79 Merc 9.8 bottom end locked up - pictures attached

[darrylgood]

Hello all,

I have taken off the bottom end and cant turn the prop by hand. Seems like there is a gear or something inside that is stuck. How can I access this? I assume there is a special tool, but is there another way in? Hopefully the picture shows what I mean,

Thanks so much

 

[Texasmark]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

You are looking at the secret in the second photo. Notice the inside of the LU housing is threaded. Between the shaft and the housing is the "spanner" nut that runs in these threads. You have to unscrew it to get at the lower unit. This was not Merc's finest hour as the housing being aluminum, corrodes and makes unscrewing the spanner nut almost an impossibility.

I think I saw an arrow in about the 2 o'clock position in photo #2 and adjacent to the arrow I think I am reading the word OFF. If that is what I am seeing, it indicates left handed threads, meaning you turn the nut CW to remove it, unlike the normal right handed threads where CCW is removal. Usually LH threads are only used in unusual circumstances where a rotating shaft/object MIGHT loosen the nut...like years ago lug nuts on autos were LH on the RT side of the vehicle (as I recall).

Once you get the nut out, the drive shaft and the rear bearing carrier, and R gear and the shift dog all come out with the shaft. F gear and bearing remains up front and the drive pinion gear is at the top of the empty housing.

I made a spanner wrench to fit mine, a much larger engine and the threads were RH. You can check the runout on the threads and determine which way they are pitched and thus whether or not you are dealing with LH or RH threads before you attempt to remove the nut.

Corrosion is going to be your enemy big time, not only in attempting to get the spanner nut out but in trying to pull the bearing carrier once you get the nut out.

Just remember you are dealing with CAST aluminum and it does not take kindly to prising on it...it is brittle and will break.

Since you are going to replace all the rubber parts any way, heat and penetrating oil will surely help as will running a thread chaser in the housing threads before you start. Give the penetrating oil time to work and use a quality oil. PB blaster is popular around here and works great.

Laddies and other mechanics on here can help you with some tips they surely learned over the years.

Good luck,
Mark

 

[oldman570]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

As Texasmark states, not one of Mercs finest devlopments of the time. The spanner nut is lefthand thread and can still be bought if broken. A special spanner socket is usally needed to mremove the nut. On some of the older motors there was a thrust washer of stainless that held a spanner that could be used to remove the nut. If yours has this type, which it dose not look like, you could luck out. Otherwise you will need to do as Tex has stated and make a spanner or beg ,borrow, or steal one. Being you cannot turn the drive shaft, I think that you have bearings that are seased up or totaly shot from water intrusion to the LU. Yor best bet might be to see if you can find another LU that is good. Could be cheaper that rebuilding the one you have now and alot quicker fix. Check out Ebay and the salavage Yards listed in the sticky FQA on motors here on Iboats for the list. JMO
Oldman570

 

[Texasmark]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

Yor best bet might be to see if you can find another LU that is good. Could be cheaper that rebuilding the one you have now and alot quicker fix. Check out Ebay and the salavage Yards listed in the sticky FQA on motors here on Iboats for the list. JMO
Oldman570

+1 Great idea.

Mark

 

[Mi duckdown]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

From the picture, someone has already been in the lower unit. .02

 

[darrylgood]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

Thanks guys, really having a hard time find a used lower end that old. I will keep looking

Also that was me with a screwdriver and hammer (those marks on pic 2) just trying to see if I could get it to budge counter clockwise, but no luck with only pushing on one of the 3 notches

 

[oldman570]

Re: 79 Merc 9.8 bottom end locked up - pictures attached

Did you look at the thrust washer to see if it had a piece that could be remove on the back inside of it?Some were a stainless cup with that pice in it and it can be used to remove the locking nut from the LU. You have to knock it out of the cup if it dose have one. If it dose let us know so we can tell you how to use it to remove the lock ring nut.
Oldman570