750 crankcase crack

I've had two Commando engines crack in this spot. I know of at least one other.

I believe the cause was using too small a gearbox sprocket over a long period, especially if the bike is used for long distance work. This allows the engine to rev too freely with not enough load in balance. Typically 19 and 20 tooth sprockets. Simply, the engine simply does more revs than it does work. At the same time the exhausts rattle loose and other bits fall off.

I had both the engine cases welded. The hard part about this is finding someone who will listen and who has skills and most importantly, someone who will care....they are not welding a Caterpillar crankcase!!!

The first case I found the right person. I stripped and cleaned the cases as perfectly as I could. I bolted the cases together with all the case studs & bolts and torqued it up as if it was a final complete assembly. I explained my fears of distortion to the welder, who understood my concerns. I got the cases back, still bolted together, the weld finished so it was invisible except to very close inspection. These cases never leaked.

The second set of cases I was in a different location, so couldn't go back to my first welder. Found someone else, went through the same process. Came time to pick up the cases and he gave them back to me seperated. Saying it was too hard to weld them assembled. I held the cases together in front of him and you could see the distortion. What could I say!! They leaked.

The repair can be done successfully, you just need to find the right person. The cases need to be bolted together and kept together through the repair process.

Bob

Previously wrote:

My 1970 Commando has a crack on the LH crankcase by the rear top cradle attachment boss. It's quite common apparently and mine has been there since I bought the bike in 75. It's not getting any worse but I plan to strip the engine soon and wonder if anybody has any experience of having this crack welded and how much distortion a welded repair might cause. I'm planning on reaming the crankcases out to 8mm and fitting some close tolerance bolts to make a proper job of fixing the cradle to the engine at the same time. the final aim is to get the engine and gearbox straight in the frame and eliminate the inaccuracies in wheel alignment. I don't think the frame holes where the front and rear isolastic bolts fit, are a. parallel to each other or b. perpendicular to the centre line of the frame. But thats scope for another thread.

hello I know kev stevens at KSR here Near Me At Howden east yorkshire He did my 650 right hand crankcase he welded the intermedant gear shaft hole up and re-machine it to interfearances fit he did a realy good job and welded my pinch bolt hole up and re-drilled it that was for my domie 88 fork leg and he did not cost me a lot ,So if you want you can post me the crankcase and I will take it round to him it may take some time as he allway very bissy , maybe two weeks Max, yours anna j dixon east yorkshire branch Secretary

Norman White at Thruxton has done a couple of sets for me over the years, always successfully. It's crucial to have them bolted up and pre-warmed to minimise distortion.

Norman also offers ground spigots and can bore the cases to suit. Spigots through both halves are the only way to be certain that the problem cannot come back as the load is spread evenly.

Running with rear bolts not fully tight is a rapid way of causing this damage (Don't talk to me about powder coating !)