Lower Unit Question 85 hp(850) Merc

[frankenmerc]

I have a distributor type 1973 85hp mercury, having to change the lower unit,the original unit has a preloaded pin on the end of the shaft and the one i have to replace it with is off a 1977 85hp Mercury and does not have the preloaded pin.
is there any reason for concern or is this not a problem?
Thanks

 

[emckelvy]

Re: Lower Unit Question 85 hp(850) Merc

Lower units with the preload pin have a tapered driveshaft bearing which requires the preload; other lower units were designed to use a straight ball bearing which does not require preloading.

One very easy way to tell with the lower unit removed is to grab hold of the driveshaft and yank it up and down; a preload-design will have a lot of free play, whereas one without a pin and with straight bearing should have little to no discernible free play.

Fitment wise, it should bolt right up.

Only one caveat, if the old lower unit's driveshaft has an O-ring groove and the newer one doesn't, you might want to grind a groove to match. If both shafts appear identical in that respect (groove or no-groove), you're good to go.

Earlier shafts used an O-ring on the driveshaft, which rode on the inner diameter of the crankshaft; newer designs used a separate stainless seal assembly pressed into the crankshaft. The seal assy holds the O-ring in place, and the seal is formed against the smooth O.D. of the driveshaft.

HTH..........ed

 

[Chris1956]

Re: Lower Unit Question 85 hp(850) Merc

Merc, I made that exact swap a few times, although I put the gearcase from my '77 Merc 1500 on my '73 Merc 850. Worked fine...

 

[frankenmerc]

Re: Lower Unit Question 85 hp(850) Merc

Thanks for the input guys.I have had concerns about this and wanted to see what was the best route to go before i ran it with the other unit ,did'nt want to dammage my powerhead,here are some pics of the two shafts the one with the pin was the one on it and the other is the one i want to put on.