In a lathe I wrap the piston with a piece of ordinary writing paper or masking tape. I set the jaws lightly and true it up. Tighten it a bit more and double check the trueness. Pistons tend to either want to run true around their OD or on the top. It's very difficult to true one well enough to get both parameters within, say .001". Too much screwing around with it will leave marks on the part.

In a mill I use a special vice. It grabs the part by three places too (actually presses two parts against one). I use this for cutting boost ports and such. If the piston is of the right type - skirt length on the front and back the same height, like a MXZ700 Rotax, the part can be carefully clamped using a pin through the wrist pin hole then the top can be cut flat. The automotive industry sells some very fancy piston vices for cutting valve reliefs. If I did more large four stroke work I'd probably buy one.

In either case it is wise to use no more tension than necessary to hold the part. It's kind of like holding an eggshell while machining it, but with shallow cuts and sharp cutters the failure rate is very low. Any machinist with a good feel for his equipment should have no trouble with this.

A flat top piston from Wiseco or OEM is heavier than a crowned top version. The problem is that the flat top acts like a drum - literally. It reflects energy - heat, sound and generally wants to make the piston "walk" around in the bore. The added material is there to prevent the "drumming." There is still advantages though.

A flat top piston will hold up. I started cutting them that way back in th early eighties. I sagged a few back then, but materials have changed and so has my experience. Wiseco changed materials and I started having fewer problems. Also, I used to go as thin as .100" (center thickness), but now I know that .155" is about the practical safe limit.

The piston crown angle can be matched very closely using a protractor. For the TRX the angle is 11°. Since we're only talk about the squish area, that angle matches very well for 8 mm to 10 mm in width. And that's about the range of the squish width you'll have for just about any engine design you can come up with (depending on bore size obviously).

Typically when a piston crown is cut it's done across the whole top of the piston. In other words, if you set an angle to just get the outer area of the piston you'll change the piston timing edge and you don't want that. If you do it like is supposed to be done, it will cut from center to edge, with more material coming off in the center.

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