Cm870 built with CM 570 internals.

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It's certainly not impossible to build an 870 with 570 pistons but the end result would not justify the headaches and costs. If you bought an 870 o/haul kit and a set of 570 pistons, you'll spend a fair bit more and have 6 870 piston kits you'll never need. If you buy a 570 o/haul kit, half the gaskets won't fit. The 570 pistons will also require a different part number injector - Which you won't have any cores for. They may also require a different injector cam. On top of that, the new injectors would need a completely different set of maps in the ECM to run correctly. By the time you're done, you could've just rebuilt it as it is and gotten the power increased and/or VG removed.

As Brock said, 870s don't really suffer from excessive cylinder pressure. As with any ISX - It's all in the injection timing & liner protrusion.