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I spin my 306 to 6000 rpm quite frequently with no problems over the last 10 years. It is balanced and uses forged TRW flat top pistons. It does use stock rods but with stronger after market Pioneer rod bolts. I think the piston speed is around 2700 f/sec. It's the g forces on that rotating assembly that makes it come apart. That's directly related to piston speed and the mass and strength of the components, just as Unit said.

Rev