Topic: Bottom end swap??? 77' and 72'

[Xander^]

Good day to all!

So, i have a 1977 cb750k and after/during the top end rebuild an issue arised with the cam chain bottom end sprocket. Long story short i need to replace the drive shaft in the bottom end.

I found a bottom end from a 1972 cb750k for really cheap so i bought it.
Wanted to know if anyone had any experience with a similar swap idea. Im thinking of keeping the whole bottom end of the 1972 motor and putting my 1977 top end thats rebuilt with that.
I was told to notice if both bottom case numbers match and they both had "BBB" ingraved.

Anyone have opinions or concerns on this??? Will this work???

Thanks

[BPellerine]

the 77 has a longer countershaft,so with the 72 bottom end you will have drive chain issues,you could put the crank out of the 72 in the 78,but would likely have to put new main bearings in unless you get real lucky

[Xander^]

the 77 has a longer countershaft,so with the 72 bottom end you will have drive chain issues,you could put the crank out of the 72 in the 78,but would likely have to put new main bearings in unless you get real lucky

hmm.. did you mean 77 not 78? (was that a typo?)

i didnt know anythign about that longer countershaft that sounds like it wouldnt work at all then.

if the crankshaft itself of the 72 works with the 77, and even if i need bearings, that will still save my project and solve the issue.

[BPellerine]

77,and 78 are the same,yes the crank will save your build,if the rod bearings in the 72 are good you can swap rods and all,but check them when you have it apart

[Xander^]

77,and 78 are the same,yes the crank will save your build,if the rod bearings in the 72 are good you can swap rods and all,but check them when you have it apart

The crankshaft swap  may be the only way to do it then. Thought the whole bottom end swap would work but the safe play is to open both motors and move over the 72' crank. Rod bearings arent too expensive so might replace them all regardless of condition theyre in.

Thanks for responses! If you have any further helpful thoughts Much appreciated.

[HondaMan]

You could just put the '77 top end onto the '72 bottom end, then find a 630 sprocket for the CB750F2 bikes, and drill some 6mm holes into the sprocket to mount it with the little floating plate. Or, maybe check with CycleX (or sometimes at Joker) to find their "630 sprocket for early CB750 engines", which they offer from time to time. These cost about $35, but solve the problem very neatly.

[754]

Just use an offset sprocket on the 72 output shaft.
 Or swap rear carrier and a few spacers.

[Xander^]

You could just put the '77 top end onto the '72 bottom end, then find a 630 sprocket for the CB750F2 bikes, and drill some 6mm holes into the sprocket to mount it with the little floating plate. Or, maybe check with CycleX (or sometimes at Joker) to find their "630 sprocket for early CB750 engines", which they offer from time to time. These cost about $35, but solve the problem very neatly.

this sound super interesting, an external fix without having to crack open the bottom end of either motor. mind if i direct message you if i need any more help if i decide to take this step?