A failed cylinder head gasket is fairly common for E46 3 series BMW’s. The root cause is usually a problem with the cooling system. Faulty thermostats, bad radiators, bad cooling fans and leaking cooling systems must be address immediately with these cars or they can cause severe engine damage. What happens is the long straight six cylinder head warps from the excess heat. This causes the cylinder head bolts to pull the threads from the aluminum engine block. The official BMW factory fix has been to replace the engine. Many dealers and quality independent shops such as DART auto have been repairing rather than replacing these damaged engines.
First the intake, camshafts and cylinder head are removed. The cylinder head is machined back to a flat surface. There is a limited amount of material that can be removed, so in severe cases a replacement cylinder head must be found. The block is also checked to make sure it is flat. Renewing the threads is the tricky part. One way to do it would be to totally disassemble the engine and have a machine shop put it on a mill to install thread inserts. We found a great little jig that lets of drill the block, re-thread it for time-cert thread inserts and install the inserts nice and straight. The jig also covers all of the cylinder bores during the operation so none of the machining materials end up in the engine.
After the new threads are in place, everything is assembled, the cooling system is flushed and carefully inspected and the car is ready for many more miles of being the ultimate driving machine.