and then i need to ask,will it change the tiller?
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It might,.....I would clamp with a maximum 1", and if it has string follow at the moment I would just clamp it straight.
I tryed several different belly tempering methods, the one that works best for me is : getting the gun farely close ( about 1" ) I keep it moving steady on a 6" area untill the back is hot, than I move up and work the next 6" area, but, I overlap 3" on the previous, than keep going on overlapping every 3" and working on a 6" area at a time, ...this way the wood takes very little color but lots of deep heat.
belly tempering, like every other tecnique will take time to get it right, can't expect to nail it right of the bat