The walls are pretty thin. I would leave the bore and do head porting and cam work. You will see more gains from that.

 

I agree they look pretty thin. My issue is that I am having my engine rebuilt and my engine guy says the cylinders are already over sized when looking at the build manuals specs. All he did was run the honing stones through it 3 passes to clean them up. No cutting, just honing. That is not a lot. I'm surprised that the cylinders would be that close to their limit without work ever being done before. Either way, it's either I have to buy a new set of jugs or put over sized pistons in. Wanted to see if anyone on this site has ever put larger pistons in an iron lined cylinder of an 850.

 

There are no jugs for the 850. The cylinders are part of the top half of the crank case. I agree with Franky on putting stock size back in and doing other mods to get HP from it. I'm not sure you can get 3mm out of the stock cylinders, but it's got to be cheaper to bore and have plated than replace the $1400 crank case assembly.

 

GVZ86 --- We at Millennium have tried to run these out to the JE +3 mm pistons and have cut through the cast liner every time.. We then have to re-sleeve the blocks for that size piston. We have had good luck with the +1mm (88mm) custom pistons that we had made. There is enough of the cylinder wall left at this size and we do stock these piston and would be glad to do the work on the block to help get you back on the trail. This will give you options until we come up with a bigger and better way to get the monster you are looking for. Feel free to call or email with any questions..