Topic: CB750 K0 shift drum doesn't seat

[jebon]

Hi!
I can't get my shift drum to seat all the way in and also have the neutral switch in and engage at the same time. Do I have a wrong shift drum for this case? I'd be surprised if this is the way it's supposed to sit

[bryanj]

You have to be assembling something not quite right

[jebon]

If I take out the neutral switch, then I can put it in all the way but the end of the drum blocks the hole for the switch. Could it be the switch or the drum that's wrong?

[Jerry Rxman Griffin aka MuthaF'er]

Chances are the spring loaded ball in the neutral switch is not perfectly engaging the shift drum. Loosen the neutral switch and wiggle the basket thingy in your picture in and out to see if it will seat.   

[jebon]

For the drum to sit right the neutral switch needs to sit at this height. But then it won't line up for the flat sided washer below. And there's a gap between the case and the switch. Is this just supposed to be squeezed tight?

[HondaMan]

The Neutral Switch must be seated, or else you'll have both an oily mess and the upper gears in the tranny will jam when shifting, misaligned.
The shifter drum does not fit flush to the inside bearing hole. If you have a late shifter drum in an early case, the shoulder of the drum will sit higher than you were used to seeing with the early shift drum (near the shift end, where the little pins and cover live). This shoulder is wider on the later drums.

If you have K0 cases and are fitting a later shift drum, the later drum may not fit because the K0 spring arm detent system is different from the later ones: sometimes the little rollers on the sprung levers will not sit at the right height. The shift drums from the K6/F0 and later bikes don't fit into the K0 cases well, if at all, and vice-versa, because the case wall got thicker during the K1 engine, and the shifter drum, while a 'delta part' design, wasn't quite a 100% replacement fit.

[BPellerine]

did the drum come out of this engine?

[flybackwards]

FYI - this is the way mine sat in my K7 prior to taking it apart - so as the Hondaman says it does not sit flush

[jebon]

The Neutral Switch must be seated, or else you'll have both an oily mess and the upper gears in the tranny will jam when shifting, misaligned.
The shifter drum does not fit flush to the inside bearing hole. If you have a late shifter drum in an early case, the shoulder of the drum will sit higher than you were used to seeing with the early shift drum (near the shift end, where the little pins and cover live). This shoulder is wider on the later drums.

If you have K0 cases and are fitting a later shift drum, the later drum may not fit because the K0 spring arm detent system is different from the later ones: sometimes the little rollers on the sprung levers will not sit at the right height. The shift drums from the K6/F0 and later bikes don't fit into the K0 cases well, if at all, and vice-versa, because the case wall got thicker during the K1 engine, and the shifter drum, while a 'delta part' design, wasn't quite a 100% replacement fit.

Thanks, yes that's what I thought.

[jebon]

did the drum come out of this engine?

Everything came from this engine when I disassembled it. I did not take note that anything fit poorly when I took it apart. But this drum clearly doesn't fit the case.

[jebon]

If the switch is to be flush below then it has to sit this high. It could be lower depending on the O-ring size. But I need the correct drum for the case

[jebon]

... Also it seems from the marks on the drum that it been sitting wrong

[jebon]

The Neutral Switch must be seated, or else you'll have both an oily mess and the upper gears in the tranny will jam when shifting, misaligned.
The shifter drum does not fit flush to the inside bearing hole. If you have a late shifter drum in an early case, the shoulder of the drum will sit higher than you were used to seeing with the early shift drum (near the shift end, where the little pins and cover live). This shoulder is wider on the later drums.

If you have K0 cases and are fitting a later shift drum, the later drum may not fit because the K0 spring arm detent system is different from the later ones: sometimes the little rollers on the sprung levers will not sit at the right height. The shift drums from the K6/F0 and later bikes don't fit into the K0 cases well, if at all, and vice-versa, because the case wall got thicker during the K1 engine, and the shifter drum, while a 'delta part' design, wasn't quite a 100% replacement fit.

So if I find the correct drum, there's a chance I also need to source the rest of the shifter mechanism - or some of it?

[jebon]

CMSNL has an alternative view with the star shaped plate. Is this maybe the one I need? There's one on ebay now.

https://www.ebay.de/itm/1970-HONDA-CB750-K0-SHIFT-DRUM-/264388921444?_trksid=p2349624.m46890.l49292

[69cb750]

24301-300-030 was used on K0,K1
24301-300-040 was used on K2
The shifter drum changed in 72, not sure what changed, someone here may know.
There is a K1 drum on ebay - https://www.ebay.com/itm/1971-Honda-CB750-K1-ENGINE-TRANSMISSION-GEAR-SHIFT-SHIFTER-CAM-DRUM/174541549891?hash=item28a37cb143:g:jPkAAOSwKc5drIqi

[BPellerine]

I thought the shifting improved on later bikes,that was the only change,some mention in hondamans book

[flybackwards]

Using the info available on the documents section of this forum and the on-line sources such as CMS check your part numbers carefully to see when and how they changed otherwise it s a gamble buying stuff.

[jebon]

After browsing a great deal I haven't seen any that looks to have a narrower flange. They all look very much like mine. Even if they state K0 or K1. One on ebay even has the engine number listed (in the 10,000's I believe) I've asked a few on ebay and CMS to measure the flange.

I'm beginning to fear what else is not original on this bike. I hope not the cam shaft etc.

[69cb750]

Your flange is 16.30 mm ?
Report the flange info you receive.

I'm beginning to fear what else is not original on this bike. I hope not the cam shaft etc.

I suspect it is the crankshaft and some of the main bearings.
You should file suit against all the previous owners and demand huge sums of money !

[69cb750]

[69cb750]

[jebon]

After trying it again I'm maybe leaning towards that I may have the right part. Either that or the correct drum is a very rare part. I was able to assemble it. But it has a gap at the end of the drum inside the case.

However, I am not overwhelmed by its willingness to shift properly. I don't know if this will improve when the upper gears are fixed by the uppercase and the clutch. It sometimes just gets stuck and even jams when trying to go into 5th. I'll try a test fitting with the upper case tomorrow.

[bryanj]

Both shafts have to rotate to shift gears

[jebon]

Both shafts have to rotate to shift gears

I'm not sure I know what you mean. I'm rotating the upper which is engaging with the lower.

[bryanj]

Not all the gears turn when you totate one shaft so the "dogs" can but up againsta solid instead of a hole, whentrying to shift rotate the other shaft slightlyand the gear usually drops in. In use the only time a shaft is not moving is at standstill and you only go into 1st from that

[jebon]

I tried with the upper case on to fix the upper shaft at the bearings. Still very unstable shifting. I took it apart again to try and show you what I'm seeing. I took a drill on the upper shaft on the clutch side and started shifting. I don't know if you can make anything of this and I'm sorry it's painful to watch. The scraping noise at 1.07 is just two of the cogs rubbing slightly because the upper shaft is not fixed at the bearings. It behaved completely the same with the shaft fixed by the upper case.

Video:

I saw that two holes in cogs in the lower shaft were not aligned but aligning them did not make any difference. I just want to say that before I took it apart there were no gear issues like this so I'm guessing either I've done something wrong or it will fix itself when there's load on the transmission. I hope to be able to find a bug though

[jebon]

I can't seem to find anything wrong or any documentation on if the holes should be in line on the lower shaft.
I'm guessing when there's load on the shafts and the final drive and continuous pressure on the lever the dogs will find their way easier.
So unless someone has some light to shine on this I'll proceed and button up.
Thanks for your comments 

[69cb750]

I can't seem to find anything wrong or any documentation on if the holes should be in line on the lower shaft.
I'm guessing when there's load on the shafts and the final drive and continuous pressure on the lever the dogs will find their way easier.
So unless someone has some light to shine on this I'll proceed and button up.

When the bike is rolling down the road the engine is spinning the clutch and the shaft even with clutch released.
The rear wheel is spinning the other shaft.
So both shafts are spinning and the dogs fly by the holes.
When you press the shift lever the dogs enter the holes and you are in gear.
You can test all of this on the bench, the goal is to shift through all the gears.

[jebon]

I can't seem to find anything wrong or any documentation on if the holes should be in line on the lower shaft.
I'm guessing when there's load on the shafts and the final drive and continuous pressure on the lever the dogs will find their way easier.
So unless someone has some light to shine on this I'll proceed and button up.

When the bike is rolling down the road the engine is spinning the clutch and the shaft even with clutch released.
The rear wheel is spinning the other shaft.
So both shafts are spinning and the dogs fly by the holes.
When you press the shift lever the dogs enter the holes and you are in gear.
You can test all of this on the bench, the goal is to shift through all the gears.

Thanks. I'll just double check that the big inner cog is flipped right and that's it then.
Thanks for all your comments

[HondaMan]

Yeah, now that I think back (WAY back, like 2006 or so) on it: I once assembled one of these from several boxes and put the inner gear on the countershaft on backward (it turns the oil pump). I only discovered my error when I had the case lying on its side and turned the crank to check shifting (no pistons in yet) and heard a scraping sound. The gear that operates the pump off that one gear biased the gear sideways just enough to make noise against the one next to it. I remember being happy that I had not yet assembled it into the frame...I forgot about that episode, sort of like one forggets about a snakebite. At least it wasn't a poisonous snake (black snake).