head and barrel skimming
 

any one got tolerances for the amount of material to remove from the head and barrel of an 86 tenere ,,,,I,ve removed 15 thou from the head and 5 thou from the barrel.....I will check if I have valve clearance by putting plasticine in the valve cutouts on top of the piston and rotate engine to see if the valves touch the plasticine....if some one has figures it will be of great help....cheers

The less material you remove the better.
I used to skim car cylinder heads and such in my lathe. The key thing is to mount what you are skimming on blocks and clamp onto the blocks. This way you dont stress the part or bend it. For instance if a head had warped a little so it was bowed and hollow in the middle, clamping it to a straight surface would bend it straight. after machining when you undo the clamps it will bend back again and yo have a nice new bent surface.

the trick is to clamp it unstressed the remove material slowly. This can be done using a lathe, mill or surface grinder. Once you have a clean machined face, stop removing material.

Can't help with actual measurements for your bike. On my alloy headed car back in the 70's I took about 3 to 5 thou each time as every few years the head gasket would leak water. On an early mini, the max you can remove is 57 thou :) definitely helps it climb hills :)