@Murdo did you have to pull the valve seats out before welding up the cracks? If not, how did you protect the seats and ensure the cracks did not go below the seats? Peter
I followed the cracks down with a 3mm drill, they weren't very deep and plugged the sparkplug hole so I could fill it with argon gas from the torch and with very small tip welded the drill holes fusing each side together. I did get a flashover from the arc onto one seat but was able to cut it out with the stones.
I wonder how bad this engine has been abused to do that? Sorry to hear of your ongoing frustrations with this build. I think these bikes had to compete with small capacity 2-strokes when they where released (1983), so I've always thought their engines would have been given a fair hiding. They were a quick little bike and the 14,000rpm redline would have been 'just a suggestion' to young riders. The later models seem to be less so abused. On the plus side, your build is helping me determine what's savable on these old bikes and how to save them. Keep up the good work. I'm looking forward to seeing it in the flesh in August (lockdowns and weather permitting).
@Murdo thanks for the details. I have an old GSXR750J cylinder head that I might have a go at fixing at some stage. But I suspect that might be a bit more work. Quite a few cracks from the plug holes to the valves.
Been working on these heads since last Saturday and after all the dramas have finally got the rear one together. Started today by cutting the valves and checking the seat contacts with bearing blue. All good positions and width. Next was to sand the head to get flat again. If I had fitted this up it would have blown the gasket in quick time. First cut shows how much it was warped. I refaced the cams and followers to smooth the surfaces and to cross sand to make some small grooves to help with oil retention. A good wash in CT18 and HOT water in the laundry sink and a blow dry and started with the assembly. Looking better. Now have to buy some new valves to be able to wash and assemble the front head. After all these dramas the next bike, IF I EVER BUY ANOTHER BIKE, will be running and rideable. I am over these major engine rebuilds.
I call BS , if there is a rusty frame tube in a field , we all know you will build a 8 cylinder 2500cc 2 stroke out of it
Over the last couple of weeks I have been able to get the engine together, painted, filled with oil and primed on the starter. Last weekend my son helped me to put the frame on the bench and fit the engine. I now have to fit all the bits to make it go and then start on the fairings.
So true to my word Grandson drove me to Scone a couple of weekends ago to collect a 2007 CBR125r that was out of rego but was RUNNING. It just needed to straighten up the headlight/fairing bracket (it has been down on both sides) and fit some new front blinkers. The boy gave it a full service (engine oil, clean filters, set valves, new spark plug, change fork oil, new steering head bearings, coolant flush, etc) and starts and runs sweet. Waiting on ordered new footpegs and will be ready for rego.
Finally got this finished and rolling. Took for a 14Km ride this morning and although engine is still a bit tight every thing runs fine and handles good. Front brake needs some bedding in of the new pads, etc. Will put some more K's on it and it will be a great little bike.
After riding this for a bit more the riding position is just no good for my back, so it has gone to live in Brisbane.