or dive-happy, or both. You can modify the damping rods if you're handy but you won't be able to get the compression damping perfect because of the design.
The only exception to this is late CB450 forks which have a real damper inside them like Norton Roadholder forks. Good luck finding a set that is decent, and of course you have the Mickey-mouse brakes to deal with.
Oh, and one more thing about dual discs - The Mickey-mouse brakes can be made into duals with the correct fork lower legs and an extra caliper and rotor, but the speedo drive has to be modified, or so I'm told. The rotor bolts to the hub with six through-bolts so you just mount another on the other side.
The good calipers (and their matching fork mounts) unfortunately only work with newer, thinner, lighter 5-bolt rotors which, also unfortunately, only come on ugly Comstar rims or, on some CB750ks and CB650s, as a single disc spoked wheel.
I have never seen a dual-disc, spoked wheel that works with the 5mm thick late model rotors and matching calipers. Oh, and most of them are 19" so you will have to search before finding an 18" wheel to match your stocker. This applies to the above too.
The best calipers for the above swap are the ones on 1980 CB750fs because they are dual-piston and will still clear spokes.
Now if you've read this far there's another swap I considered but ultimately rejected: GL1000 goldwing. It's got 37mm sliders and solid-mounted calipers, a 19" spoke wheel with an alloy rim and dual discs. The problem for CB400f/CB450/CB550 etc. bikes is the extreme length of these forks - about 2" longer than CB750 or almost 4" longer than stock. Yuck. Also, the top triple has cast-in risers almost 4" tall that cannot be cut off without seriously weakening the top triple.
There are guys on here with a hell of a lot more experience than me, and I'm sure they will add to (or contradict) what I've listed.
This information is what I've collected over the past 6 months by hanging out at the bike junkyard and scouring Honda parts catalogs.
Good luck!
CB450/500t stems are probably the same length as yours but might not be. Later model forks including various CBR/GSXR, etc. are cool but the length, offset, front wheel design and various other things make them a much harder swap. Plus they don't look vintage, if that's the look you're going for.
The funky swinging brake link disc brakes on 70s Hondas suck. I don't care what anyone else says, it's a Mickey-mouse setup that was designed to avoid having a floating caliper mount directly to the fork. I've never been able to figure out why they didn't just design a floating caliper but whatever.
To avoid said sucky brakes, you want the forks (or at least the fork tubes and legs) from a later CB750F, CB650 or likewise. They will be too long but you can either slide them up or have them shortened. Or use a spacer under the top out spring like I did (which is not the best way to do it because you lose travel but it's cheap). CB450 and 500/550/650 forks are close to the same length as CB400f, but they have the aforementioned sucky brakes. Use the longer forks but just watch the travel so they don't bottom on the triple clamp.
Anyway, the other problem is offset. If your bike uses the same or similar forks to a CB400f, it has the same offset as CB450/CB500T, CB650 Custom and early CB750k models so you want those triple clamps to maintain stock offset. CB750f, CB500/550, CB650 Custom and CBX have different (less) fork offset that will give you more trail and heavier steering. CB450/500t have rubber-mounted handlebars which is cool or you can ditch the mounting and have a cleaner top clamp with clip-ons. CB750k lower triples are almost the same as CB450/500t but the stem is longer and the top triple has solid bar mounts. CBX triples have less offset and are aluminum with no upper bar mounts which is even cooler, but they have a weird fork stop which means using them is hard.
Still with me?
For some reason Honda uses a million different gauge mounting patterns and brackets, so the chances of finding one that lets you use your stock gauges with the new fork are minimal to none. You will most likely have to fabricate it.
All of these forks are simple damping rod forks which means they're either harsh in compression or dive-happy, or both. You can modify the damping rods if you're handy but you won't be able to get the compression damping perfect because of the design.
The only exception to this is late CB450 forks which have a real damper inside them like Norton Roadholder forks. Good luck finding a set that is decent, and of course you have the Mickey-mouse brakes to deal with.
Oh, and one more thing about dual discs - The Mickey-mouse brakes can be made into duals with the correct fork lower legs and an extra caliper and rotor, but the speedo drive has to be modified, or so I'm told. The rotor bolts to the hub with six through-bolts so you just mount another on the other side.
The good calipers (and their matching fork mounts) unfortunately only work with newer, thinner, lighter 5-bolt rotors which, also unfortunately, only come on ugly Comstar rims or, on some CB750ks and CB650s, as a single disc spoked wheel.
I have never seen a dual-disc, spoked wheel that works with the 5mm thick late model rotors and matching calipers. Oh, and most of them are 19" so you will have to search before finding an 18" wheel to match your stocker. This applies to the above too.
The best calipers for the above swap are the ones on 1980 CB750fs because they are dual-piston and will still clear spokes.
Now if you've read this far there's another swap I considered but ultimately rejected: GL1000 goldwing. It's got 37mm sliders and solid-mounted calipers, a 19" spoke wheel with an alloy rim and dual discs. The problem for CB400f/CB450/CB550 etc. bikes is the extreme length of these forks - about 2" longer than CB750 or almost 4" longer than stock. Yuck. Also, the top triple has cast-in risers almost 4" tall that cannot be cut off without seriously weakening the top triple.
There are guys on here with a hell of a lot more experience than me, and I'm sure they will add to (or contradict) what I've listed.
This information is what I've collected over the past 6 months by hanging out at the bike junkyard and scouring Honda parts catalogs.
Good luck!
