11 mm to 12 MM head bolt Conversion A common problem has been cracking of the block leading to coolant leaking into the oil pan. Other complaints are cracks at the base of the knock sensor bosses, which is where the 11 mm head bolt terminates. This is due to the high pressures caused by high power applying large stress to the 11 mm threads which end at the coolant jacket and cracking the block. The 12 MM head bolt, as used in 2011 model year blocks, extends past the coolant jacket and ends in the main web where the stress originates at the crankshaft. The 2011 MY 12 MM blocks are not prone to cracks as the stud terminates at the main web. Recommended by Ford Racing for high horsepower and twin turbo applications With the now common complaint of coolant leaks into the oil pan we have identified one of the causes. Other complaints are cracks at the base of the knock sensor bosses, which is where the 11 mm head bolt terminates. The 11 mm head bolt blocks have the threads terminate at the base of the coolant jacket of the block, as shown in the photo. The remedy is the modification to 12 MM bolt threads which extend into the main bearing web. The combustion force is always trying to lift the head and those forces are transmitted to the head bolt. As we increase pressure (Torque) we increase the stress to the bolt at the local area, eventually exceeding the tensile strength of the block material and forming cracks. All the forces originate at the crankshaft and the reactive force is at the bolt. If we tie the bolt to the crankshaft area, we transmit that force back to the crank instead of thru the aluminum coolant jacket or knock sensor boss area, which is thin and non-structural. The remedy is to convert to 12 mm head bolts which tie the head forces to the crankshaft via the main web. The challenge in conversion is to obtain a 90 degree head stud to the deck. The reason for the challenge is it is very difficult to move an existing hole center and to drill off-center creates an angled (cockeyed) hole. The existing holes in blocks do not meet the Ford blueprint after measuring 100’s of head bolt holes, which results in always drilling off-center to the existing hole creating a cockeyed hole not 90 degrees to the deck. An experienced machinist knows very well the challenge of drilling into an existing hole. L&M Engines has developed tooling to move any existing hole to the Ford blueprint and affect a 90 degree hole to the deck which will result in proper head gasket clamp.