Lucio,
The "Belt stretcher housing," (metal casting) is in fact locked in one place when you are happy with the pre tension , but the piston/plunger is free to press against the "Belt stretcher plate" which holds the pulley. The pulley is on an offset hub so it acts like a cam/lever.
Along with some very strong springs, and added oil pressure the faster it goes the tighter the belt gets, but when the oil gets hot and the block expands some, then the pressure is less. As much as needed. It's a neat system..
David ,
"
They are not a tight fit," where you used silicone, I decided to put a thick "O"ring inside of that rubber seal between the rubber scrapers. Figured that since it was always going to be soaked in oil, it might not wear out or bake as fast as the original edges. It has only been 2300 km, so far so good. 27 000km to the next TB and water pump and..and.......
Any idea of puting grease in there was only for the install, it would melt and dissolve with hot motor oil after a few cycles of pressurizing that cylinder at run cycle and relaxing when off.
The only tricky thing was getting that timing belt back on. A handy tool for slipping the belt back over the idler pully , The spine of your socket holder set, cut down to about 7" it's perfect.( some one else may have mentioned that somewhere)
Glen