Chainsaw Repair

You could always make long aluminum internal jaws for a three jaw chuck and use those inside a cylinder to cut the base.

Also for cutting pop up pistons you can make/purchase piston jaws that clamp on the inside of the ring land and use the ring land surface to square the piston up.

Either of these on a good quality 3 jaw chuck will achieve acceptable accuracy since you'll make the final cut on the clamping surface of the jaws on the lathe so they are matched to the run out of the chuck. Make sure to mark them so you always get them bolted back up in the correct position.

There must be a thousand ways to skin the same cat .Whatever works, works .

I have a Monarch 10" EE lathe with Cushman chucks . A 6",4 jaw, face plate and an 8" with long reversible jaws .Plus several  more I haven't had the time to modify to take a D1 3 mount .

What works for me is clamping right near the ring groove using brass  shim stock to soften the jaw bite so as not to mar the piston .I've even used the aluminum from a beverage can . With that you have to be very careful not to ding the part .Once again though methods mean very little ,results are where it's at .

Yup....remember making a perfect triangle to tease my boss once...cranked up the jaw pressure (air chuck), machined and threaded the inside diameter of a fitting....at 3500rpm...with iron chuck jaws....then cut the rpms to zero. The chuck crushed that part into a triangle now that the wall thickness was much smaller. Handed it to my boss and asked him how did I THREAD a TRIAGLE. He asked years later when I met him as an AE for a CAD/CAM company how I did that.

Part of the reason I took the approach I did was not to risk anything along the side of the piston and ring grooves. Since the cuts are small, the material aluminum, the parts relatively short, it works.