Hole in crankcase help

[GofastGardner]

So I have been restoring a 1980 CB750f and finally was making headway. I got it running and got the carbs relatively balanced. The other day I went to start her up and heard a loud bang and the engine appeared to seize. It would not turn over for a few tries. Then all of a sudden it worked fine and started right up. Then I noticed that oil was pooling on the ground. There is a pretty sizeable hole in the lower right side of the crankcase and the pieces that broke out have a noticeable point of impact from the inside. However, nothing appears to be loose and the engine still runs fine. What could have caused this? Its like something was loose in there and got in between the crank and the crankcase wall. Can I JB weld it back together and hope for the best? Is my project over? Thoughts and guidance would be awesome.

Ryan

 

[amc49]

Ain't no way jack.

Probably spit out a connecting rod, occasionally they can go back up running after but no way is it right as you should be down to 3 instead of 4 cylinders now.

Drain the hole and yank the bottom oil pan, that'll tell the tale and most likely will be ugly. If ground up shrapnel in the pan, motor is scrap. Most likely the seize was bearing spinning, then insistence on starting it up spits the rod out from drag on bearing.

 

[GofastGardner]

Ok, so removed the engine and took off the oil pan. There is definitely some shrapnel in there. But not a ton. Scrap either way? Any hope in rebuilding it, or should I just look for a new motor? I was hoping to split the case and replace just the bottom portion of the crankcase. Is the issue that there will be little pieces of metal in my engine destroying things?

Thanks for the help, I have next to no experience with motors...

 

[amc49]

If the bits are either aluminum looking or copper then rod bearing pieces. Crank will be scrap most likely. There is no lower case changing, the upper and lower halves are machined together to make matched parts, mismatch a new one and odds incredibly high you do the same thing five minutes after new motor is started. You have to find a new PAIR of matched cases or new engine.

New engine for you, please understand no ill intention or insult intended with what I say next at all, but if you can't identify a rod coming out of engine you will not have the skills to rebuild one using new cases. Trying to help you, not tear you down.

 

[GofastGardner]

No insult taken. I'm pretty handy mechanically but this is new territory for me. Im really doing this to learn more than anything, figure I need to start somewhere. I really appreciate the info. I think I'm going to tear into the engine anyway, just to get my head around whats going on, while I look for a new one. Maybe start to build up the skill set a bit! Such a bummer though! Motor didn't really have that many miles on it, what causes the rod bearings to fail like that?

 

[amc49]

Dirty oil, engine abuse, the like...............normally the engines are pretty durable in stock form except for the 900 which will spit rods ultra easy due to the long stroke.

At higher loads and on the bigger motors the #3 rod can spit out when the crank flexes since the only rod with no main right next to it to stabilize the crank from bending due to primary chain drive loads.