In order to prove a product liability claim, the plaintiff must show that the product was defective and that the defect was the cause of the injury. Therefore, if you modify a product and then it injures you, it will be very difficult to sue for damages since the injury may not have had to do with the product defect at all. That said, if you can prove that the defect was there before the modification and the original defect caused the injury, you may be able to sue. But you wouldn’t be able to sue for injuries caused by your modification of the product.