Cracked crank cases.

[royhall]

Hi all.

I am rebuilding a CB750K4 engine for Alan (MCTID).

Got the engine apart and am cleaning up the parts. After removing the thick goo from around the drive sprocket area (looks like the goo was deliberately spread out to cover it)I have discovered deep score marks and a large crack in the lower case, caused by a chain breakage.

The crack is beyond welding up as the alloy has been pushed inwards and is twisted.

Does anybody have a spare set of cases (obviously I will need both halves as they are a matched set) they would be willing to sell.

The bike is a CB750K4 USA import. I think (not certain) that K3 to K5 cases are all the same.

Thanks. Roy.

[Bryanj]

Roy a decent alloy welder should be able to tap back the broken bit and weld up the damage, may be easier than newish cases

[royhall]

Cheers Bryan. Am I right in saying the K3 to K5 cases are the same, and possibly K2 as well.

A few pictures of the damage.

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[philward]

Hi Roy
I've got a set - I go in the shed and get you an engine number - I'm in Wigan
Phil

[philward]

Hi again Roy
Engine number is CB750E - 2727871. They need a good clean up as have surface corrosion. If not suitable, I have a complete engine from a K5 (from memory) that is weather seized - I don't know internal condition. Don't want a lot for either
Phil

[Bryanj]

That's a lot less damage than I have seen repaired by a welder.

There were several changes of output shaft and bearings you would need to check at somewhere like motogrid for part numbers and what models they were used on mate

[royhall]

Cheers Phil. That number is reasonably close to ours (2333840) is that a K5 set? That would get Alan's project back on track, I'm sure he wont mind driving up to Wigan from St.Annes. Not been able to contact him yet, so the problem may be sorted before he even finds out about it.   Thanks Phil will be in touch.

[steff750]

  i have had good results with chemical metal i think it was called ? but all depends on the damage
 the problem you got is the lower case is pushed in, if its  not put back in place, it will catch the gearbox sprocket not much room behind there, i would not like to try it with them cases unless you can get that crack closed up

[royhall]

Looks like yours are K7 Phil. As Bryan says the output shaft bearing is larger on those. What a shame. Its 341 product code I need that appears to span K2 to K6. Thanks anyway.

[philward]

Ok Roy - if your struggling, don't forget that K5 engine
Phil

[royhall]

Ok Roy - if your struggling, don't forget that K5 engine
Phil

Thanks Phil

[royhall]

Cases are all fixed now. Couldn't tap the piece back into shape without severe stress on the cases. So I cut out the offending piece and prepared it for welding. Alan took it to some guy near Blackpool airport who made a great job of welding it up. Thanks for all the help guys, again. Alan's project now back on track.

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[Rozabikes Tim]

Thats tidy. Wish i could weld like that insead of the pigeon  dodo I usually produce!

[AshimotoK0]

Thats tidy. Wish i could weld like that insead of the pigeon  dodo I usually produce!

+1 on quality of that repair.