Topic: K to F2 Transmission Interchangeability

[SuperDave]

Hi everyone! This is my first post, but I've been messing with CB750's for about 13 year now. I've got a concern that I haven't been able to find an answer for so maybe you guys can enlighten me.

I recently took my 1978 CB750 F3 (CB750E-3101408)motor out to fix a broken 5th gear. It was worn dogs and a bent shift fork. No big deal, I'll split the case, and swap some parts and be back on the road in a couple days. I already had a complete K2 transmission on the shelf.

My F3 trans looked great besides the damaged parts so I decided to leave most of it in there and swap out the bad fork, the 33T main shaft "5th" gear, and the 24T/27T mainshaft "2nd & 3rd" gear that engages with 5th. At the time, I assumed all CB750 transmissions were the same but I still counted the teeth on my worn gears vs my donor gears to be sure. 33 tooth, 24 tooth, and 27 tooth...yep all good right? So I plugged them in and sealed/torqued the case.

A failed leakdown test kept me from installing the motor that same day. While I was looking for new valves I came across Cycle-X website showing the different gears the F models used. Well crap! I just installed K gears mixed with my F3 gears! Fortunately, the 5th gear I installed did not change part numbers over the years even though it is meshed with a "new" 32 T  "5th" gear on the countershaft.

However, the 24T/27T gear I installed changed part numbers with the F models and so did all the gears it meshes with as well as the "new" 30T "4th" mainshaft gear which the 24T/27T dogs engage with.

I didn't have any issue installing the gears or closing the case. My transmission SEEMS to rotate and change gears fine by hand on the bench. 5th gear engages fine but has a noticeably small amount of backlash compared to the other gears(this could be the different dog type).

So my questions to any gurus out there is this: Did I do a bad thing or am I good to go? Can I ignore the part number changes since the tooth counts are the same? Is there a change in dog shape that is going to cause a premature failure?

See the attachments for my understanding of the gear changes.

[bryanj]

Tooth profiles altered

[SuperDave]

Honda must have changed the tooth pressure angles to add or subtract a tooth here and there and not change the meshing gear. I understand how that works, but the part number changes on the others has me spooked. Did something else change on the gears with the SAME tooth count that I can't see? I held them side by side in my hand but I didn't notice a difference.

[PeWe]

I have a slow build of a CB750 K2, engine cases from 1975.

It has a later transmission.
The only thing I changed is the final gear of the late trans, mounted on a K final shaft.

I guess you can do the opposite using an earlier K trans in later case as long as you use the earlier final gear on the later shaft.

I had some early thoughts here
http://forums.sohc4.net/index.php/topic,180088.msg2097194.html#msg2097194

The final shaft visible in the end of that thread.
Will soon add more. Do the complete bottom end.

I hope it will work
Trans is undercutted, gear 2-4.
Late clutch K7/8, F2 ...

[SuperDave]

I have a slow build of a CB750 K2, engine cases from 1975.

It has a later transmission.
The only thing I changed is the final gear of the late trans, mounted on a K final shaft.

I guess you can do the opposite using an earlier K trans in later case as long as you use the earlier final gear on the later shaft.

I had some early thoughts here
http://forums.sohc4.net/index.php/topic,180088.msg2097194.html#msg2097194

The final shaft visible in the end of that thread.
Will soon add more. Do the complete bottom end.

That makes sense and is probably what I'll try if I determine this installed combo is a no go.

[SuperDave]

Does anyone know if I'm good to go? I'm about done with the top end. I need to find out soon if I need to open the bottom end again.

[SuperDave]

Update: I decided to just install the motor and try out this F3/K gear combination an so far it rides and shifts just fine.

[PeWe]

Good news
You did the opposite I'll do in my build.

You used a K transmission in an F3 engine.
Final gear also K  that replaced the F3 final gear on the F3 shaft that fits the engine cases.

You will notice that gear 4 and 5 will rev slightly less compared with before.

[SuperDave]

Actually, I only replaced the 33T and the 24 T/ 27 T from the K donor trans into my F3 trans. The part numbers were different but the tooth count were the same as the F3. So I should still have the F3 ratios. I was concerned that there was something different about them that I didn't see. Judging by how smooth everything is working I'd say the gears are the same design.

[PeWe]

OK,
These 2 marked in red are the different gears if I got it right.

(Plus the final gear with different teeth, both sides)

[SuperDave]

You are correct that those are the gears that changed number of teeth. Those stayed in my engine. I changed gears #6 and #11 on your drawing. If you look at my drawing in my first post I have different colored labels for the gears that changed part number. It is the gears that had the tooth count the same but had a new part number that got me confused. My gears I was replacing had the same tooth count but meshed with gears that had a part number change. It was those gear that I didn't bother to look at when I was in there. BUT, so far it seems to work! (fingers crossed)