because a block guard goes on the top of the cylinders, over the combustion chamber, where the coolant is most needed. it blocks the coolant and causes hot spots on the cylinder walls. this causes warping and detonation. to anyone who knows motors, they know sleeves fail in the center when the rod is at the greatest angle to the sleeve and putting the most side load on it, not at the top at the combustion chamber. if you want to properly support the cylidner walls, get the block sleeved. otherwise, dotn bother, leave it stock. i wont even mention the horrible idea of just punding the block guard in, which will almost guarantee that you will warp the sleeves and you'll sieze the motor. the only correct way to install a block guard is to tear the motor down to the bare block, press in the block guard, weld it in, and then have the cylinders overbored to be brought back into perfect round. as you can probably tell, thats about as much in cost of parts and labor as it is to just have the block sleeved, which is aboslutely superior.