FS5W71B Transmission Failure. How and Why

[zKars]

Friend had an exciting day yesterday. 75 280 with the early Z (78-80ish) 5speed (the one with no reverse lockout).  Pulled away from a stop light in 1st, got about 10 ft, and BANG!!!!!!! And the car stopped dead. Needed a tow home. 

He drives the car 50,000 km a year, drives it daily. Has a modified L28 “maybe” 200hp. Not a sense of anything going wrong, just WHAM. I rebuilt this trans over a year ago with the usual bearing and sychro refresh. 

He pulled it last night, brought it over this morning, we tear it down. Found something I’ve never seen before.

[SteveJ]

Does that mean we're going to see a photo of a broken shaft?

[zKars]

The tranny was indeed locked up. Nothing that you can see on inspection of gears, syncros, shift rods etc. Trans shifts ok into every gear except 5th. My first guess was a loose big main shaft nut.  Nope.

When in neutral, you turn the input shaft by hand the output shaft turns and is hard connected to the output. Oh oh. Something under one of the gears or synchros has a failed (sleeve bearing?) and is not allowing each idler gear for each speed to spin free in neutral.  Hmmmmmm

Well, start at the back end and start to take 5th gear apart. Get the big nut off (it was still staked and tight), and then try to slip  off the fat washer behind it. No move-y. Get a hammer. No move-y. Get a chisel to get between that washer and 5th gear. Hit it. No move-ness. Nada.  This is the one with the little ball and groove that prevents it from moving.

Well after using more and more “assistance”, the washer finally comes off. And what should I find?  Behold

[zKars]

Seems the little ball decided to try to leave home for greener pastures. In doing so, it deformed the groove in the washer, causing it to lock against the gear face next to it, effectively locking 5th gear to the main shaft. Usually it hits the sleeve under there where the sleeve bearings run, and not touch 5th gear. 

I hope you can see the deformed area at the top of the groove arch. This is enough to touch and lock quite solidly against 5th gear.

[zKars]

Now in the next trans revision, this was apparently a known issue because , here is what they did.

Big fat pin and much larger slot in the washer. 

[zKars]

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…..

Edited June 26, 2023 by zKars

[zKars]

And of course we will speculate endlessly about what caused this in the first place. Notice the presence of what I can only say is rust near the failure area. It was also under the bearing sleeve that this washer tightens against Only spot I see any such rust.

Trans was filled with Redline MT 90. 

I will say the big nut wasn’t as tight as I likely left it. Peen area was still peened, but I’ve seen trans with very loose big nuts where the peen was still in place, as if it backed off one (or more) full turn then stopped back at the peen area. I know from experience, I no longer un-peen before backing the nuts off, they quite happily un-peen themselves as I turn them off (hand wrench, yes long hand wrench), so that collar/peen area is really pretty soft.  This nut was NOT backed of to loose, just not to torque spec. 

Edited June 26, 2023 by zKars

[Zed Head]

3 hours ago, zKars said:

Once the big nuts are tight these washer ain’t gonna go no where, and so what if they did rotate some?

Seems like they just want to make sure the "washer" (l'd call it a spacer) spins with the shaft.  Edit - obviously.  Might be that any dimensional differences across the "washer" could cause some weirdness if it moved.  

Funny, but the 5 speed has more of those balls than the 4 speed even though, supposedly, the 5 speed is just a 4 speed with an extra gear.   Actually, a lot more differences than I thought.  Edit - looks like 4 speed shafts might not work for 5 speeds and vice-versa.  The washer you're looking at is 21?

Edited June 26, 2023 by Zed Head

[Zed Head]

Edit - I found them, thought they weren't there.   Hard to find, I had to search for the word to get the drawing number then look at the drawing again.  One of the 5 speed balls has (1/4) after it, don't know what that means.  Glad I'm not rebuilding any transmissions.

Edit again - I notice that ball #66 has a different part number.  Any chance you put a small ball where a big ball should go?

https://www.carpartsmanual.com/datsun/Z-1969-1978/power-train/transmission-gears

Edited June 26, 2023 by Zed Head

[Zed Head]

[zKars]

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. 

Regarding the possibility that a too-small ball was used when I replaced it, well it’s possible. If I lost that ball and replaced it with another, but my replacements are the “same” size within measureable limits.

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. Only guess I have at the moment. 

 

Edited June 27, 2023 by zKars

[Captain Obvious]

Yikes!    That's not good!

Glad it didn't shatter anything and send chunks between gears!