You need to measure the bottom of the bb850 sleeve and mill the crankcase to that size. No other porting is needed unless you want to
I built a bb850, it's not difficult.
Used cr500 pistons which require a shim under the cylinder.
I don't know about groupK pistons
The cr500 pistons are 5mm taller than the 62t pistons. There is a plate under the cylinder to compensate for this. This is a standard 850cc BigBore mod
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