Using a lathe to turn down a piston crown is a bad idea. tell me how do I know? In the seventies when I was a lad I used a guy that had a lathe and was a machinist to turn down a set of 340 pistons for a 360 but he broke one piston and the rest were all over the place.I eventually got those pistons to work with a new piston and further machined by my engine specialist that I still know to this day. He used a special adapter piece to hold the bottom of the piston solid and milled them. I later sent him some pistons for my 340 that were 13to one forged pistons. they were obviously too high of comp for street use so I took him one 10.5 to one 340 pistons and told him to cut the other ones down to about 11 to 1 . He laughed and said he would give it a go. At the time I was a green horned twenty year old but now as a older gent in my late sixties I have learnt a lot about my younger years mistakes. That 340 ran great and did low thirteens back then on not so good tyres.