Ive never used block guards but Ive read they will eventually make your cylinder walls oval. If this happens your rings wont seal and youll be losing power. Now if you plan on using it for some other purpose later down the line, you might just have to sleeve it for it to be any use to you or like a guy I met, ditch the block and start from scratch. The other problem with these things is that they cause hot spots at the top of the cylinders at the ring pack. I think a block guard is so easy to manufacture, and its easy to sell bec they are such cheap "insurance" for novice builders, They provide peace of mind knowing that you can boost your motor and not worry about cracking your sleeves all the while a different kind of damage is being done to the motor...