Those are Big Bore MS660 cylinders with a 56mm bore. In order to fit them to a standard bottom end they need a decent amount of work. The case halves need to be opened up to clear the 56mm piston. The original stud holes need to be filled in and the deck re-drilled for the new bolt pattern. Often the deck of the case halves and/or the cylinder need to be milled down to set the correct compression height/squish gap as well as port timing.
The transfers on the case need to be filled in when using an MS380, 460, or 660 cylinder.
Then there is the issue with the wrist pin bearing. Most of the stihl saws use a 12mm wrist pin while the CG bottom ends run a 10mm pin, so most often people run bronze bushings to run the smaller wrist pin. There are YD/LD100 bottom ends that run a 12mm pin, as well as the 110cc engine. The YD100 rods can and have also been opened up to the 15.8-16mm ID in order to run the larger wrist pin needle bearings.
The YD100 bottom ends have enough space to run up to a 54mm piston without clearance issues and the 110cc engines can run the 56mm piston with no clearance issues.
None of this work is overly complicated, but it is a lot of tedious work to get it right. Small details like ensuring the cylinder is centered before marking your holes to drill and tap.
This is actually one reason the Phantom 85 engine was created. It gives you the chance to have what is essentially an MS460 Saw cylinder on a YD100 case with none of the work mentioned above. Unfortunately, that doesn't mean you won't be doing a bunch of work anyway to get a good engine, but the work you do have to do is much simpler. I love my Phantom, but I had to rebuild it and rework it to get it to a point where it isn't a time bomb (hopefully).
. Now, I don't even bother talking to people.
