I have a mercury lower unit with the pinion gear and the forward gear busted. I also have parts from another mercury lower unit though, that I bought at a repair shop. The other lower unit has a messed up drive shaft and housing so I bought it cheap thinking i could use the internal parts but the mechanic working on my outboard says that the gears are different that the teeth on one lower units set are straight and that the other ones are spiral. Does it matter though because all the internal parts should match because they came off the same lower unit(the one bought cheap with the messed up drive shaft and housing). The things I would be substituting is the housing and a compatible drive shaft.
