Re: 64' lower unit on a 71' 65hp?
The bolt pattern is the same, but the older lower unit doesn't have provisions for the exhaust tube which is present in your '71 motor, also the shift shaft and reverse lockout mechanism are different.
Your motor has a 2.3:1 gear ratio which works best for prop selection, the older motor has a 2:1. Your best bet is to find another 65 hp of similar vintage, or any of these 66 c.i. I-4 models: "800", "850" "80" or "75". These have the same gear ratio and fitment as your motor. Later models do not have the O-ring groove in the end of the driveshaft and if yours does, all you need to do is grind a groove in the same place as the old one. Easily done with a cutoff wheel chucked in an electric drill, or air motor if you've got one.
Check your local
www.craigslist.org or eBay Motors as there's always something kicking around. You'll likely find a complete parts motor on Craigslist, however, for cheaper than most lower units on eBay.
If you can't find a compatible 2.3:1 unit, you can use a 2:1 (found in most I-6's) but you'll need to reduce prop pitch by at least 2" to get the same performance you have now. Personally I'd hold out for the 2.3:1 gears, they work so much better with these motors.
You can tell the gear ratio by turning the driveshaft 2 revolutions and counting the number of propshaft turns. 2:1 gets you one turn of the propshaft. 2.3:1 gets you less than one turn.
If you should find a lower unit that gets more than one turn of the propshaft for every 2 turns of the driveshaft, you don't want that one, it's an "overdrive" unit used on certain early 70's I-6's such as the 1350, it won't work well on your motor.
Here's a lower unit currently listed on eBay that would work, but would need the O-ring groove ground into the driveshaft (a bit on the $$pendy side, though):
http://cgi.ebay.com/ebaymotors/MERC...0-85HP-4-CYL-LOWER-UNIT_W0QQitemZ370363971185
Later models like this one have a redesigned gearcase with swept-back fin and stainless driveshaft, both are nice upgrades to your motor.
One last caveat, the lower unit must come from an I-4 or I-6, later model Small Inch I-3's and late 80's Big Foot I-3's and I-4's all have different units that are not interchangeable with your motor. I'm assuming you have a longshaft motor (measured at approx 20" from where it sits on the transom to the anti-ventilation plate on the lower unit). Make sure you get a lower unit with the correct shaft length for your motor.
HTH..........ed
