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smashed cases

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cbnovascotia:
 Hello, just new to the forum, I'm writing from Halifax, Nova Scotia,Canada. I'm building a 1973 cb750k3 engine. I have two to work with, one has had an 836cc bore kit put in, with a larger cam, bigger valve springs, 90 psi comp. in all cylinders. The other seems stock with 160 psi comp. in all cyl. Both have damaged cases, in front of the sprocket, smashed opened from the chain snapping. One has a hole about 1.5"x 1.5", the other case has a much bigger hole. Both engines are tore down and the cases split. Should I...
(a) repair the less damage case by a professional welder
(b) look for cases with no damage, if so, must the cases be from another k3?
Any input is greatly appreciated! The game plan is to have a fully rebuilt 836cc in a cafe style cb, using a stock frame.....years from now!

kaceyk2:
HYA there , welcome cbnovascotia ,
Unless you absolutley want your bike to have a k3 motor in it, for authenticity say, then there is no reason why you couldn't drop any cb lump in, as you have allready got K3 internals though, there is a good case for having your cases repaired, allthough the internals will fit into other cb750 cases..
I guess you will have to way up the cost of replacement cases against the cost of your cases repair...do you have the broken bit from the "hole", its possible it may be inside the cases still?? That could make a repair easier, but it's not essential...

steff750:
 ::) you might get a better answer later on, but here is my tip of the day
   i have done this myself but yet to see the results as the engine is still out of the frame
 i bought some chemical metal and filled the cases myself it is easy to mould and cut to shape BTW don't mix up the cases as they are made as a matching pair the same goes for the main bearings  ::)  i would say welding is the route to go if you really want a perfect job , but i cant see any problems with chemical metal there is no oil or back pressure behind the cases and as long as it don't leak its job done for me

cbnovascotia:
Thanks for the input. The k3 engine case that i want to repair is split, guts are still in the two halves, and do have the missing piece, under the gears. I do have decent 1977 and 1978 cases. kaceyk2 mentioned the gears/crank from a k3 could be swapped into one of these?Are all 1969-1978 cb750k engine cases the same? it's strange that the two k3's that i aquired had the same damage.

kaceyk2:
The motor will be damaged in this place if the chain lets go.... some folk will change to a 630 X link chain and sprockets, but its a hot topic of debate, believe me, I have had more than one person telling me the stock 520 chain is fine if its looked after.... but by the number of damaged cases, it seems obvious, that the chains that caused that damage werent looked after!!!  Plus of course you have to oil them so you get oil over your new restoration....its a matter of personal choice and what you want out of your bike...
Modern alluminium welding is very very good, and not too expensive...you get to keep the motor that came with the bike.....
Steff,
I was on (lets just say "another" sohc site)  and I got a real dishing by a guy when I told someone not to "mix and match" the cases.... This is what Honda says just like you and I believe, after all why do they have different codes on the cases ....This is because each set of cases is finished to the specs that those codes relate to, crank journal size for that bike...
When I was telling someone that, this dude says " we used to mix and match em in racing all the time" and had a go at me..... I nearly said "if you didnt mix and match You wouldn't have had to be doing it all of the time!"   
Don't mix and match...is very good advice.....

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