diagnose broken lower (clutch) shaft - 1957 7.5hp

[chri0891]

I recently rebuilt the lower unit, magneto, and carb on a non-working 1957 7.5hp. After reassembling lower unit the driveshaft turned but hard. Other (smarter) members advised this shouldn't be the case so I pulled it apart again, visually checked everything seem to align and mesh properly, but same thing once assembled again. I saw surface rust on the lower shaft and figured (naively) maybe it was just tight from sitting dry for so many years and needed to be run to lube/loosen up the interface between parts, seals, etc.

Sure thing it fired up- had a little rattle from the recoil spring but I didn't sense a strain on the motor. Had a few good outings and maybe 5th time out felt like I could hear a rub, felt like motor was struggling against throttle. Stop being paranoid, I told myself. A minute later wham - it threw hard to the side, and seized. Confirmed this week that the lower shaft sheared inside of that wider collar. Other gears/splines look undamaged, but a couple things strike out at me:
1) My lower shaft doesn't match the schematic. Mine has a narrowed segment that would sit above the sleeve in the gear case, with a washer (see photo). Is this normal, so the shaft doesn't spin directly on the aluminum gear case?
2) Where that washer would sit (looking down the gear case from above) the surface is pitted/damaged. Almost looks like two tabs on one side, or like the aluminum wall is torn up. But I didn't find any metal fragments.

Kinda unsure on next steps. I'd rather not install a used replacement shaft just to repeat the process. Except for the surface where the lower shaft/washer sits, the gear case looks good but I don't know if it's salvagable. And from what I hear the quality of used cases are hit or miss.

??


Looking down into gear case housing:

 

[racerone]

Yes that is the way they are from the factory.----That is what is called a " slip clutch "-----Fairly common for the spring inside to break.-----Then the thing gets longer and jams up.----Last year they used it was the 1965 model 6 HP.-----Lots of used ones around if you know where to look.

 

[chri0891]

Well that's good news about the washer. As far as the condition of the housing, the inner surface looks chewed up, but since the washer should be seated on it maybe that's ok. But are those ragged ears/tabs a concern? Or again a normal condition from how the housing was cast - or even purposefully there to align the washer. The ridge above the shiny seat looks a little off center in that it's narrower on one side and wider on the other, but hard to tell whether that's from the factory or if it has been worn down.

 

[F_R]

I've had many hundreds of those lower units apart, and must say I've rarely if ever been concerned about the issues you mention. Far more common is the damage done to the upper bearing, even to the point of breaking the thin bearing plate.

 

[racerone]

Agreed with the above post.------I am too far away likely.----Have many of those shock absorbers.-----Just find yourself a good used one and assemble it.----Likely to need a good inspection of that upper bearing plate as stated.

 

[chri0891]

Thank you for the replies! This makes sense about the shock absorber. I don't know how I missed it earlier but I found an older post that had a good image explaining how it works, and breaks. And I found a used one here in MN, so that's a start.

With better lighting and more time I picked through the whole lower unit. Just as you said, found more damage above.
1) Lower pump housing was shoved up and broke around its collar. It pushed against the upper (thin) plate, cupping it just a tiny bit, 0.03" up in the center (maybe salvagable?). But I do have minor pitting on the upper housing piece and now I'm thinking I should maybe replace the whole assembly with one of those newer ones with plastic housing.
2) Upper drive shaft, top edge of the flat, surface is rough, looks like corner is sheared a bit. Exact same on the other side. Bad enough that it should be replaced?

 

[racerone]

Oh so typical of the damage.----Just need a new bearing plate.----That shaft will last another 36 years I think.

 

[F_R]

Those shaft flats are almost always worn like that. No problem. Have you discovered how rare and expensive those bearing plates are? If you find one at a reasonable price grab it.