Yamaha F100 TLRY - 2 lower units but only one shaft fits?

[westcoastrib]

I recently bought a used lower unit from a 2003 Yamaha 75hp. According to boats.net, and the seller, this unit is compatible with my F100.

However, when I went to install the "new" (used) lower unit, I can't get the drive shaft splines aligned. What's frustrating is that I have no problem re-installing the old lower unit. (the old lower unit has a damaged bearing carrier that I can't get into quite yet)

I've watched dozens of videos, searched for answers on forums and can't figure this out, so any advice on things to try will be appreciated.

(note: I know to be in forward gear, gently more the prop to try and align splines, also that I need to align the shift shaft and water tube ... but I'm not even getting that far with new lower unit drive shaft)

The only thing I notice, is that the old drive shaft splines seem to be slightly more narrow than the new splines? I'm stumped.

 

[dingbat]

Looking at the parts breakdowns…. I’m seeing different part numbers for the 75 and 100 and the 100 has lists two different part numbers for the shaft.

Don’t know the differences but your dealing with three possible part numbers

 

[westcoastrib]

Thanks dingbat. May I ask where to go to try and determine if there are differences between those driveshaft part numbers? I used boats.net to look at the parts breakdown, and for the 2000 model year, they do list that the 75hp Yamaha lower unit is compatible. I also measured the shaft lengths and diameters (used a micrometer for diameter) and they seem to be exactly the same. I counted the splines and each has 18.

Is it possible that the differences in those 3 part numbers are simply spline thickness?

 

[boscoe99]

I am seeing the exact same part number for a 2003 F75 and and 2000 F100.

I have always swapped the gear case on the larger Yamaha motors with the motor in neutral. But I don't think that is your problem.

 

[dingbat]

I am seeing the exact same part number for a 2003 F75 and and 2000 F100.

I have always swapped the gear case on the larger Yamaha motors with the motor in neutral. But I don't think that is your problem.

I was comparing 2003 models. Didn’t catch the switch to 2000

 

[westcoastrib]

I am seeing the exact same part number for a 2003 F75 and and 2000 F100.

I have always swapped the gear case on the larger Yamaha motors with the motor in neutral. But I don't think that is your problem.

Thanks boscoe. I appreciate the help. I’m trying to eliminate items that could be causing the problem. I have a marine mechanic friend coming over tomorrow to give me a hand. I’ll post the outcome afterwards.

 

[mickyryan]

i think i know problem as i kinda had a similar issue the oil pump spine isnt lined up to the shaft spline that was my issue when i replaced a lower somehow i spun bottom shaft before it was engaged into the top and the splines werent lining up and would not drop in

 

[westcoastrib]

i think i know problem as i kinda had a similar issue the oil pump spine isnt lined up to the shaft spline that was my issue when i replaced a lower somehow i spun bottom shaft before it was engaged into the top and the splines werent lining up and would not drop in

Thanks, mickyryan. If I understand you correctly, the reason the old lower unit driveshaft will fit into the upper unit, is that perhaps the splines are damaged slightly, but damaged in the "right way" so that they still slide up into the upper and align? The consequence is that the new lower unit driveshaft I purchased isn't damaged at all and won't line up with the splines in the upper unit?

 

[mickyryan]

try spinning as you slide it up you will engage oil pump first then the crank once you feel it click into crank push it on rest of way

 

[mickyryan]

at least thats how it worked for me after fighting it for about a hour trying to figure out what was holding it out of place almost exactly as yours looks

 

[stresspoint]

im in for a look here to see what the issue turns out to be , iv swapped f80 to f70 , f 80 to f100 no issues never gone to f70 from f100.so i am keen to see the outcome here .

sorry mod s , i pressed report instead of edit

 

[stresspoint]

any results yet on this LU fitment ?

 

[westcoastrib]

Thanks again for the people that took the time to suggest ideas. Spent more time with the engine this weekend… unfortunately still won’t go. I had a marine mechanic neighbour help and he is stumped also. He though there might be some slight damage to top of new shaft splines. We did some light filing, but no luck. I did measure the spline width on the two drive shafts… my eyes didn’t deceive me. The new drive shaft splines are 1/64th larger. (1/16th vs 3/64th) Not sure that’s enough to stop progress… nor do I understand why they’d be slightly different in size.

We also used a brighter flashlight to look up into shaft cavity and flywheel splines. My marine mechanic neighbour is a bit concerned it doesn’t look great. Hard to see though… and again, a good connection with the old lower unit that still slides in with no problem.

Next steps I hope to take are using a buddy’s camera scope to look at condition of flywheel splines. Other than that, I could file the new driveshaft splines some more, but will likely wait to see if I can use a camera scope to see what I’m dealing with first.

Won’t be easy to take off the power head to look.

 

[mickyryan]

your problem sounds and looks exactly like mine and even the spacing i still say the oil pump and drive shaft arent aligned

 

[saltchuckmatt]

Thanks again for the people that took the time to suggest ideas. Spent more time with the engine this weekend… unfortunately still won’t go. I had a marine mechanic neighbour help and he is stumped also. He though there might be some slight damage to top of new shaft splines. We did some light filing, but no luck. I did measure the spline width on the two drive shafts… my eyes didn’t deceive me. The new drive shaft splines are 1/64th larger. (1/16th vs 3/64th) Not sure that’s enough to stop progress… nor do I understand why they’d be slightly different in size.

We also used a brighter flashlight to look up into shaft cavity and flywheel splines. My marine mechanic neighbour is a bit concerned it doesn’t look great. Hard to see though… and again, a good connection with the old lower unit that still slides in with no problem.

Next steps I hope to take are using a buddy’s camera scope to look at condition of flywheel splines. Other than that, I could file the new driveshaft splines some more, but will likely wait to see if I can use a camera scope to see what I’m dealing with first.

Won’t be easy to take off the power head to look.

Not sure on your motor but I have swapped driveshafts in lower units where the top is different but the bottom was the same.

 

[stresspoint]

i would not recommend filing the surface off the splines , that could lead to rust.
thank you for the update

 

[westcoastrib]

try spinning as you slide it up you will engage oil pump first then the crank once you feel it click into crank push it on rest of way

Thanks, mickyryan. I'm not clear on what you mean to do? It gets "blocked" about an inch and a half from going all the way up ... which matches the length on the top splines on the drive shaft. To me, it means that even if i twist or spin the shaft, I still bump into a metal-on-metal problem and can't get it to go further. Definitely don't hear or feel any click. (again, I can get the old lower unit in and know what that nice "click" feeling is when it seats nicely into the splines) With enough force, I seem to be able to wedge it in, so part of the splines are meshing but feels way too tight. A couple of times in frustration, I push hard enough and it actually takes me yanking on the lower unit to get it back out.

 

[saltchuckmatt]

Thanks, mickyryan. I'm not clear on what you mean to do? It gets "blocked" about an inch and a half from going all the way up ... which matches the length on the top splines on the drive shaft. To me, it means that even if i twist or spin the shaft, I still bump into a metal-on-metal problem and can't get it to go further. Definitely don't hear or feel any click. (again, I can get the old lower unit in and know what that nice "click" feeling is when it seats nicely into the splines) With enough force, I seem to be able to wedge it in, so part of the splines are meshing but feels way too tight. A couple of times in frustration, I push hard enough and it actually takes me yanking on the lower unit to get it back out.

Yes, it's not right. You can't beat a SQ peg into a round hole.

Have you thought about swapping the shafts?

 

[lakensea]

Remove spark plugs, rotate flywheel while inserting drive shaft until lower splines line up with oil pump drive.

 

[westcoastrib]

Yes, it's not right. You can't beat a SQ peg into a round hole.

Have you thought about swapping the shafts?

I have thought about swapping shafts. The pain point is that I'll need to drill out the bearing case retaining nut, and probably the bearing case as its seized right into the lower unit. (tried lube and heat and force and no luck) I was hoping to avoid this by buying the lower unit I'm now struggling to put in. But that's an option I'm considering.