Re: Vintage 1950's 7.5hp Lower Unit Fitment/Interchange Help
From the looks of things you should probably be o.k. to use a lower unit from a 1956-58 7?, 1956-64 5?, 1965 6. In all of those instances the ratio is .578 or 12:26. The 1958-63 10 also shares the lower unit & 12:26 ratio. In all of those instances the drive shaft is protected by a shock absorber; the prop does not have a rubber hub. The 1966 & later 6 horse, though it looks the same, is different. The shock absorber was eliminated and they went with a rubber hub in the prop for protection. OMC made sure to make the drive pin sit at a different depth so you couldn't interchange the early & late props; you don't want to run a non-hubbed prop together with a non-shock absorber drive shaft! By deliberately making the dimensions different OMC made sure you couldn't.
You'll want to keep the prop from your 7? if you grab a lower unit from a 5? or 10 (or 1st year 6).
PLEASE NOTE: I have not personally performed the swap so take the above information as an informed guess. You should be o.k. though.
EDIT: You also asked about the case; I assume you mean the midsection. In that case you can eliminate the 10 from the list since only the lower unit is shared.
