Topic: Splitting the Cases Without Removing the Top End

[Ellz10]

Is it possible?

If so, is it worth the time/hassle?

1977 Honda CB750K

[Floshenbarnical]

I read a really, really long thread on here recently detailing the process. A forumite was having some transmission issues (his lifter fork was damaged if I recall, and some of his gear dogs had sheared off). He was in and out of the bottom end several times without removing the top end. It involved pulling the engine and turning it upside down. I'm sorry I can't be more helpful but the Search function might save you, or maybe a vet can link you to the thread. It was a really informative read that I hope never to have to try myself.

[denward17]

I found:

http://forums.sohc4.net/index.php?topic=105038.0

[Ellz10]

I read a really, really long thread on here recently detailing the process. A forumite was having some transmission issues (his lifter fork was damaged if I recall, and some of his gear dogs had sheared off). He was in and out of the bottom end several times without removing the top end. It involved pulling the engine and turning it upside down. I'm sorry I can't be more helpful but the Search function might save you, or maybe a vet can link you to the thread. It was a really informative read that I hope never to have to try myself.

Ironically enough, I have a feeling that's what's going on with mine!

I found:

http://forums.sohc4.net/index.php?topic=105038.0

Thank you Dennis!!!!!

[Tracksnblades1]

You’ll find it E10…

It’s making enough noise, you’ll see it.

[Ellz10]

You’ll find it E10…

It’s making enough noise, you’ll see it.

Huh? What's E10? Like post #10 on that thread?

[MauiK3]

You shouldn’t have much trouble spotting bad forks etc.. I suggest getting Hondaman’s book as a guide, will save you time, money and heartache.

[Ellz10]

You shouldn’t have much trouble spotting bad forks etc.. I suggest getting Hondaman’s book as a guide, will save you time, money and heartache.

Oh I bought Mark's book before I even started tearing the motor apart! It's invaluable!

[newday777]

You’ll find it E10…

It’s making enough noise, you’ll see it.

Huh? What's E10? Like post #10 on that thread?

Looks to be shortened Ellz10

[HondaMan]

Absolutely!
I do it often enough. In truth, if I'm having trouble getting new crank bearings for a given rebuild, I'll put the top end together when I have the chance, then turn it upside down with a special wood block I made to protect the breather cover, and a stack of 4x4 + 2x4 to hold the back up level, resting on the rear cross-hole bungs. Then I do whatever I can until the bearings come in, and close it up last.

[PeWe]

It's good that the CB750 engine can be opened without taking off head and cylinder.

My K6 weep a little oil between case halves so it need to be sealed. Last time with Permatex Aviation no 3 since I did not have good Hondabond.

Pull engine out and remove all M6 bolts and bigger crank bolts.
Good chance to check crank bearings wear too. Prim chain tensioner not old.

[70CB750]

Been there with Red.  She was leaking and thats how I resealed her.  Maybe done it second time whenshe would not shift pass 3rd gear, not sure anymore. 

Engine stand helps but you can hack a wooden stand that will work just fine. 

[Ellz10]

Good to know fella, thanks!

After talking to a few people, I'm guessing I have bent gear forks and/or bad gear(s) somewhere. I had my gears undercut at CycleX and I remember inspecting them beforehand and they looked great, but I guess you just never know.

[HondaMan]

I had my gears undercut at CycleX and I remember inspecting them beforehand and they looked great, but I guess you just never know.

This could well account for the extra-loud gearbox noises you are hearing, if the scenario I outlined above (about the "C" fork not being engaged with the mainshaft) is the case. The extra slot clearance in recut gearboxes will make them 'rattle' quite a bit more than the normal ones, which was always apparent in the racing 750s I remember from their days. Some of those riders/wrenchers would open the slots in the receiving gears wider so they could "shift faster" (translation: avoid using the clutch at all) and a precious few also reduced the size of the dogs (which broke off if shifted often enough without the clutch) so as to make the moves quicker. In truth, the guys who actually WON the races did it with stock gearboxes or the "racing kit" 6-speed versions from Yoshimura.

[Floshenbarnical]

I had my gears undercut at CycleX and I remember inspecting them beforehand and they looked great, but I guess you just never know.

This could well account for the extra-loud gearbox noises you are hearing, if the scenario I outlined above (about the "C" fork not being engaged with the mainshaft) is the case. The extra slot clearance in recut gearboxes will make them 'rattle' quite a bit more than the normal ones, which was always apparent in the racing 750s I remember from their days. Some of those riders/wrenchers would open the slots in the receiving gears wider so they could "shift faster" (translation: avoid using the clutch at all) and a precious few also reduced the size of the dogs (which broke off if shifted often enough without the clutch) so as to make the moves quicker. In truth, the guys who actually WON the races did it with stock gearboxes or the "racing kit" 6-speed versions from Yoshimura.

I can shift my 750F2 without using the clutch, is this suggestive of a transmission problem or previous work having been done to it?

[bryanj]

Neither, most can do it

[HondaMan]

I had my gears undercut at CycleX and I remember inspecting them beforehand and they looked great, but I guess you just never know.

This could well account for the extra-loud gearbox noises you are hearing, if the scenario I outlined above (about the "C" fork not being engaged with the mainshaft) is the case. The extra slot clearance in recut gearboxes will make them 'rattle' quite a bit more than the normal ones, which was always apparent in the racing 750s I remember from their days. Some of those riders/wrenchers would open the slots in the receiving gears wider so they could "shift faster" (translation: avoid using the clutch at all) and a precious few also reduced the size of the dogs (which broke off if shifted often enough without the clutch) so as to make the moves quicker. In truth, the guys who actually WON the races did it with stock gearboxes or the "racing kit" 6-speed versions from Yoshimura.

I can shift my 750F2 without using the clutch, is this suggestive of a transmission problem or previous work having been done to it?

Normally we can shift sans clutch if we let off the throttle just-so and shift quickly and firmly. The racers didn't want to let up on their throttles, though: with the 500 this worked OK but the 750 needed some help, like tapered dogs, to get it there. I didn't work with any 550 roadracers that I remember, so I can't speak to them: they have a more sophisticated gearbox than the 500.

[Floshenbarnical]

Thanks again mate. What does it mean if I can find neutral between most gears, lol

[HondaMan]

Thanks again mate. What does it mean if I can find neutral between most gears, lol

Mine's like that, too. I have a Neutral between 2-3 and 4-5 if I'm lazy and don't pull the lever all the way. It always had the 2-3 Neutral (many 750s do) but the 4-5 Neutral showed up at about 110,000 miles for the first time. I means the shims on the end of mine's mainshaft are wearing a little thin. But, at over 150k miles, I'm not unhappy about it.

[Floshenbarnical]

Thanks again mate. What does it mean if I can find neutral between most gears, lol

Mine's like that, too. I have a Neutral between 2-3 and 4-5 if I'm lazy and don't pull the lever all the way. It always had the 2-3 Neutral (many 750s do) but the 4-5 Neutral showed up at about 110,000 miles for the first time. I means the shims on the end of mine's mainshaft are wearing a little thin. But, at over 150k miles, I'm not unhappy about it.

Brilliant, sounds like mine may not be totally buggered then.

[Geoff Hastings]

Quite a simple job to strip the bottom end without disturbing the top end. I had rebuilt my engine not realising the previous person to rebuild the engine had left a spacer out of the gearbox. I took it for a brief test ride and all seemed ok but was aware of a knock and the drive chain snatched when ticking over on the side stand. To take the engine back out and strip it again after 10 mins riding was annoying to say the least.