You have to machine the seats to accommodate the new valves, and open up the ports as well.
This is a precision job requiring precision tools and knowledge, so get a shop knowing how, to do it.
The valve job is important in the quest for horsepower, so don't skimp on it.
i wouldn't do it...was going to get 1mm oversize myself but decided against ..it can actually choke up the motor if everything is not right..more valve shrouding for starters...money better spent getting a performance valve job instead
I know that the valve seats are the part of the head but I saw that it can be changed. With bore the valve seats, I meant that you enlarge the iner diameter whitch is now for example 26mm.... and after it will be 27mm.
An local engine shop(germany) did the valve job. They said its no problem with the Honda valve seals. I rev to 7500rpm and had no problem, till an valve spring was broken. But the engine is running soon again. Only one valve was damaged