Topic: Engine question - need some guidance

[arncruz]

I recently bought a 1976 CB750 for $250 and a stuck engine.  The fins on the engine are mildly damaged.  I have a 1978 CB750 K engine that has a crack upper crank case.  I was thinking about taking the top end ( cylinder jugs, pistons, and cylinder head/cover)  of the 1978 CB750 K and transplanting it onto the transmission of the 1976 CB750 engine.  Does anyone know if this is possible?  The 1978 CB750 K engine was running a month ago, but was spilling a lot of oil.  I removed it and found a hair line fracture on the upper crankcase near the front sprocket probably from a chain that broke loose. 

Thanks for any advice in advance

Arnold

[hsas.69]

Why not have the case welded?

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[arncruz]

Thanks, I never thought of doing that.  I always thought that if the case was cracked, it was no good. I guess I just wanted to try this top end swap and wanted to see if anyone has tried it.

[Jerry Rxman Griffin aka MuthaF'er]

Is the 76 a K model too? 76 F has a better cam than the 76 K. The 78 K cam is on par with the 76 F.

Use the 78 cam and springs too. Both better. Use the 78 transmission too. Better gearing.

Sure its VERY doable. Should be NO problem.

[arncruz]

Thanks Jerry.  I think the 1976 CB750 is an F model just because of the rear disc brake.  So you think it would be fine to use the top end from the 1978 CB750 K and place it onto the 1976 CB750F bottom end, correct?

[flybox1]

Thanks Jerry.  I think the 1976 CB750 is an F model just because of the rear disc brake.  So you think it would be fine to use the top end from the 1978 CB750 K and place it onto the 1976 CB750F bottom end, correct?

Why not drop all of the 78 into the 76 cases?  crank, rods, trans and all....
Wouldnt you only have to size crank bearings shells at that point?

[arncruz]

I was hoping not to crack the crankcase on the 1976 engine and just transplant the top end of the  1978 cb750k

[CB750F2]

Yes, the swap can be done. However, to do this without splitting the crankcases you will need to free up the pistons on the 76 motor so why not try and get the motor running prior to proceeding with the swap? This will enable you to check the condition of the bottom end, transmission and clutch operation. I would also try and repair the crack in the K8 crankcase. If the crack is minor this should be able to be done without dis-assembling the crankcases and may able to be done with the motor in the frame. Use the search engine to find out how the crack can be repaired. Pat

[Jerry Rxman Griffin aka MuthaF'er]

Thanks Jerry.  I think the 1976 CB750 is an F model just because of the rear disc brake.  So you think it would be fine to use the top end from the 1978 CB750 K and place it onto the 1976 CB750F bottom end, correct?

Yes. The 78 K top end is a carry over from the 75/76F. Cam timing slightly different by 5 degrees. Either cam would be fine. The 78 K springs are stronger so use them. The 78 K has different valve keepers too so those valves with their valve keepers are only a 1 year part. That is not relevant to what you are doing just if you were looking for replacement parts. Either valve set would work.   

Double check your engine serial numbers to be sure that is what you have.

[arncruz]

You guys are awesome!  I'll try to do the crankcase repair since I already know the engine works and runs well.  I did not know that I could do that.  Hopefully who ever fixes it can do it without opening the crankcase.  In addition, I'm gonna rebuild the 1976 CB750 K engine and try to learn this engine.  These engines are truly amazing.  I bought a 1971 CB750 that was garaged for 12 years.  Changed the oil, new battery and electrics, and rebuilt the carbs.  It started up on the first push of the starter button.  all it needs is a new seat cover and rebuild of the brakes and it should be rideable.