Topic: Picked up a 74 cb750 what would cause this engine crack.

[Mcwilliams570]

Hello all bought a 1974 cb750 that has a bad engine comes with a spare 73 so wasnt worried. I am just curious what would have caused this to break and could a break like this be fixed.

Matt

[Stev-o]

Thinkin someone wasn't too careful trying to pull the bearing out.

[HondaMan]

That looks a lot like one of those missed-the-clutchless-shift-and-lost-the-street-race sort of accidents. 

When the gear dogs lock face-to-mate face (and not into holes) and the fork is pushing hard and the power comes back on, the cases get a mighty shove right about there...

It may have also twisted or shattered the retaining ring (half-moon shape) that holds the bearing in the case. While it might be fixable, I think different cases would be cheaper? There is not a lot of room under the shift arm to make a plate to replace it, so welding in other things, or just weld, then machining it back, then maybe re-cutting the retainer groove, gets expensive!

[Terry in Australia]

Yep, it looks like those cases have never been apart, or else I'd have guessed that someone wasn't too careful when they reassembled the top case onto the bottom. Cheers, Terry.

[CB750F2]

Maybe that is a replacement bearing and the groove is too tight for the half-moon retaining ring so when the halves were tightened down - crack! Pat

[Don R]

Was the loose metal there or did it go inside and get ground up?

[Mcwilliams570]

Thanks for the responses guys. The engine top and bottom were rebuilt. I have no idea when or what was done.

Was the loose metal there or did it go inside and get ground up?

nope it was sitting there one big piece when the cover was pulled off.

[Terry in Australia]

If the engine cases were rebuilt, then I think the problem was simple, the person assembling it didn't have the bearing locater in the slot when he belted the cases back together. Cheers, Terry.

[CB750F2]

+1 Terry but also see my previous reply. That bearing locator looks is if it is sitting high on one side although that may look that way because the camera is not pointed straight at the bearing. Pat

[Terry in Australia]

Yep, it could be Pat, but I've never seen one break from a missed shift, I've missed lots of shifts and pulled apart lots of CB750 engines, and I've never seen that damage before. Cheers, Terry.

[Dunk]

Not sure on the crack, that's an odd one. I assume the cases are scrap and the engine for parts? If so I could really use that rubber plug in the oil galley below the shift drum. If you can part with it let me know.

[MoMo]

If the engine cases were rebuilt, then I think the problem was simple, the person assembling it didn't have the bearing locater in the slot when he belted the cases back together. Cheers, Terry.

a PO supposedly had the trans rebuilt, went less than a mile when shifting became difficult and the case started to crack....Larry

[HondaMan]

Not sure on the crack, that's an odd one. I assume the cases are scrap and the engine for parts? If so I could really use that rubber plug in the oil galley below the shift drum. If you can part with it let me know.

Cadunkle: I posted a while ago about how to fix that problem. You can drill and tap the hole, then make a plug from a piece of a bolt, and set it in place with some Red Loctite. I've done a couple or 3 engines like this, and it works fine. I'm not the best at searching the forums to find it, or I'd send you a link to it: maybe someone else here might be more adept at that?

[Dunk]

Jackpot, my other K1 had one. Yet another part scavenged. If I can come up with another plug before I restore that then great, if not, I'll do a tap and plug as Hondaman suggests.