RT....I am using a GX head as well and did the conversion from mm to thousandths..had my shop check the head and it has been shaved down 20 thousandths over the past 40 years which comes out to 0.50 mm. I have been concerned with piston to valve clearance with the stock "offset dish" A15 pistons. Don't think I need to be concerned?? My cam has been reconditioned by Isky. Specs are 260/.425. I still plan to do the clay test where you put clay on top of the piston, set a used head gasket and head on top of the block and turn the engine by hand and check the clearance. Hopefully no contact..... Unless you know of a better way to do this test.