Topic: Lower Case Swap- Tips?

[Mikey G.]

Well, I done did it.  Be it lack of maintenance, messed up sprockets, or whatnot, I've joined the Broke Chain club.





So, I'm still exploring options (because this laid up my 750 until winter, no question), but I've got a set of engine cases in the parts pile that I might be able to just swap all the internals over to.  Assuming I go this direction, is there anything to watch out for?  Major obstacles?  Are bearing shells matched to the case, or the shafts (or assume nothing and plastigauge them regardless)?

[scottly]

Bearing shells are matched to both the case and the crankshaft; there is a chart that shows which case and crank sizes go with which inserts. One option is to clean the area up and patch the hole with a piece of aluminum and your favorite flavor of "glue".

[HondaMan]

You will need to at least disassemble those cases to remove the chunks that are now hidden inside. I got one of these some years ago where the owner just glued the hole shut and rode on for some more years, after which the internals of the engine were wiped out with a lapping compound made up of the ground-up aluminum. There was nothing left to rebuild on, as even the transmission's bearing mounts were worn loose! The low oil pressure, brought on by finely-ground aluminum eating the scavenge side of the oil pump, could not return oil to the tank, so every time he tried to ride it, the tank ran low. Adding more oil eventually got it blowing out the top vent tube, big time. He said there was 2 gallons of oil inside when he finally drained it all!

[PeWe]

This will bring cylinder, head inspections with eventual need of refurbish.

Entering the "not good enough to assemble but no reason to take it apart for refurbish" before the chain accident.
Internal chains with tensioners, head job when ex guides + valves might be too worn.
Crank bearings....

[Mikey G.]

The sad thing is, I had planned on a bunch of engine-out work this winter anyway.  It would've resulted in a top-end rebuild, so going that much deeper isn't that much farther.  Thing is, I've never done a bottom end before.  Just swapping parts I feel like I could do, but things like bearings give me pause.

What's the bearing availability for these bikes, anyway (talking early K2/K3-ish specifically)?  I know crank bearings for GL1000s are practically nonexistent.  What's the supply look like for CB750s?

I've never been a fan of the "glue a patch on" solution, personally.  It just feels way too band-aid for me.

[PeWe]

Crank bearings no problems from cmsnl.
Check letter on top case at front from beneath.
Very vague letters on crank.
There is a chart in CB750 shop manual.
I guess green will work

[HondaMan]

The sad thing is, I had planned on a bunch of engine-out work this winter anyway.  It would've resulted in a top-end rebuild, so going that much deeper isn't that much farther.  Thing is, I've never done a bottom end before.  Just swapping parts I feel like I could do, but things like bearings give me pause.

What's the bearing availability for these bikes, anyway (talking early K2/K3-ish specifically)?  I know crank bearings for GL1000s are practically nonexistent.  What's the supply look like for CB750s?

I've never been a fan of the "glue a patch on" solution, personally.  It just feels way too band-aid for me.

If you don't have my book, I'd suggest it: there's help for this, there.
If you get another crankcase (cases) then you'll just have to change the crankshaft main bearings (maybe...). Most of the main bearings in the cases were "A" or "B" sizes in production (Green or Yellow colors) because Honda techs were good at keeping the bearings right on the cranks. In the K7 and later engines, this changed a lot as they accepted anything that could be fit up via bearing shell changes: I have seen many K7/8 and F2/3 engines with Brown bearing numbers, factory-installed. lmost 95% of the K0/1 engines I have worked on had Yellow or Green bearings, and if one was different, it was a single Brown somewhere in the 5.

[Translations]

I did a case swap on mine it's not terribly difficult to do. What helped me is just laying both cases on the shop table side by side and carefully swapping everything into the new case the same way they were in the original. While your in there do the main bearings, tensioner (nos from CycleX), chains and buy a quality gasket set you should be in good shape. The biggest PITA for me was swapping over the damn kick start mechanism 

[PeWe]

Make sure that the small rubber plug sit in the lower case close to the bearing holder on the shifter side.