The diffrence between M30 heads is five fold.
Old M30 -78 , crack like crazy on account of thin material around the coolant section close to the chamber. Namely when they overheat. Chamber design is for plat top pistons, not dome shaped, same head casting on all M30 models no matter what size.
Old M30 79-81 , Better around the coolant paths, still pretty bad if they overheat, same chamber design
Mid M30 81-88(E24,E28,E23) Same chamber design but best coolant passage strength. And all old ones share same size valves no matter engine size.
Later M30B30 E34 and E32, Bigger exhaust valve, old intake valve size. Dome Chamber design, non US market only I belive.
Later M30B35 E34 and E32, Bigger exhaust and intake valves by 1mm, Dome chamber design, all markets,
Old M30 -78 , crack like crazy on account of thin material around the coolant section close to the chamber. Namely when they overheat. Chamber design is for plat top pistons, not dome shaped, same head casting on all M30 models no matter what size.
Old M30 79-81 , Better around the coolant paths, still pretty bad if they overheat, same chamber design
Mid M30 81-88(E24,E28,E23) Same chamber design but best coolant passage strength. And all old ones share same size valves no matter engine size.
Later M30B30 E34 and E32, Bigger exhaust valve, old intake valve size. Dome Chamber design, non US market only I belive.
Later M30B35 E34 and E32, Bigger exhaust and intake valves by 1mm, Dome chamber design, all markets,
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