Topic: Cracked Lower Crankcase

[74CB750Chopper]

So today was the day I was going to put my engine back together after being in pieces for the last two months for cleaning, painting, inspection. When I was tightening the bolts I failed to check that everything was properly seated and I cracked the case pretty bad.

 In my defense this was the second time I closed a CB750 case back up and I was a bit distracted by my 18 month old boy running all over my garage trying to keep him from hitting things with the hammer and going for a swim in a pan of engine degreaser.

Anyways my question is when I get a new case (I am pretty sure based on what I have read on the forum that they have to match and I can't just replace the bottom half ) I have a K4 so do I have to use a K4 case or are they identical for a certain range of years. I know alot of things are interchangeable for pretty much everything 70-76 but the case seems like it might have minor yet significant differences. I need all the guts to fit back in the new case.

Once I get the new case do I need to replace the journals? I am worried about that because I have no idea how to do that stuff, I am pretty sure I can figure it out if I can find some guidance. Anyways I am just looking for any pointers on how to deal with this situation from someone who may have had a damaged crankcase that they replaced. I will most likely start a build thread since I have had so many complications I figure that maybe someone can learn from my many mistakes.

[Cqyqte]

Can't say for sure but I would try to find a replacement case part that you cracked.  I had a CB450 and found on reassembly that the lowercase was cracked and I sourced a year correct replacement for $40 and simply switched  the broken one out.  Being the lowercase nothing was mounted in it as everything is held in the uppercase when you dismantle.  When I button everything up everything was fine, the important thing to remember is that Honda has incredibly high levels of accuracy so the tolerances should be the same for each case.

For the cost I would try it first before doing a complete teardown and replace both top and bottom cases. 

[MCRider]

Can't say for sure but I would try to find a replacement case part that you cracked.  I had a CB450 and found on reassembly that the lowercase was cracked and I sourced a year correct replacement for $40 and simply switched  the broken one out.  Being the lowercase nothing was mounted in it as everything is held in the uppercase when you dismantle.  When I button everything up everything was fine, the important thing to remember is that Honda has incredibly high levels of accuracy so the tolerances should be the same for each case.

For the cost I would try it first before doing a complete teardown and replace both top and bottom cases. 

The CB450 has a roller bearing crank, so you can switch just the bottom half. The CB750 has a plane (plain) bearing crank and the cases must travel as a set.  They are line bored at the factory for the plane bearings, and that requires they be a set only.

[74CB750Chopper]

Here are some pictures of the crack,

[Don R]

Was a shift fork out of place? It may be bent if so.

[74CB750Chopper]

Replacement is on the way lol. Ill check out the forks, thanks for the tip! Hopefully they are all good.