Please tell me this isn't as bad as I think...78 cb750f problem

[sb750]

Hey everyone, So I recently got a 1978 cb750f and have been loving it! Such a nice bike and I love this history and most of all the the thing I love about this bike is the super helpful online community I have found. Literally every road block I have hit along the way I have been able to get online and find the help I need.

I'm hoping this will be the case for today but I fear there may be no good answer.

I decided to put a nice new chain and sprockets on my cb cause the ones it had when I bought it were looking a little worn. Also, I have had a slow oil leak coming from around the sprocket so I thought this would be a good chance to investigate that and maybe order and put a new front sprocket o-ring seal on.

But i found a terrible surprise when I pulled the sprocket off. The front sprocket leak is not coming from the o-ring as I thought, but from what appears to be chain break damage that cracked the engine case. Its a hairline fracture that has been oozing oil. Here are some photos. First shows the damage, It doesn't look to bad, or like it would leak, but the second shows the bike running, you can see the oil oozing.



so i guess my question is what can I do now if anything to fix this? and until the fix is it a bad idea to ride the bike in this condition. Its my daily driver and only means of transportation at this moment.

I did a little web search and see that chain break damage is not too uncommon, and most people said it needs to be welded. Is this my best option? or replacing this part of the case? if so where would I find one?

Would any of you experienced guys out there know of any product I could pick up to temporarliy patch the leak tell a more permanent solution can be financed?

Hoping and praying for some good news but not expecting it!

Thanks guys.

 

[dirtdigger]

Welding would be my second option, first would be replacing the cases. But for temporary get some JB Weld, clean it with some good brake clean or acetone. Then scratch it good with rough sand paper all around the broken area. Clean it again and spread JB Weld on the area. The key is to make sure you get all of the oil off and let it dry for 1 to 2 days before starting it so you dont get oil around the crack until it is cured. If you do a good job it will last a long time with the JB. I dont like the welding unless you have a very good welder do it as it will warp the cases if the welder lets it get too hot. If you replace the cases you need the correct year and you need the upper and lower as they are matched sets.

 

[tommycarl]

I had one break in '72... put a hole through the case about the size of a dime (although it was rectangular, not round). I epoxied a thin piece of aluminum (1/8" if I remember correctly) over the hole and had no problem after that. Just be sure to check clearance with the chain and sprocket before you put something too thick there. I probably put another 25k on the bike before I wrecked it... now THERE was a problem!

 

[Marty]

JB Weld is great stuff! Just clean it up real good to bare metal, maybe stop drill a little in crack end points so it wont spread any more from vibrations. I would file the crack to make a v inside so that the JB has some bit into it and spread it over the bare metal you cleaned around the area. I think it will be fine for a long time. At least you could enjoy the bike till you decide if you want to re case or build up another engine down the road.

 

[grepper]

I've read about this happening before. If it's a real hole, then I think the only solution is to pull the engine, split the case and have it welded.

Given it's just a crack, I like Marty's suggestion. Let the JB Weld cure for a good long time. Since it's winter time and you're in no hurry to ride again, I'd give it a week to cure before filling it with oil again and starting it.

 

[sb750]

Thanks so much guys. I Have some time off finally so I'm going to be working on the bike this weekend. As far as filing a v for the jb weld, how large of v do you think would be needed. And for stop drilling the crack ends, does anyone know how think the case is and how deep I should set the stop? Just worried I'll make it worse trying to make it better

 

[tommycarl]

It's been years since I fixed mine, but I'm pretty sure the safe is very thin at that spot... at least it was on my '71. By thin I mean 1/8" or even less. There's no pressure in the case, so no reason to worry about something pushing the patch off.

Seriously, I wouldn't make a big deal of it. I'd clean it and put an epoxy/JB weld coat on it, and maybe cover it with a thin piece of aluminum... and by thin I would even consider something as thin as roof flashing. There's probably really no need for the aluminum "cover", it's just that I did it on mine and no problem, so know it worked for me.

The most important part of this job is to clean the area really well.

 

[Marty]

If you were to stop drill the crack ends, just use a small bit a little larger than the crack like maybe 1/8th inch. You don't need to go thru the case with the drill, just a little touch so as to stop the tear in the metal. Then as far as the v filed in the crack, just a light score to give the JB weld a little more grip to seal. Clean the area around to bare metal and remove all traces of grease oil . Put your JB weld on and you can put that thin plate on for added security if you want . will help with debris being slung from the chain on your repair.

 

[kal-el428]

In the Fall, my chain snapped and on its way out, it slapped the case.

We use a product at work(devcon aluminum putty) to repair aluminum castings, and I decided to give it a try.

Wont know for sure till the spring if it will hold up.
But seeing how it applied and cured, I have high hopes!!

 

[Marty]

Ouch! Nice repair job though. I think it will be fine also. It really pays to keep a eye on your chain to prevent this. Which reminds me , I need a new one too.