The J-3 and J-8 are very easy to shim as there is only one shim to play with. You basically grab the lens from both ends and unscrew the optical block from the focusing block. Focusing block is the one with the screw mount. The shim goes between the two parts. Very easy.
Originally Posted by Red Robin
Only real problem with this may be if the lens has not been taken apart for, say, 50 years, then they might be a bit stuck together. I recently made a 0.8m close focus mount following Brian Sweeney's instructions on my own J-3 and it took some courage and grip to unscrew the two halves from each other!
I have adjusted these with shims cut from paper as I do not have access to ones made from metal (tips welcome!). The J-9 is a whole different kettle of fish and I believe Brian Sweeney is correct in stating that may be impossible to make a J-9 to focus correctly from 1.15m until infinity on a Leica body. After playing with mine for many hours (there are two shims that you need to play with in a J-9, one between the two blocks like with J-3 and J-8 and another one with which you can adjust the focal length of the lens) mine is now correct from about 1.8m until infinity.
ps. Hello to everyone from Helsinki! I used to lurk around RFF quite actively but as RFF seems to have vanished I thought to start lurking here instead!