Use a table ban saw to hack off the 340 piston crown .

Righty Tighty said
I'm still stuck on the fact he wants to trade what he calls a "junk" piston he got for less than $10, for a usable 4-5" outside mic. Yep, even Steven.
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No worry's, Walmart has the micrometer for $26.00...and that piston that says R&D might be a collectors item in a few years worth more.