crankcase broken from chain jumping

tooljunkie · May 30, 2012

so,as title says.
the crankcase is broken behind the chain cover.i figure a stone or something rolled in and jammed up in the case.
plan on welding it and building it up a little to toughen the case.
i ride a lot of gravel,and have a feeling the tires pick up a lot of stones-i see them flying up in front of the bike quite often.
a brush of some kind may be in order to keep stones out of the housing.
bad idea?
new tires waiting to go on as well.
or a fender-like guard to keep the pebbles out of the chain all together?
lathe,plasma,mig,e-wheel all in the garage waiting to get on this project.
suggestions,opinions appreciated.
thanks in advance.

P.S. i ordered a replacement engine to keep me rollin,old engine will go on another bike. cflg

2door · May 30, 2012

A brush might work. Or maybe a 'curtain' made of thin rubber sheet to help deflect small stones. This isn't a common problem. You're sure your chain isn't too loose? I know you have the mechanical sense to know what a roller chain needs to run true but make sure it isn't running out and interfering with the engine case.

Let us know what you come up with and post a photo or two if you can.
Good luck, Junkie. Keep us posted.
Tom

tooljunkie · May 30, 2012

will do.
pulling engine tomorrow.
will give me a chance to look inside,and do a couple minor mods as well.and hardware update.

biknut · May 31, 2012

The most likely cause was your chain was too loose. What happens is, when you let off the gas, the chain bunches up between the sprocket, and case if it's a little too loose.

It's easy to let it happen. Even a mb super genius like me, self proclaimed, has heard it happening a few times.

biknut · May 31, 2012

If the hole isn't too big, you could consider JB weld. If you do end up welding it, don't build it up much because it's already a tight fit between the sprocket, and block. You've got to have as much room as possible, for the reasons I mentioned about what the does when you decelerate.

MotorBicycleRacing · May 31, 2012

tooljunkie said:
so,as title says.
the crankcase is broken behind the chain cover.i figure a stone or something rolled in and jammed up in the case.
plan on welding it and building it up a little to toughen the case.
i ride a lot of gravel,and have a feeling the tires pick up a lot of stones-i see them flying up in front of the bike quite often.
a brush of some kind may be in order to keep stones out of the housing.
bad idea?
new tires waiting to go on as well.
or a fender-like guard to keep the pebbles out of the chain all together?

It might just be the chain bunching up and jamming inside the case.

ArrowMotorizedCycles.com makes a case saver that would fix your problem.

tooljunkie · May 31, 2012

where the chain cover bolts on-the casting inboard of the sprocket-broken clean off-as in if i pulled the clutch i would have broken the rest of it off.
pictures and forums and me arent quite on speaking terms ,but i will try.
i do like the guide.
i was on the throttle when it broke,was a bang and i stopped.
chain was still on engine sprocket.

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crankcase broken from chain jumping

tooljunkie

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2door

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tooljunkie

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biknut

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biknut

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MotorBicycleRacing

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tooljunkie

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