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1980 CB750K Drive Sprocket Oil Seal Replacement

NormanWI

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Hey all,

First time poster. Restoring my father's 1980 CB750K and I have hit a snag in the project. I have a pretty bad oil leak under the cover of the front sprocket and shifting lever. I have purchased new seals and I did my homework on how to remove the seal. Unfortunately, its not as easy as most of the videos I have watched make it seem. My first approach was drilling pilot holes into the gasket and sinking screws as pull points. I was not able to get it to budge. I am at a loss. Thanks in advance for any advice!

Cheers, Jared

Gasket 1.jpgGasket 2.jpg1980 CB750K.jpg
 
Case splitting time. The seal there does NOT pull straight out, it has a lip on the back end that traps in a groove in the case seal bore. A weird 2 part seal too, one piece presses up inside another to make an oil passage to lube the output bearing there. Look at your new seal, it better have that ridge. To get seal out with no splitting you pretty much have to damage the cases.

The center sprocket holding bolt has an o-ring on it too, the leak could have simply been that o-ring and not the bigger seal.

You can split the cases without disassembling the top end but the alternator rotor must come off to get cases fully apart. Rear axle of bike makes a good rotor pulling tool.
 
I would suggest getting a shop manual so you can look through and see what you are up against. You will need torque specs and torque sequences when going back together.
 
I would suggest getting a shop manual so you can look through and see what you are up against. You will need torque specs and torque sequences when going back together.

I was afraid that would be the answer I get. I am going to talk to my local gear heads and see about having it worked on for me. I don't have the proper tools or experience for that involved of a job unfortunately. Thanks for taking the time to reply. I will keep y'all posted as the situation plays out.

Cheers,

J
 
Oil seal update

Well, I ended up taking the bike to a repair shop to have that seal replaced. The owner told me he could do it without splitting the case. When I finally put the rest of the bike back together, got it started and did a test ride around the block, low and behold I had heavy leakage from the drive sprocket. When I pulled the sprocket off to check, this is what I find...

seal is installed crooked and super recessed. Clearly botched. Time to take it back and get my money back for the repair. Now way I will trust him twice.
 
No need for a pic, imagination works well enough for that type of butchery. Best hope he hasn't ruined your cases now. That lip forced in will destroy your true sealing flat there by making it oversized.

There could be a major pressure leak inside too now as well if the outside seal did not pilot correctly on the inside section. Recessed says he crushed the inside one. There MUST be an o-ring on the center sprocket bolt as well.

Repair shop owner no less, that's pretty rich..............he never stood a chance, more useless damaging bullshit.
 
No need for a pic, imagination works well enough for that type of butchery. Best hope he hasn't ruined your cases now. That lip forced in will destroy your true sealing flat there by making it oversized.

There could be a major pressure leak inside too now as well if the outside seal did not pilot correctly on the inside section. Recessed says he crushed the inside one. There MUST be an o-ring on the center sprocket bolt as well.

Repair shop owner no less, that's pretty rich..............he never stood a chance, more useless damaging bullshit.

Well shit... Fingers crossed. I have an appointment on Oct 4th to take it into the more reputable shop in Nashville. Just out of curiosity, how long do you think it will take an experienced mechanic to do this repair properly, i.e. splitting the case, etc...? Just trying to brace myself for how much this $20 seal is actually going to cost me to have replaced.
 
I cannot find which exploded diagram/service manual covers this particular oil seal! The only one that gets remotely close is the Transmission section. Could someone please provide a link so I can actually see what's going on inside the engin right behind the drive sprocket oil seal? I'm terrified my case might be fu*cked now because of this bone head shop owner. Am I basically screwed if he did something detrimental to the internals? This bike has been in the family for decades, I will be livid if this so called "professional" causes me to scrape the bike...
 
DryClutchAssyetc066.jpgoriginal_zps8atsh6p9.jpg


You're looking at the seal bore there. The deep groove is what the seal ridge locks into. This engine heavily modified by Mr. Hyde of NZ and his pic. The blue screw plugs the OEM oil passage, pretend that screw is not there. Pressured oil comes out of that hole to lube the bearing, you can see the bearing shell double imprint in the case there. The seal had to be ridged, a simple press like most and the oil pressure would blow it out, there is pressured oil between the seal and the bearing too, look at the connecting groove to back of seal. Seal has another piece pressed into the back that butts flat against bearing to seal that oil chamber off, the ridge holds all that in place.

Trying to press the seal in results in a major oil leak, one that could damage motor if run long enough.
 
countershaft_1_zpsjg0u1axo.png


See how the seal seems to be spaced away from the bearing in that pic? The inner piece that butts up to bearing side to seal it does that, the seal is tightly against the bearing there even though does not look like it.

Seals are shown as one piece in any parts breakdowns as they are sold with both pieces pressed together but not hard to get apart.
 
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Look at the biggest seal there, you can clearly see the ridge, you NEVER try to press or push in a ridged seal like that, the ridge says there is a locator groove for it.
 
Well this guy was a real dumbass for doing it...twice. 😐

How hard is it to crack the case and access that seal? Is there a video walk through somewhere online? Part of me wants to dive in and get it done, another part of me is holding back in fear of messing something up irreversibly. I have a pretty high level of mechanical competency, I just don't understand all the idiosyncrasies of my engine and its inner workings. I'd love to learn though.
 
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