Just chucked up a hw95 piston in my lathe (4 jaw chuck) and it's the first time if had a serious attempt at setting up a 4 jaw.
I have it held chuck end by the seal locating flange and have made a mandrel that slides inside the piston and is supported by a live centre in the tail stock .
The best I can manage is 6 thou runout , Does this sound about right.
Jonny