73 50hp mercury lower unit interchange with 1982 40hp mercury?

Need lower unit for 40 hp 1982 Mercury. Will a 1973 50 hp mercury lower unit interchange? Thanks

Generally speaking, should interchange, but Merc did make some sneaky changes over the years. Best to verify the driveshaft length (easy to measure from the flange at the top of the gearcase).

Also there may be a difference in the width of the 2 front studs. Older gearboxes (& the mid-section) had a narrower span between studs, this could lead to fitment issues on a newer motor. In a pinch you should be able to enlarge inwards the holes in your mid-section to fit, but it's always better to find a lower unit that fits perfectly.

At any rate, those 2 things are easily measured. The only other strange thing I can think of is for those lower units that have a shift shaft with a square on the end instead of splines. In that case, the lengths are the same, you just have to swap shafts. And there's a trick to that as well; be sure to ask for specific instructions before attempting.

HTH............ed

Is the 40hp a 2 or 4 cylinder??

Make sure you find a lower unit with a stainless shaft in it !