while rebuilding the cylinder head, during problem that i had with the decomp(see prev thread), i found the reason to the problem: i put the valves in their position and checking the valve lifters. in the right exhaust valve, that the decomp is manipulate, the valve lifter didn't fit smoothly as it should, i can put it inside with force. after checkink this lifter with the other (insert smoothly), my conclusion was that during excessive heat of the head by the dented header that distorted the bore that the lifter comes into. i polish the bore gently with sendpaper 600 mesh and smooth it with mesh 1000. (maybee remove 0.1 mm of). the lifter is now coming smoothly inside. for the dented pipe, i treat it with the freezing method that mention here and you can see the results with the pics at my garage.