crankcase broken from chain jumping

[tooljunkie]

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.

 

[2door]

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]

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]

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]

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]

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]

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.