Sure, I am in the process of doing the same thing at the moment. It's generally easier to turn down the cylinders than to bore the cases as you would have to take all the bearings out to prevent them from swarf.

Thanks you for the good advice, I also think it is best to enlarge the crankcase, because I don't have a milling machine, I bored out an old 920R cylinder at 105mm, and try to enlarge the hole with a hole saw (105mm), this is no precision fitting, cylinder BT1100 is 104mm, hole crankcase105mm .
Can I use the clutch of the BT1100 , do I have to replace the axle for that ?

THAT is a bad*ss approach. Unfortunately one that might bite you. Unlike on your 920 the BT seals with a metal gasket and I am not sure that it will seal well, when you open it out this far.

Turning down the liners sounds bad, but we're talking only about 0.5mm reduction in wall thickness...

If you want to run the BT-clutch, which you could use as a basis for a 9-disk clutch, you'd have to swap in the complete BT-gearbox. Added benefit would be a slightly longer 5th gear. (I did the math in another thread, it's somewhere around 3 to 5 percent. Same as with an older XV1100 gearbox, which I currently have in and yes, you do notice it, if you have to do some Autobahn-touring every now and again.)

Cheers,
Greg

So correct me if I misunderstand, I use the axles, clutch of the BT1100, gears and outer clutch plate of the TR1

Badass solution no so bad after all , 105,5mm ,normal 105mm BT1100