Re: merc 70 lower unit
Well, the first 70-3's came out around '76. Sometime mid-80's Merc went to prop-rated HP instead of crankshaft-rated and the motor was re-listed (and probably detuned) as a 60HP. Production of the 49.8 Cubic Inch Three continued into the 90's and included 50HP & 45HP flavors as well.
You'd have to do a bit of research to find all the various HP's but there's a wide array of models and years to choose from. It appears that there may also be some cross-over with 2-cyl and 4-cly models but you'd be wise to take careful measurements if getting parts from other than a 3-cyl donor.
There should be plenty of parts around, too, considering the Self-Destructing nature of these motors!
You definitely do not want to overprop, this motor should be propped to achieve WOT rpm at the high-end of its recommended operating rpm, with a lightly-loaded boat & only the driver aboard.
There is a water-circulation mod recommended from the factory, which moves the telltale water outlet from the side of the block or from the cyl block water cover, to the top of the block (this ensures a fully-flooded block with any entrapped air vented out the top). This should be done if not already there. You'll need to drill/tap the top of the block for the 3/8" N.P.T. telltale fitting and plug off the old hole.
Last and most important of all, to prevent Thermonuclear Meltdown you oughta replace the impeller every year. These motors will not tolerate a loss of water, and will actually crack the block around #1 cyl if water pumping is lost at speed.
Anyway, since you're replacing the lower unit it's a good time to renew the impeller. At the same time, check the pump plate and cupped insert for wear and replace as necessary.
I spotted a few L/U's in a short 'cruise' on eBay:
http://cgi.ebay.com/ebaymotors/Bran...at_Parts_Accessories_Gear?hash=item414a585634
This one has a new water pump and reasonable shipping:
http://cgi.ebay.com/ebaymotors/Merc...at_Parts_Accessories_Gear?hash=item2a0263ec94
HTH & G'luck with the lower unit search..........ed
