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Discussion Starter #1
hey I gotta 1987 Quadzilla 500 that I've had for about a year and a half never did have it running correctlytook it apart one day notice at the left case was cracked around the crank bearing so I had it welded up put back together ran it about 2 hours and it cracked again so I ordered a l different set of cases off of ebay and was wondering if I might experience the same problem due to a different issue or ? could it have anything to do with the crank itself? any suggestions would be greatly appreciated thanks
 

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Which side is cracking?

Check the crank runout, aka "wobble". If there is wobble, it will transfer load onto the bearing. At some point, something has to give and in this "case" it seems the aluminum case is the weakest link. The manual shows the run-out measurement procedure and spec values. Super easy method to check wobble on the stator side is to...
1. remove spark plug
2. remove stator cover (case)
3. kick over the engine with one hand using the kicker, while watching the flywheel spin. If the flywheel is wobbling at all, you likely have excess crank run-out.

Measure the bearing seat ID vs the bearing OD. Make sure you aren't over-doing it with the press-fit.

Check the roundness of the bearing seat, as these can wear out over time. The rotation of the bearing inside the engine case seat can cause heat, strange forces, etc on the engine case.
 

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If you do have crank run-out, it can be corrected. But if you're going that far, you might as well get a new rod bearing, rod, etc and have it all done at a local 2-stroke shop.

Once all the parts are running right & proper including proper break-in & initial tune, these Zillas will give you many hours of ride time. If you ride it hard, expect to work on it more. Get one of the puzzle pieces wrong, and it will suck.
 

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Discussion Starter #4
I did have new crank bearings installed when I had the left case welded. That's why I am wondering if the case was already weak or if maybe there's something wrong with the crank? I don't know enough about this to make that call. What do you think? Thanks
 

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Sounds like you need to check the crank run-out (per manual, not the quick way), especially since you'll have the engine completely torn down anyways. Next time you might break something else. Spec the crank out, spec the main bearing holes...do it once and do it right.
 
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