Re: mercury 115 lower unit
If you do tear the lower unit down for inspection pay close attention to fwd gear as it'll probably be damaged too, and require replacement. Check all bearings for damage from all the metal filings. Check to make sure the shift plunger is in good shape and not binding, also I'd replace the shift cam.
There are a lot of lower units on eBay and unless you do the work yourself, it'd probably be cheaper to pick up a good used one there (or on Craigslist.org ).
Merc started using the swept-back fin design in '79 so anything that year or newer would be an identical replacement for yours. If you just need something to get you going again, about anything from '66 and up (all Inline Sixes and 66 C.I. Inline Fours) will fit. Some have different gear ratios so watch for that.
And L/U's older than mid-70's have carbon-steel driveshaft which are subject to corrosion around the oil seal area. If you find one of these be aware that repairs may be required and pay accordingly. The later models with stainless shafts are better.
One last thought, be sure to check your control box and shift cable for proper operation/adjustment. It'd be a shame to ruin a lower unit because of problems there.
HTH & G'luck........ed
