Mercury 85HP 850 Thunderbolt lower unit compatibility

[efdog]

My forward gear is bad and needs replacement. Upon searching around, it seems would be cheaper to replace the lower unit / gearbox. I have a 1976 850. Will any 850 lower unit fit? I've heard that the fleetmaster is built for a racing model. Does anyone know if it will still fit as I've found one cheap. Any tips or advise is greatly appreciated!

 

[Chris1956]

Re: Mercury 85HP 850 Thunderbolt lower unit compatibility

Any 4 or 6 cylinder inline lower unit from the early 70s to the mid 80s will fit. Check your gear ratio. It will be either 2::1 or 2.3::1. All the inline six motors have the 2::1 gear ratio, so that will be easy. If your gear ratio is 2.3::1, it will be harder to match, as you need to find a particular LU from an inline 4. Your alternative is to buy a new prop with 2" less pitch, to compensate.

 

[capri15068]

Re: Mercury 85HP 850 Thunderbolt lower unit compatibility

I have an 85hp(850 merc) that i am putting a new lower unit on . i have the lower unit set in forward. shift lever set in forward but when i push the unit up on it shives the shift linkage out of its hole in the power head .My question is . Is there an easy way to align the splines in the shift linkage ? I am pulling my hair out and i am almost bald as it is . please help

 

[emckelvy]

Re: Mercury 85HP 850 Thunderbolt lower unit compatibility

Disconnect the shift cable from the upper shift arm then reposition the shift arm in the general location for forward gear (arm towards the front of the motor). With the cable disconnected, you'll be able to freely move the arm just a bit to align with the shift shaft. What's happening is, in Fwd there's a bit of preload in the shift cable adjustment. This is necessary for correct shifting action with everything put together. However, when you're trying to reinstall the L/U this preload puts the upper shift rod slightly out-of-position from the L/U shift shaft.

An alternative to disconnecting the shift cable is to place gearbox and remote control both in Neutral then try again. Since Neutral has a positive detent both in the L/U and remote control box, you're more likely to get things lined up. Of course you'll have to have long arms or a helper to rotate the flywheel a bit CW to align the crankshaft splines and the end of the driveshaft.

To shift to Neutral, wrap a layer of tape around a pair of Vice Grips or slip-jaw pliers (Vice Grips work better). Very Very carefully grab the L/U shift shaft splines and rotate CCW. You'll feel resistance then it'll shift into Neutral (providing you're still in Fwd gear now).

Verify you're in Neutral by moving the prop, it should frewheel in both directions.

With the control box in Neutral you should be able to get 'er installed! Be sure to test-shift once you've got the L/U sucked-up to the center section. Shift to Fwd then back to Neutral then leave it in Fwd and you've got it!

Sometimes they'll just slide back together in Fwd gear like nobody's bizness; then you've got the ones that want to fight you. I've found disconnecting the shift cable to be the best solution in that case. Have never failed to reassemble a 'cranky' one after doing that!

HTH & G'luck on the reassembly..........ed