This is a custom cylinder head repair we did at PRE. Spark plug on cylinder 6 was not torqued down properly, vibrated loose, and finally ripped out all the thread. Heli-coil spark plug thread repair kit was of no help in this situation; too much material was torn out. This kind of repair cannot be done with the cylinder head installed on the block. We removed the head from the block, completely disassembled it, and inspected all components for wear. Cylinder head was checked for cracks with a Magna-Flux system. BWM M50/M52/S50/S52 head are notorious for cracking into the water jacket near the exhaust valves. After passing crack inspection the head was cleaned and valve seats checked for concentricity. Cylinder head was also checked for straightness of the deck mating surface. After passing all of the above tests we began the repair process. Cylinder head was mounted in a Bridgeport style mill and spark plug hole counter bored to a specific size. Additional counter bored “step” was machined and 4 spots for laser welding of the fitting. An aluminum fitting was precise machined in a lathe with a standard M14x1.5 spark plug thread pitch on the inside and a precise side on the outside diameter. Aluminum fitting was machined to have a 0.002″ inch interference press fit into the head. Also a pilot tool was machined to ease fitting installation. After all machining work was done the cylinder was heated up, fitting cooled, and pressed in with a light coat of green “sleeve type” Loc-tite. Afterwards the fitting was laser welded in 4 designated spots in the head. Combustion chamber was vacuum checked for seal and volume checked to make sure all cylinders maintained same compression ratio. Cylinder head was deck mill faced and hand stoned to ensure perfect mating surface for a factory or performance multi layer steel head gasket. All valves were cleaned and refaced using a Kwikway valve grinding machine. Valve stem seals replaced with genuine OEM BMW parts. All valve springs we examined and spring pressures checked using a Rimac valve spring tester.
Pictures of individual machined components and finished work: