Topic: Differences between CB750 crankcases

[DuckFan]

Hi all,

So I have a 1976 CB750F1 and the crankcase has that massive hole behind the primary drive sprocket that everyone knows and loves. I need to replace it and my question is, what is the difference between the various years as far as the crankcases? Is there a difference between the K and F models with crankcases?
There are a lot of 77' and 78' cases on eBay, and I haven't gotten a straight enough answer to make me feel comfortable enough to pull the trigger and buy one. Will a 77'/78' work just fine or am I terribly mistaken? Thanks everyone!

-DuckFan

[drumstyx]

On a similar note -- that hole is from a broken chain, right? Has anyone ever thought of welding in some reinforcement there? Another 1/4" of aluminum and it could probably withstand a breakage easily.

[DuckFan]

Unfortunately this hole is MASSIVE, through both blocks, and destroying one of the holes where the bolts insert. I'm not sure how salvageable it is...
 

[drumstyx]

As far as salvaging, I'm not sure. I've heard of people welding the piece back in (or welding up some missing bits) and then machining the surface flat.

I was just thinking as a preventative measure. Not much use to you, I know, but just a thought that occurred to me when I read your situation.

[HondaMan]

As far as salvaging, I'm not sure. I've heard of people welding the piece back in (or welding up some missing bits) and then machining the surface flat.

I was just thinking as a preventative measure. Not much use to you, I know, but just a thought that occurred to me when I read your situation.

Honda has occasionally had a "guide" that held the loose chain onto the front sprocket's teeth so it would feed out the back, laying it harmlessly on the pavement. If you took care of the chain, it did not bunch up or kink the links, and it worked. But...most of the broken chains were due to NOT taking care of them, in which case the guide got crumpled up by the 50+ HP driving the kinked links into a chainsaw, then pushed the whole mass thru the cases anyway.

Moral: oil the drivechain?

[DuckFan]

Advice taken for sure! Although I doubt the previous owner did. There was also a resin build up within the combustion chambers, almost like sugar. I'm not sure what happened previously before i got ahold of it!

But back to my original question, what are the differences between the crankcases from a 76' F1 and other years of the SOHC?

[pmanning]

I bought my first 750 because the owner had thrown a chain through the cases and couldn't afford the repair cost. The hole was also large and included one of the crankcase mounting bolts.  Maybe I was lucky but I found a shop that could repair the damage including the mounting bolt location.  Rode the bike for 70,000 miles after that with no problems.

[754]

 The main difference in cases is around 1973 they went to a wider counter shaft bearing. Then for 77/78 they used a longer sprocket shaft, bearing area  might have differences those years... this is as far as just having a running motor, not being correct.
 So look for 73-76 if you can.

[Jerry Rxman Griffin aka MuthaF'er]

Hi all,

So I have a 1976 CB750F1 and the crankcase has that massive hole behind the primary drive sprocket that everyone knows and loves. I need to replace it and my question is, what is the difference between the various years as far as the crankcases? Is there a difference between the K and F models with crankcases?
There are a lot of 77' and 78' cases on eBay, and I haven't gotten a straight enough answer to make me feel comfortable enough to pull the trigger and buy one. Will a 77'/78' work just fine or am I terribly mistaken? Thanks everyone!

-DuckFan

I am using a 78 K case on my 75 F hotrod build. It will work but you have to overcome a few obstacles. The 77/78 cases have a 10mm offset for the double row bearing on the output shaft. Everything to it's rear also needs the 10mm offset ie rear sprocket carrier and thus the swing arm and chain guard. If you wanted to do it right you'd also want to offset the left shock mount too. Then there's the 630 chain mount on the 77/78. I used a CB650 530 front sprocket which does not have the double shoulder (one on each side) as it has only a shoulder on one side and mounts on the end of the shaft so I could use a 530 chain. Now with this said you can 'rig it up' and try not to do everything the 'right way' but you need chain alignment one way or another.