Well, to me it looks like a weak construction but it shouldn't have to take any loads to talk about. I've hade this braking down and the ratchet wheel on the kick starter axle. I think that the issue lies more with the fact that we are talking about 35 year old engines that's had several owners over the years. The XT/TT models probably also lived their lives a bit harder than the regular SRs. Over all I think the engines are solid pieces of Japanese engineering and as long as you keep them maintained and don't abuse them to much they are good for well over 100,000 Km. My bike has about 65,000 Km on the clock and the gearbox, clutch and lower end looks good, at least when inspected visually.
Bottom line, if you do take care of your bike it should be around for at least 25 years more.
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