Topic: Would CB900F Forks Work On My CB500?

[rabidelf]

Who knows if the Forks and Triple Tree from a 1980 CB900F fit on my 1971 CB500? Looks like a good way to upgrade to dual disc brakes and air assisted shocks.

[bryanj]

Not without transfering the steering stem from the 500 lower to the 900 lower

[ProTeal55]

Same as putting a newer sportbike front-end on one of these older bikes...

[rabidelf]

So I would have to grind the weld off my current stem, press it out of my 500 lower, press it into my new 900 lower and re-weld? Just like the thread for the Goldwing thread a week or so ago, huh? Sounds like a lot of trouble. Is there a front end with dual discs from any other bike that would just swap in without a lot of fuss? What about a CB750 Custom? Remember, I'm a cheap bastard.

[82CB650SC]

How close are the 82 650sc and the 500 neck size.  I have a spare 650sc fork I could measure the stem tonight.  I see those forks on ebay all the time cheap.  You would have to switch to comstar rims though.

[rabidelf]

Cool. I will measure the 500 stem size this evening. If everyone with an unused frame and/or forks could measure the dimensions maybe we could come up with a steering stem compatability FAQ. Hub and caliper dimensions would be helpful too. Too tall an order? I would hate to give up wire wheels for the swap though.

[wallstreet]

Cool. I will measure the 500 stem size this evening. If everyone with an unused frame and/or forks could measure the dimensions maybe we could come up with a steering stem compatability FAQ. Hub and caliper dimensions would be helpful too. Too tall an order? I would hate to give up wire wheels for the swap though.

I installed a 1980 CB900F (Super Sport) set of forks on a K3 frame. That is replacing the K3 35mm tubes for 37mm CB900 tubes.
 I used the 1980 CB900  OEM lower and upper clamps.
I used the CB900 axle and the K3 hub/bearings and spacer.
 As I did not wish to replace the frame races,  I installed the ball bearing races on the CB900 stem.
I did not exchange the stems.
I used the gauges from the CB900 on the K3.
I removed the bump stops from the frame as the stops on the CB900 stem are different.
I used the headlamp bracket from a 1981 CB750 as I did not have the CB900 healamp bracket.
The CB900 stem appears to set the tubes slightly wider than the K3 or the 1981 CB750/F (Super Sport).
You could use the CB750 Super Sport assemble; the headlamp bracket would fit.
The cap nut and bearing adjustment nuts seem to be a different design.
The last issue is using the K3 6 hole rotors which match the K3 hub in the CB900 calipers. The min thickness of the K3's is 6mm. The CB900 rotors are 5 hole and min thick is 4mm. I am planning to machine the K3's to the thickness of the CB900's so they install into the CB900 rotors. Anyone ever machined Honda rotors?
I would select your tubes and a stem to match their diameter.
Remove the front assembly from the CB500 or 550/F. Install/test the stem onto the frame to see if it matches the frame. IF your frame has ball bearing then remove the races etc from the OEM stem(easier and quicker for tests)  and install it on new stem. If you use the OEM wheel, which caliper/rotor will you use. Is it possible to have thinner rotors mounted/riveted on the OEM rotor hobs? Check OEM fork length and angle. If new forks are longer the rake will change. See if you can lower the forks thru the clamps to regain the OEM rake angle.
If the CBX stem  w/ 35mm forks will fit a K# CB750 it is possible it might fit the 550/F. I believe the stem bearing for all CB's fit 1969 to 1981 or '82. Check the All Balls specs for all CB's/CBX.

[82CB650SC]

Measured my stem last night this is what I got.  Measured from the top of the bottom bearing race.

[wallstreet]

Cool. I will measure the 500 stem size this evening. If everyone with an unused frame and/or forks could measure the dimensions maybe we could come up with a steering stem compatability FAQ. Hub and caliper dimensions would be helpful too. Too tall an order? I would hate to give up wire wheels for the swap though.

If you view the Dennis Kirk website under the All Balls vendor listing, the chart will show ALL motorcycles using a certain bearing. The CB550 and CB750A (many others)  use the 121431 and the CB750K, CBX the VF750 (many others) series use the 121439 number. Are these numbers interchangeable? If they are, why does a vendor make one number for several motorcycles and a different number for a different series if they are the same. The stem length can be shorted or lengthened.  In the case of the CB550, you might have to change the stem at the clamps  then make it fit. Will you frame take the change?  I fit a 1983 VF750 clamp to a CBX merely shortening the stem one inch. Bearings were the same.
Wire wheels should interchange with Comstar. Use different axle spacer. OR use wire wheel spacer for wire wheel or the Comstar spacer on the CS wheel.
BTW are there 18 front Comstar wheels available on any Honda's?

[rbowen]

Just for kicks I measured my CB450twin fork steerer tube and it appears to be a tiny bit shorter than the CB650SC one.

From race to first threads = 6 7/16
From race to top of first threads = 6 7/8
From race to second threads = 7 7/16
From race to top 8" even

So it's about 3/16 overall shorter than the CB650SC

Anyone with a CB550 or 750 fork want to measure for us?