FS5W71B Transmission Failure. How and Why

[Captain Obvious]

Thinking about it a little mire... I've not been inside a Z transmission, but here's some thoughts:

47 minutes ago, zKars said:

I plan to spot weld the big nut to the big washer to prevent it from turning, effectively duplicating the function of the ball. Drilling the main shaft for the later 5mm pin upgrade is not possible due to incredible hardness of the main shaft. 

I had to re-surface both the washer and the face of the gear it touches (sand them flat) in order to get them to turn freely when the nut is torqued. Both were a bit chewed up but there is virtually no clearance when tight. Just enough for an oil film. Can’t let any defects remain that can get the fit in trouble again. 

Seems the presence of a bit of rust and slightly loose nut allowed that washer to turn under the tremendous torque of a mighty L series and caused the deformation that turned into a lock up.

I'm not so sure welding the washer and nut together is a good idea.

My read on the way it was designed is that locating feature on the washer (whether it's a ball or a pin like the later design) is that feature exists exactly to isolate the nut from any rotational forces from the gear on the other side. In other words... You've got this gear spinning on one side and a nut that could potentially be loosened (or tightened) on the other side. They wanted something in addition to the peen on the nut to prevent that nut position from ever changing. So they used that locating feature to keep the washer from ever spinning with respect to the nut.

If you weld the nut to the washer and completely skip the locating feature, you will allow whatever rotational forces are ever generated to be directly translated to the nut. You would be counting solely and completely on the peen to prevent the nut from changing position and I'm thinking that's not a good idea.

In fact, I'm thinking you've got a chicken and the egg thing going on with the failure... Did the nut loosen up and allow the ball to slip out of place? Or did the gear transmit too much torque to the washer somehow (lack of oil, burrs on the surface, debris between the two, something) and start to spin the washer. And then when that washer spun, it ripped the ball out of the hole and started to rotate the nut.

So I'm wondering what happened first, did the washer (and ball) slip first which loosened the nut, or did the nut loosen which allowed the washer (and ball) to slip?

[zKars]

Cap’N thanks for the excellent thought process, yes, hard to say what caused what. 

So plainly I should weld the nut to the main shaft AND them both to the washer, and nothing bad can happen. Right?

Anyone want to buy a slightly used 5 speed?

 

[EuroDat]

On 6/26/2023 at 9:05 PM, zKars said:

So. How to “fix” this. I might have one of these larger pins and matching washer, I guess I can drill out that little half moon hole to the PIN size and put it back together.

So why is it such a big deal to not have these washers rotate as you tighten the main nut or in road use? Once the big nuts are tight these washer ain’t gonna go no where, and so what if they did rotate some?

Same ball and groove on the big washer on the front of the counter shaft main bearing. I’m tempted to just leave the ball out and bolt it all up…..

I wouldn't leave the ball out. If the washer gets the chance to spin it will weld itself to the main shaft, trust me☹

I remember a guy bringing in a seized tranny and the washer was friction welded to the main shaft, after he rebuild it himself.

Our conclusion was the main nut was not torqued correctly. We had seen it in a couple of our rebuild transmissions where it became hard to use the gears and it still wanted to move forward in neutral. To make sure that didn't happen again, we torqued the nut with a homemade (workshop) tube socket that fitted over the length of the shaft.

We tested it a couple of times by tightening the nut first by hand and then checking with the tube socket and torque wrench. It was then we found that by hand we were tightening the nuts to about 3/4 specified torque iirc. This is all going back to the eighties.

The guy with the welded washer. The main shaft and fifth gear were a throw away, but for the rest of the transmission it was in goed condition.

 

[Captain Obvious]

10 hours ago, zKars said:

So plainly I should weld the nut to the main shaft AND them both to the washer, and nothing bad can happen. Right?

Exactly. That will take care of it.  

Failures like this are really intriguing. When the "impossible" happens. I will have to look at the gear train and flow of power, but I wouldn't have thought that portion of the gear train was even under stress when that failure happened. He was in 1st gear, right? And the wheels locked up and skidded to a stop after the "bang"?