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Discussion Starter · #1 ·
I bought my first bike, 1973 Honda CB750 Custom Bobber. Haven't owned it just a couple weeks and one day after riding I noticed it was leaking oil. Not a normal small leak, a substantial oil leak. After investigating I noticed was basically looks like a whole in part of the engine or gearbox cover. To be clear the bike will start up and get up and down the road but loses a lot of oil in the process. I know enough about motorcycles and engines to be dangerous but still very ignorant. Any mechanics here or anyone more mechanically inclined than I am that can tell me how screwed I am based of the pictures attached? What exactly are my options here? Appreciate any help you can offer.
 

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I bought my first bike, 1973 Honda CB750 Custom Bobber. Haven't owned it just a couple weeks and one day after riding I noticed it was leaking oil. Not a normal small leak, a substantial oil leak. After investigating I noticed was basically looks like a whole in part of the engine or gearbox cover. To be clear the bike will start up and get up and down the road but loses a lot of oil in the process. I know enough about motorcycles and engines to be dangerous but still very ignorant. Any mechanics here or anyone more mechanically inclined than I am that can tell me how screwed I am based of the pictures attached? What exactly are my options here? Appreciate any help you can offer.
That's common on the 750 same thing happened to my neighbor when the chain gets loose it will expand the faster you go. I fixed his although it wasn't that bad , I cleaned the area really good and got epoxy puddy and used it to fill in the spot then I got epoxy in a tube and went over the puddy waited 24 hours before starting it to ride . He drove it later sold to a friend that drove a couple of years , sold to a friend that drove for years before he sold it. Got to be clean and wait for it to cure .
 

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Ouch looks like it's engine replacement time. That hole is catastrophic if you have the piece, and it is whole not broken up into many pieces it might be able to be tig welded back together. Good luck.
 

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That's some serious damage ... You should not ride it like this .Jb weld may or may not work who knows , a new engine case is the best option.
Anyway if you ride it like this at least your chain will always be well lubed 🤪.
 

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Discussion Starter · #5 ·
That's some serious damage ... You should not ride it like this .Jb weld may or may not work who knows , a new engine case is the best option.
Anyway if you ride it like this at least your chain will always be well lubed 🤪.
I haven't ridden since I found this, but I've seriously been contemplating JB weld or Steel Weld, not sure what else can go wrong if these products fail. 🤷‍♂️ But I'm not really sure if this is a certain section of the engine case that can just be replaced or if the entire engine needs to be swapped.
 

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Owner if he did the damage would have known he did it, that is some serious damage. Engine is done as of now, you have transmission gears showing there. No glue will hold that and don't care what the other guy said. You must have the pieces to begin with but the damage goes across case split line and that requires crush to seal properly. If you have the pieces welding at the least to fix it and by the best, amateur work there will just bring more grief. The main cases are matched pieces and bought in pairs. Pic here......


Some incompetent let the chain get way too loose to jump or break, it did and the pinch point there then wrecked the cases.
 

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Unfortunately that's a lot of work even if you buy the engine case . If you are able to do it on your own then I would say go for it ... Otherwise the cost for a mechanic will be high and I don't know if you want to spend a lot of money on this bike .
 

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Discussion Starter · #10 ·
It's time for a new bike. In all seriousness that motorcycle engine is toast. And would cost more to repaire than the bike will ever be worth. Did the previous owner sell it to you like that??
Well I'm not sure I'd go as far as just getting rid of the bike. I'm finding that old cb750 motors are out there for roughly $300-400. Then it's just a matter of swapping out the engine...correct me if I'm wrong please. But the previous owners claims this wasn't an issue when I bought it. It may have been a cracked casing but it wasn't a full blown hole in the engine. It 100% would've been noticeable during the test ride. It regurgitates oil from that hole.
 

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The only SOHC 750 engine you are going to get will be a dead one at that price. A good useable CB750 engine in long running shape is worth easily $1000, they are highly valued by the collector market. Just the bare GOOD cases alone can go for $300 easy.
 

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Was the chain loose? Did you adjust the chain?Its common for people to do cheap repairs in order to sell a bike and then its not their problem anymore.
Also 300$ seems way too low for a good engine!
 

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I've got DOHC for that that then ran for years and even free ones but those are not SOHC which has been locked onto by collectors as worth far more while making less power than DOHC. Any SOHC stuff figure out a good price then 3X-5X that as what they commonly go for.

Like Chrysler true 426 Hemi (I worked on them), nothing special there but just buy parts for one.
 

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Discussion Starter · #14 ·
I've got DOHC for that that then ran for years and even free ones but those are not SOHC which has been locked onto by collectors as worth far more while making less power than DOHC. Any SOHC stuff figure out a good price then 3X-5X that as what they commonly go for.

Like Chrysler true 426 Hemi (I worked on them), nothing special there but just buy parts for one.
Sorry you'll have to educate me. What's the difference between SOHC and DOHC? And to be perfectly clear I'm in western NY so riding season is limited and I really only plan to ride my commute to work which is 4 miles one way and occasional joy ride. So I'm not looking to put a lot of miles on said engine.
 

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One needs the pieces that came loose, they include part of the case split line and you will play h-ll getting around that. Go back and look at the pic showing trans gears showing, it won't matter how good the welding guy is. First thing he will ask you is where are the pieces? BTDT that type of massive repair before on $20K cost race heads.
 

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Let’s look at the logic and facts And options. 1. You can’t drive it like that. 2. A good welder might be able to fix it but probably not after you put on JBWeld. 3. JB Weld putty stick might work. I fixed the bottom of my crank case last year and it’s still holding. Not as bad as yours. 4. New engine.
If a welder can’t help, or you can’t find one, or don’t want to, I’d try JB.
Take off the chain and gear to give yourself room to work. Clean the area really good. Sand where you want it to stick. Clean it with alcohol, not tequila. Make a ball into a pancake with the putty and slap it on good. Don’t plan on it being pretty, you won’t have time to do that. The putty hardens FAST. Practice with a marble sized ball first so you can see what it does.
You can see I had to cover a bolt to hold it in. I put a slice of scotch tape on the exposed shaft of the bolt so the putty wouldn’t stick to it in case I ever have to crack the case. This is the under side of the engine. I hit it on the trailer ramp picking it up from the seller. I cried for a while but then I fixed it. I was pretty lucky.
Good luck and Best,
Steve
Motor vehicle Material property Bumper Auto part Automotive exterior

Daytime Motor vehicle Mode of transport Font Gas
 

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There is no logic or fact there, clearly that situation does not match the OP here at all. The fracture here does not go off into fluid at all and the bolt boss is a localized stiffener that is not present in the OP situation. So much for the 'facts'.This one was fixable but the other is not, not without those filler pieces. Still no mention of them.
 

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Yeah. Ok. Just my 2 cents on something to try before spending a bunch of money. I didn’t see mention of the missing pieces in the original or subsequent posts. If he can find a welder/machinist who can fix it that’s great. If my choices were buy an engine or try jb weld, i would try it first.
Best,
Steve
 
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