Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Blasta on October 26, 2017, 01:59:05 AM
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Hi Guys,
My K4 has developed a leak from the crankcase seal behind the ignition. Can I replace that seal without splitting the cases?
Thanks
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Simple answer is nope. The seal is rubber over metal and has a lip that sits in a groove in the case. How bad is the leak ? I have never seen these leak on the contact to the case, only on the ID of the seal to the crank. it will have to be the big engine out.
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The leak is bad enough that I want to fix it, and I think it is leaking from the seal to the crank.
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The leak is bad enough that I want to fix it, and I think it is leaking from the seal to the crank.
While you have it apart, change all the engine seals, they do go hard over time ;)
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I will replace all the seals while I'm at it, but I'm still a bit confused.
I bought a seal kit in advance. Here's 2 pics of it, one from each side;
[attachimg=1][attachimg=2]
And here's the seal in the bike.
[attachimg=3]
The seal appears to have no lips or ridges or anything on the outside surface. The seal in the bike is flush with the casings and they don't seem to be have any lip or anything holding the seal in. I looked in the manual I have and I can't see what's holding the seal in.
After checking all that I thought the seal would come straight out and had a go, didn't matter if I damaged it it was already leaking and coming out one way or another. You can see some damage to it in the pic.
So, I still don't understand why it won't just come straight out. Can someone explain what holds the seal in?
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Hi,
They usually have a rounded "lip" running round the outside diameter that that sits in a groove machined in the crankcase seal housing and trapped in place when everything is bolted up, yours look like standard oil seals.
https://www.davidsilverspares.co.uk/graphics/parts/91201300003P_large.jpg
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OK, I see now. They must have sent me the wrong seal eventhough it has the correct part number on the bag.
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Put it in and see if it works... if it does, all well and good.
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Yeah I think I will, maybe with a tiny smear of sealant around the outside edge.
Now I just have to get the old one out. Any ideas?
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Put it in and see if it works... if it does, all well and good.
How can it work?. It won't be sealed, it will just be a seal in a hole with nothing clamping it in there, ie, the casings.
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Not all lip-seals are locked in place, most just press in.... so long as its the correct size and material...
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Agree, but I think Blasta will find that this seal is locked in place. Also, the only, non bodge way of removal is by splitting the crank cases and the same goes for insertion.
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That's how 99.9% of seals work by relying on a tight fit. Very few have a locating lip. Its purpose will be to ensure correct placement when building the engine.
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If its only a rubber lip I would expect to be able to get it out and fit a standard seal. Just make sure it is the same thickness and note how far the original seal is in. If need be make some sort of tapered seal installation tool to make sure you don't damage the lip seal that rides on the crank. As already said you have nothing to lose and everything to gain.
Certainly easier than pulling the engine and stripping, never mind the costs involved.
What I will say is the end of the crank doesn't look very happy and there could oxidisation on the crank where the seal sits. But that will be revealed when you get the seal out.
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I'd follow the initial advice of dismantling it to change seals, you really don't want to make it worse by risk of compromising the surfaces or the new seal.
But if you really want to remove it in situ then some do it by drilling and using self tapping screws in the face of the seal. Another is to drill two holes opossite each other and putting an Allen key small end into each so you can get a straight pull on them, put mole grips on the Allen keys and use something like a slide hammer to drift it out.
In clamped cases like this application the steel internal and rubber coating is usually getting close to an interference fit in the cases, that generally makes them very reluctant to come out. And hard to get replacement in without damage to it's structure. You'd need something like a flat faced drift with hollow centre to clear the crank.
As moorey points out, it looks like the original seal may be compromised by corrosion of these crank surface.
Blocked crankcase breathers wouldn't help it either, so worth looking at.
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Not all lip-seals are locked in place, most just press in.... so long as its the correct size and material...
Not all seals have 50 psi of oil pushing on the back of it. The lip is there for a reason.
And the seal in the picture is not a crank seal, I feel a bodge coming on and some damaged crank cases >:( looks like Fred Flintstone has already been in there.
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I agree with Trigger, Honda did not use that type of seal just to be awkward, they were used because they keep crankcase pressure/oil in as opposed to keeping oil splashes in, would hate one to "pop" out at high revs.
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That picture shows the wrong seal, there should be a definite "Lip" on the side with the metal washer. The one in there has been bodged in and is probably why its leaking BUT why was it done in the first place as these don't often leak, is there a bigger problem inside? I have a NOS one in my hand as its the same as a 500
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Just for Paul Gledhill >>>>https://www.youtube.com/watch?v=tRsm2G7bulI
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If you go to the effort of prising out the old seal and replacing it with new seal only to find it still leaks you'll have to remove the engine anyway. There's an old but true saying 'it takes longer to bodge a job than to do it properly in the first place'. If Honda could have saved a few yen by fitting non lipped seals they would have done. Save yourself serious frustration and a potentially wrecked engine and fit genuine Honda seals, you can renew the cam and primary chains whilst you are in there and have a fresh engine.
Good luck with whatever option you go for and keep us posted.
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Just out of interest blasta, you may want to look at the link below. The seal you have bought has no lip but has the correct part number?, i’m no expert on honda parts but it may be that the replacement seals are lip free to enable fitting without stripping the motor, if this works then you will have saved a ton of work or money.... so imo worth a try. However it does look to be pretty crappy around that seal so if it looks bad in there once you remove the old one it may be better to strip the motor anyway. Have a dig and see what you think. Good luck!
http://forums.sohc4.net/index.php?topic=74771.0 (http://forums.sohc4.net/index.php?topic=74771.0)
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That one on the net site's interesting, as pointed out above by Bryan they don't usually leak there. It's possible that both this example and Rob's posted one area suffering initially from crankcase pressure build up as someone has mucked about with the breathers on the USA one.
We had on here about 4 or so years ago a (can't remember who's it was) blue 750 K2 in Kent that had the breathers connected wrongly and kept pushing oil out of different places, corrected those and went away.
It looks like previous owners go straight for the seal and make a dog's dinner of it.
If you want to put an original lipped seal into it, put one of those smooth stainless jubilee clips around it, after first greasing with silicone, then place this against the cases and push the seal through with a drift. The rubber flange is just that and not supported by a steel lip inside (you'd have to check this).
The purpose of the lip in car crank applications is to make it, to an extent, self sealing against crankcase pressure buildup and stop oil pushing past the outer edges. They are normal at the flywheel end to make sure the clutch doesn't get contaminated.
I'd still split this one to get a better longterm fix though, it's not that hard...... Is it?
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I think if you wrap some draft excluder round the end of the crank it may work, well it keeps drafts out ;D ;D ;D
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If the engine's that windy, may need that on the piston rings ;D
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Thanks for all the advice, and thanks Rob62 for that link. All very interesting.
I've had the bike for about 6 months and the guy I bought it from had rebuilt the motor. He's in his 60s and has been riding/racing/rebuilding 754s since they came out in the 70s.
He did the motor many years before I bought it off him and sorted all the rest of the bike, I've done 1600km since.
I spoke to him after I posted this thread and he's offered to help me take the motor out and replace the seal. He also thought that the alternative option would probably work, and after reading the thread that Rob62 posted I think I agree. I think it's worth trying, if it leaks I'm really no worse off and I'll take him up on his offer.
So now I just have to get the old seal out, and as you can see from the pic I had a go yesterday and was unsuccessful. I'll have another go and let you guys know how it turns out.
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Just for Paul Gledhill >>>>https://www.youtube.com/watch?v=tRsm2G7bulI
Not my favoriye episode ;) ;)
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If you really can’t be arsed to do a full strip down you could take the motor out, turn it upside down and undo the bottom case. You’d have to remove anything that attaches to both, I can only think of the ignition, clutch / alternator covers and gear change mechanism.
You could then lift the end of the crank to slip the old seal off and the new one on. Assembly is the reverse of disassembly, as Mr Haynes would say. The cases use sealant so getting a gasket in there isn’t an issue.
The are other seals you could / should change while you are there such as the final drive seal, but this looks like the minimum work route, the sort of thing I used to see going on in race paddocks when working to a deadline. There are potentially disastrous pitfalls such as the cam chain moving a tooth so you’d have to be careful there.
Others may be along soon in a state of shock to tell you why this wouldn’t work, it certainly isn’t the best way, but I am good at spotting a short cut.
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I know lads who've used that method but not even gone as far removing the engine from the frame, albeit not on CB750's, but on other Jap fours.
I'd still have a go at doing it with the engine in one piece first, as said, what's there to lose, just be careful to not damage the crank. After all you have documented evidence of it working (posted by Rob 62)
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I always used to use the upside down engine route when working on CB750 gearboxes, why strip the top end when there is no need, just remove the side covers, gear change mechanism etc off and take the bottom crankcase off. Saves a load of work and hassle.
Cheers
Dennis
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Yes, I'd spoken to a few people and was going to do the upside down thing if required.......but I don't think it is.
I had another go at the seal and it actually came out fairly easily and I managed not to damage anything.
The new seal was a nice snug fit and I put a little sealant around the edge.[attachimg=1]
Looks pretty good to me and I'm pretty confident it's not going to leak (famous last words :) ).
I've got a few other things to do to it before it's back on the road, but I let you know if it's worked when I get a few kms on it.
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Nice one blasta... hopefully you have solved your problem without major surgery !!
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Good man at least you have tried it. Hopefully all will be good and you can think how much money you have saved yourself.
Nothing ventured.........
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Hope it works, good job done if it has.
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Looks alot better than the initial photos so hopefully you'll get a reasonable assessment of it now.
As some of the others have said, it's really not hard to split the bottom with it upside down if you need to go that route so either way you'll get it sorted.
It's quite often looked at as a major task to remove the engine and carry out further work but I've changed gearbox components on these in about 3 hours from stop to running again. I'd rather do one of these than lay under a car to get a gearbox out for a clutch change, which is after all a wearable consumer item.
If you needed eventually to go further, then the output sprocket shaft is the other major one to replace at the same time while it's apart.
Be interesting to see how it gets on now with that new seal in it.
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My CB72 engine output shaft seal leaks a little if the bikes been stood for a week, but only when you start it up, not while standing. It has the original 1962 seals in it, I quite like this feature though, it puts a little oil onto the chain until the engine is warm and the seal softens then stops leaking.
Or was that a unique Honda design chain oiling feature? ;D
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Fingers crossed
Cheers
Dennis
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Well, I got the bike back together and took it for a ride yesterday. I did about 250 km on it and the seal is holding and there's no oil leak. Hopefully it continues that way :)
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Good news, great when you can get away with a dodge like that 👍
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Report back regularly please. It's still early days yet. :)
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Not all lip-seals are locked in place, most just press in.... so long as its the correct size and material...
Not all seals have 50 psi of oil pushing on the back of it. The lip is there for a reason.
And the seal in the picture is not a crank seal, I feel a bodge coming on and some damaged crank cases >:( looks like Fred Flintstone has already been in there.
Just read this topic, correct me if i am wrong, if the OP had 50psi going on in the crankcase this seal would be the least of his worries would it not, unless the seal is part of the lubrication system which i doubt because crank end float would reduce any oil pressure on the seal, i am not saying this is the correct way to replace the seal, just wondering where the 50psi came from.
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The crankcase isn't pressurised, just the oil in the oilways.
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The crankcase isn't pressurised, just the oil in the oilways.
We know the case is not pressurised, and as the seal is static and the crank has end float the seal isn't either, so where did the 50psi come from?
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The weather here is perfect for riding at the moment and I can't keep off the bike :).....so I've put about 700km on it now and it's still ok.
Will report back as I get more kms on it.
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It will be ok, so long as you managed to avoid damaging the lip as you pressed the seal over the shaft... which you appear to have done successfully..... if it was gonna leak it would have done by now. Relax, job done 8)....
Hope I haven’t just put a curse on the job :o
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Report back next week when you have done 5k on it. Enjoy. ;)
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Nice to hear of a good outcome
Cheers
Dennis
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UPDATE.
I've got busy with some other projects, but have managed to put about 3000km on the bike since posting last. The oil seal seems to be holding and no oil has leaked. Seems to have been a permanent fix.
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A good outcome for very little money or effort. Keep updating please. Let’s see what happens long term. I think you will be OK . ;)
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I would of also tried the fix you did as there was nothing to loose
Well done, I bet your well chuffed it's a success
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