Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model?

[fomoco-gnatsum]

Here's my problem! My 85hp merc inline 4, I think is a 1977 according to the ser. # 4846571<br />The lower unit is history. I got another used lower from a marine mechanic. Now here is the dilema: this driveshaft on the used lower has the preload pin on the top of the shaft, the mechanic said it would go in even though my 85hp unit I took off didn't have this (I believe the gears are cut different,this wasn't the OEM unit it had also been changed once before)<br />So my question, what type is supposed to go with my original powerhead? I got sources for a couple of other units but I want to know which one is the correct one for my engine? before I spend the? and which ones can be swapped. I've seen many different types on EBAY. The one that came off my engine has the plastic insert for the water intake, the one I picked up has the holes cast right into the unit!<br />Any help would be greatly appreciated! <br />Thanks

 

[OhNoMrBill1960]

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Hello FoMo,<br />I'll pass on what I have learned this summer being as noone else has answered you. I bought a boat with a 64 650 mercury and I knew at purchase the lower unit was kaput. I then bought a used 1974 850 motor and removed the lower unit off of it. There was a pin on the top of the shaft and the mechanic told me to cut it off. There was an o-ring on the old shaft and none on the new he said it didn't matter. One had a sort of rubber box on the bottom around the water pump the other didn't. Long story short I made sure the water pipe was hooked up and it all lined up well and we have been running it all summer with no probs. Maybe I was just lucky. But at least you are going from an 850 to an 850. If the mechanic says it'll fit, then maybe it will.<br /><br />Good luck<br />Bill from Cornwall

 

[fomoco-gnatsum]

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Hey Mr. Bill<br />Small world, I know Cornwall well I had a friend that lived in Ingleside, been to the city a few times Racing my 5.0L Stang. Anyway funny thing I too got this used lower unit (I think a 65hp) it has this preload pin and I cut it off because I couldn't get it to index or fit up into the crank (I thought it was to long) so I too cut it off!. It mated up then! so I buttoned her up, filled the gearcase and fired her up w/earmuffs. Ran fine in both fwd. & rev. then I thought I'd check the gear oil level. So hears the bad news it was nothing but cream So I knew the seals were no good. So when I removed it and took it to the marine mechanic and told him what I did, he said cutting off the pin was a bad thing! and that eventually the gears would self destruct w/o the gears being preloaded! who knew. So I'm at a loss what to do. Change the seals and reinstall and hope it last or just find the proper l/u for mu engine? but funny thing I saw on EBAY a guy had a l/u for a '75 *%hp that had the pin so I give what is proper for my year and when did they change things?<br />Thanks again!

 

[dvchristoff]

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Re: Mercury 850 Thunderbolt (85hp) What is the correct Lower Unit for this model

Driveshaft Arrangements,<br /><br />The three types of driveshafts are:<br />"Old" driveshaft with a preload pin at upper end.<br />"Old" driveshaft without preload pin.<br />New "Alpha" driveshaft without a preload pin and a new cut to gears.<br /><br />All three are interchangeable and newer is better. This is from a 1965-1989 Repair Manual. I just put a 1988 Alpha Drive in my 1974 1150 that had a "Old" driveshaft with no preload pin and it works great. As far as when was used during what years the manual does not say. My guess, 60's with pin, 70's no pin, 80's alpha.