Help with early 70's Merc 800 Lower unit choice

[Moody Blue]

Have three lower units for this motor, all of the same vintage.

Two do not have the spring loaded pin on the end of the driveshaft and the third does have the spring loaded pin.

Trying to select the BEST of the three units to install on the motor. All three units have clear clean lube oil and no visible oil leaks. The unit with the spring pin has lots of vertical free play in the driveshaft, about 1/8". Another unit (no spring pin) has a little less than 1/16" vertical play while the last unit has no perceivable vertical free play. Don't know the first thing about these units so can't tell what is good or bad.

Prop shaft turns freely on all motors but seems "free-er" on one of the units. Can't measure any runout on the prop shaft with my crude test setup. There are two types of driveshaft bearings in these units. Looks like ball bearings in two of them and tapered roller bearings in the other.

Any advice from the experts?

Are there certain things to look for or check?

 

[emckelvy]

Re: Help with early 70's Merc 800 Lower unit choice

The spring-loaded pin is supposed to exert force on the driveshaft when the lower unit is inserted into the crankshaft. This preloads the tapered bearing. So, when you pull upwards on the driveshaft, you're putting it in an abnormal condition.

Grab the driveshaft and push down firmly THEN see how it feels when you rotate.

Whichever one, regardless of pre-loaded pin or not, feels the best is probably the right one.

Some other deciding factors would be if the gear ratio was other than 2.3:1 which was the standard ratio for this motor; or if any of them had stainless cladding down by the water pump/oil seal area, instead of a mild steel driveshaft.

One last thing to think about, how was the original driveshaft sealed to the crankshaft? Older designs had an O-ring groove in the driveshaft. Later models had no groove, but also had a seal assy pressed into the bottom of the crankshaft which held an O-ring. You have to have one or the other, or water will get in the splines and rot away the crankshaft and/or driveshaft.

You can see the end of the crankshaft from the bottom of the motor (or if you've had the powerhead off, I can't recall) to check if the seal assy is there. If not, you need a lower unit with an O-ring groove or get creative with a Nukie grinder and get your own groove on!,,,,ed