Pistons are Aftermarket or OEM and come with Pistons, Pins, And Snap Rings. KP will automatically provide ultra 300 Pistons as a replacement for ALL 310’s due to their better strength and durability. When you purchase pistons on this listing, they will be Ultra 300 Pistons. If you pistons have a 0755 stamped on the side, we HIGHLY recommend upgrading to forged if is within your budget. Or at the very least Ultra 300 pistons we would provide here. Otherwise the next option is pistons stamped with 0787, which is better. The 0755 pistons have issues with the ring lands breaking under use. The 0787 does have this issue, but it is less common because it is revised. If you have ultra 310 pistons in your machine and are replacing some, but not all. You should replace with 310 pistons. So when you purchase this, just let us know. The compression ratio is slightly different from 300 to 310.