Originally Posted by
rccardr
With 120, I generally cold set instead of spreading. And I generally advise against going up more than one 'size', so 120 to 126 or 126 to 130, but not 120 to 130. 10 mm of cold set or spread can cause problems with the brake bridge and/or chainstay bridge if there is one.
When I am spreading rear triangles I use a pair of blocks with grooves to fit the stays which I clamp across the brake bridge to keep from stressing the bridge's welds/braze. They were made by drilling an appropriate hole through a wood block, then sawing the block in half leaving two pieces with half-round grooves. Cinch them down with a bar clamp and spread away!