Hi fellas,my first trip out on my rebuilt ct3 resulted in this. It was going really well and then I had no 3rd gear.has anyone got a suggestion as to what caused this. 

It might help if you describe exactly what happened...  broke when shifting?  Just cruising?  What gear were you in when it happened?

First thing that I thought of was shift drum retainer was missing or end of shaft broken off under plastic cover just forward and above sprocket.

Good point MarkT. I've never seen 'exactly that' before. How's the cases.

If someone was a trained metallurgist, they might be able to identify if it was a latent defect, impact, fatigue or other failure.  If you look at the failure points you might be able to tell where the crack started.  The metal might look exposed longer at that point if it was a crack that was exposed for some time before the catastrophic failure.  Both failure areas look like the weakest points of the gear; between teeth and where the reliefs are for the dogs to engage.

My guess is that the gear had some internal defect.  I have never seen a gear failure like that.  More often broken dogs or chipped/missing teeth.

Showing a good picture face on view of the newly exposed metal might allow better diagnosis.

Hi Fellas, the series of events was that i was out riding this bike for the first time since rebuild and she was running well. running in so nice and slow ,about five minutes in i was shifting from 2 to 3 and couldnt find 3 .so limped back home and engine out and stripped again .i found a small amount of metal in the sump and the gear in 2 pieces as shown. I have to say Mark T has hit the nail on the head, i had forgotten to fit the shift drum retainer under the plastic cover so the drum has not been held in position properly. a costly error and frustrating but a valuable lesson learned. thanks everyone for your help . this site is so useful . best wishes to all . im off to the shed again to put her back together .....

Top marks to--Mark!! Double check everything that meshes, dogs, teeth etc. I've mentioned lots over the years about the end of the drum being smacked off by a thrown chain & the internal damage that can occur. But--never quite seen it damaged like that. Frame that photo, we'll probably never see another one.

just having one more look at the pic. Man imagine the pressure to do that but i guess its the weakest point through there

Check shafts for straight too.