smashed cases

[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.....

[cbnovascotia]

yeah, it does seem like a road to endless problems to mix n' match. A highly recommended welder offered to fix it and glass bead the cases for about $100 Cd. He probably bead the barrels and head for another $25. I'll leave it at that! Thanks for your input kaceyk2. I'll figure out how to put up some pics soon of what I'm working with, here across the pond.

[AshimotoK0]

K0's and K1's had different cases for the output shaft bearings (not sure on K2)  K0's and K1's had the same two bearing sizes but they are grooved on the K0 and ungrooved on K1. Suspect K2's used the K1 arrangement

Also quote from ChrisR

'All CB750's from about the K3 model on used the same output shaft bearings according to the parts lists, ie, the one with the twin row ball bearings against the oil seal.
The F2 did use a larger - 630 - output sprocket, and hence a different output shaft, but the bearings were the same as on the later K models'

Cheers!

ash

[Spitfire]

I also would stick to the cases that came with the bike and get them repaired, you mentioned bead blasting the cases, I would be very carefull of that as you certainly do not want abrasive material getting into the oil ways.

Cheers

Den

[SamCR750]

Poor maintenance and the small sprocket size was the main causes of early chain breakages.
Modern chains are much better than they were 40 years ago and After Honda fitted larger sprockets to slow the chain speed down, the problem all but disapered. Maintenance is the key though.

Sam.

[Spitfire]

It didn't help that we thrashed them to death, I remember going down to Brands Hatch (Ago and Phil Read on MV's if I remember right) for the day (from the Wirral) and on the way back looking at my chain when we stopped for petrol. It was all siezed up, I had to buy some oil, soak a rag with it and hold this to the chain while spinning the back wheel, luckily it worked and the chain freed off. Mind you there was 4 of us all on CB750's (mine was a K1 converted to 900) and we were cruising at 100 mph, which probably did not help much.

Cheers

Den

[SamCR750]

I think back then Dennis, we all thought they would last for ever without lubricant

Aintree is back on this year if you're up for it, have been offered a ride on a 836 750F, might bring one of my old Benlys as well, would be good to see you again.

Sam.

[Spitfire]

Hi Sam, yes could do with a day out, when is it ?

Cheers

Den

[SamCR750]

I think It's in July Dennis, will let you know.

Sam.

[Spitfire]

Thanks Sam.

[K2-K6]

To repair the cases seems the right way to go as eventually they are going to be short supply.

I would empty and degrease them then push the broken bit back in from the inside and tack-weld it on the inside, then bolt the two halves together to help against distortion, keep the cases warm and weld from the outside to re-instate the casting and let if coooool slooooowly.

Don't be tempted to grind off the weld if it clears everthing, just leave it as a good quality fillet.

Chains rarely break if looked after and lubricated, it's usually too worn wich is another issue in that the chain pitch gets too long to fit the sprocket and on throttle closed overun the slack transfers from the bottom run to the top and tries to wrap round the drive sprocket punching a hole in the casting.

Iv'e seen all sorts of junk used including industrial chains which are not usually tolerant of high speed and wear really quickly when used on a bike.

If you try to pull a good condition chain away from the rear sprocket it's almost impossible as the pitch is a close match to the teeth (just try it you'll see), If you can to any degree it's knackered with a capital F.