Anyone know of a good mechanic in the Chicago area to rebuild the motor in my GS 500 race bike?  It started making a rattling noise in the bottom end the last track day I did.    Also, it has metal shavings in the oil filter, so that can't be good.  :x

Did it start to vibrate more too, but still runs OK? If so, it is likely the countershaft bearings, and the cases are likely toast. Easier to find another engine.

The countershaft bearings are a weak point of the GS for racing use. I think you'd have to replace them every season to avoid ruining the cases.

The CS bearings wear-out, and then stick to the shaft, and spin in the cases. This ruins the precision machined surfaces where the shell bearing sit.

I agree with Kevin on the counterbalancer diagnosis, thats the usual suspect...

However I wouldn't write off the cases just yet. If it is the CB, it is possible to re machine the cases to use regular bearings or even ditch the CB all together and have the crank balanced instead.  Of couse, that requires putting many hundreds to thousands of dollars into that motor that you will never be able to recoup.  So if you want to get rolling again on the cheap, finding another stock motor is the way to go. But if you are willing to build that motor, it should be salvageable.

good luck,

what, for sake of info, is a inexpensive crank reballencing job?how smooth would it run for a street engine no bumpin compression, and no headwork?

inexpensive? $200-$300.

The problem with balancing the crank is that it has will only be balanced for a specific rpm range usually less than 1000 rpm either way from the "sweet spot". Race motors are usually done at 9~10K rpm, but for a street bike much lower.  
Actually I wouldn't recommend this for a street bike.  They spend WAY to much time at idle and all the GSs I've seen without their balancers shake like paint mixers at idle.  

Thanks for the responses, but once again, any suggestions for a place to have my motor rebuilt?  Who balances crankshafts?

Thanks again.