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Taylor888

Damaged crankcase

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My son and I are restoring a 1986 XR100R and as we got into disassembling the engine I found damage to the left side of the crankcase. The attached pic isn’t mine but one I pulled from the internet. Apparently I’m not the only one with this problem. The mating surface between the left crankcase and the crankcase cover has a chunk missing. There’s no way to seal off the flywheel area when the crankcase cover is installed. 

Any suggestions? I’m assuming it can’t be repaired. Anybody ever buy one half of a crankcase and installed it? What kind of job is splitting the crankcase and putting it all back together correctly? Should I just look for a new crankcase or a whole new engine? Kinda bummed out about this  

I appreciate any advice you might have. 

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That is common damage to many bikes that are neglected on chain tension and lube. I have picked up several used engines and bikes that have this type of damage. Just poorly maintained. Closest thing to a simple fix is to fill in the chunked area with a hand fit piece of metal and then JB Weld or epoxy it in place. File, sand or die grind to fit . Otherwise an expensive and laborious weld repair can be done. Splitting the cases and such. I have found the DID  ER  428nz3 chain to be superior to all others on my little XRs. Big bikes too. Factory spec adjustment of 1 1/4" at lower mid span never fails on XR100 if you have a well lubed and not badly stretched chain. Also wheel bearings, swingarm bushings, engine mount bolts and sprocket condition can cause derailment gnarl. Good luck.

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That's what happens to these bikes since they don't come with a proper chain guide and people run loose chains.  You can buy cases on ebay, although you need to look at the left side.  90% of  cases sold on ebay have left side damage.  Plus, it will be kinda hard to find a clean set of pre-1992 cases.  If you not too sure on your mechanical skill, you can buy a bottom end. Then you can simply swap top ends.  

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Thank you all for your advice and insight. I decided to order a new crankcase cover since it was also damaged. For the damaged area on the actual crankcase, I’m going to fit a piece of metal and JB Weld it into place. I’ll use a dremel to shape it so the crankcase cover will seat well. Hopefully that will work. 

Thanks again!

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I would get a BBR chain guide too.  Then it won't happen again.

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