is it all right to gring valve reliefs a little bigger with a small air grinder I doen`t have to go deeper just bigger the valve justs rubbs the edge of the hole

 

If on the piston you may want a machine shop ti "mill" the valve reliefs in order to get a preset amount taken off....

this way you don't throw the weight of the pistons off by too much....making balancing difficult not to mention expensive.

 

what have you done to get this condition? larger valves,higher lift cam?

 

larger valves and the heads are milled about 60-70 thou
just bought the heads they are c9`s with all this done

 

there are flycutters available that fit into your valve guides and use a electric drill to cut valve reliefs , this may be done while the piston is in the block while a machine shop will need the pistons removed.

 

who makes them and how much

 

I use a flycutter with removable carbide cutters but I saw a article in Carcraft recently and they were using a specialized unit made for that purpose, If you can't find the article try the manley website or one of the piston mfgrs. No idea on cost --sorry