Re: 1976 85hp to 1972 80hp lower unit swap?
Blow out the telltale with compressed air. It should be just a matter of 5-10 seconds (typically) after starting the motor 'til you get water out the telltale.
The lower units are interchangeable and even have the same gear ratios.
One slight diff is that the older lower unit may have a groove for an O-ring seal just under the driveshaft splines, this seals to the crankshaft.
Later models use a seal assy pressed into the crankshaft and have no groove. If you find this to be the case, measure the location of the old groove. If this is to be a permanent "upgrade", you'll want to grind a new groove in the donor lower unit. This O-ring seal is vital to keeping water out of the drivehaft/crankshaft splines. Otherwise water is gonna get in there and rust out the splines or lock them up so bad that eventually the lower unit will be 'permanently' installed (stuck).
If this is just a temporary thing, grease up the shaft real good (but none on the very end), and install.
The later-model L/U may also have a tapered driveshaft bearing which requires a 'preload pin' at the top of the shaft. This makes no diff to the installation on an older motor. If the pin is present, just make sure it's free to move in-and-out. This preload is necessary and if the pin is stuck inwards with no spring pressure, the L/U gears won't last long.
HTH........ed
