Autopsy of a 4 speed stuck in 4th

[Digger48]

I have a 4-speed transmission (F4W63) from a '73 PL620 that got stuck in 4th gear. I'm trying to figure out why.  I bought this one for $100 from a guy who was planning to put a 5-speed in his truck.  I took it home, drained the fluid from it, opened up the bottom cover plate to examine it, didn't see any metal pieces, or any obvious problems, so I installed it in my truck and filled it with gear oil and it seemed to work adequately.  However, I noticed right away it tended to grind gears when I shifted, unless I slowly eased it into gear.  After driving it around town a few times (well under 100 miles) it got stuck in 4th gear.  (I found a 5-speed from a '77 620 (a FS5W71B) but am going to have to rebuild it -- but that's another story.) 

 

Before taking on the 5-speed I decided I would see if I could figure out why my 4 speed got stuck in 4th.  So I started to disassemble it.  Mind you, I have no experience, skill or proper tools (like a press or bearing puller) to attempt a transmission rebuild but I thought I might sharpen my teeth (bad gear pun) on the 4-speed. I am part way into it and have a few questions I'm hoping one or more of you experienced folks may have some answers to.

 

What I found:

-- This transmission had been opened up before (excessive gasket cement, a few marks like someone had banged on a piece or two with a tool or hammer)

-- Upon removal of the detent plugs, the springs came out, but the check-balls would not roll out despite my efforts of turning the case over and tapping with a non-metal mallet, spraying solvent up into the tubes, blowing compressed air in the tubes, and trying to loosen them with an ice pick.  I later got them out after removing the selector rods. They were apparently gunked with dirty grease and would not release.

-- In removing the selector rods I may have damaged one of the interlock plungers and at least scraped one of the selector rods..  In hind-sight I should have removed the tension pin on the 3rd-4th selector fork and tried to move the rod to a neutral position before driving out any of the rods.

https://i.postimg.cc/cJHmvmpX/revrse-selector-rod-damage.jpg][/url][/img]

https://i.postimg.cc/c4yXFf3L/Damaged-interlock-plunger.jpg][/url][/img]

-- The reason it was stuck in 4th was the 3rd-4th coupling sleeve had moved so far forward (onto 4th gear) that it would not come back to neutral -- likely because it had gone beyond the edge of the synchronizer hub and one or more of the shifting inserts had moved and blocked  it.  I could not even pry it back with a screw-driver.

[/img]

https://i.postimg.cc/YCt8Hkbc/stuck-in-4th-label-comprssd.jpg][/url][/img]

-- When I withdrew the main shaft assembly (which separates from the main drive gear between the 3rd-4th synchro assembly and 4th gear) 2 of the 3 shifting inserts popped out along with the forward spread spring.  The coupling sleeve remained on the main drive gear (4th gear) but came off easily once the synchro hub and inserts were out of the way.

-- Reverse spur gear had chipped teeth (yet no metal was found in bottom of case).

 

My Questions:

-- Why would the coupling sleeve have moved so far onto 4th gear?  Is the only thing preventing that from happening the check balls and indents in the selector rod?  If so, is it possible that the gunked up check-balls might have failed to stop the selector rod from going too far?  And, if I cleaned the detent tubes and assured the springs were good,

-- Was the original grinding when shifting gears likely due to worn baulk rings? Would that have had any relation to the coupling sleeve going too far?

 

The big question:  Assuming I can even get a new interlock plunger; in light of the damage to the selector rods should I bother trying to rebuild this?

[Digger48]

A couple pictures that I'd intended to include in the original post-- (I hope) -- am not doing something right for copying the url....

[datzenmike]

I think the detents are on the far side. From L to R it's 3dr, neutral and 4th. The large one on this side acts like an inter lock to prevent the 1st / 2nd shift rod from moving when in 3rd or 4th but another detente ball drops out of the way in neutral. This rod is back in 3rd and forward in 4th. That dimple looks like the ball has worn the shift rod from running in forth a lot. This wouldn't cause the sticking in gear.

 

Probably wear. I had a 521 4 speed stick in 4th and nothing could get it out. I had to find another.

 

The reverse idler is always going to have chipped teeth. There is no synchronize to help mesh the gears. Reverse may make more noise but you don't use it much anyway. Lack of chips could be they came out with an oil change or when it was taken apart before.

[Stoffregen Motorsports]

To get the sleeve back over the hub, you need to depress each of the detent pawls so the sleeve will slide back over. You can likely get two at a time, but then you'll have to hold pressure onit while turning it over for the third.

 

This is a solid case in point of why it's important to set the shift stops at the front cover of the trans. As I've mentioned before, you can simply not run a gasket on the front cover, which will tighten up the shifting by .050" or so, but you can also weld a small bead onto the inside of the front cover and grind it to fit. Doing this will make a stopping point for the shift rod and you'll never have this problem again.

 

I wouldn't worry too much about the small nick you made in the shift rod. Just file it smooth and re-install it.

 

Also, I use a small magnet to get the detent balls out of their sockets.

[Digger48]

Thanks to both of you for your replies.  I will see if I can figure out how to implement a "stop" to prevent the rod from going too far forward into 4th.  But before I do, do either of you have an answer to the question: What was the likely cause of the grinding when shifting?  Was it merely wear of the baulk rings?   Thanks.

 

[Stoffregen Motorsports]

Grinding into gear is usually associated with bad synchros, but it can also be caused by other things. Worn engagement teeth on the shift sleeve can cause grinding, but more likely it's the gear oil that you're using. I use Redline MT90 in all of my Japanese manual transmissions. It is slightly less slippery than conventional gear oil which helps the synchro grab the gear a bit quicker.

 

Mike has another brand of oil that he likes, and I always forget what it is. Maybe he can tell you.

[datzenmike]

AC Delco Synchromesh Transmission Fluid" ... General Motors Canada Part # 89021808.... MSDS #CN35916
AC Delco Synchromesh Transmission Fluid - Friction Modified" -GM USA Part # 10953509

 

Different countries, different labels, same contents. Totally GL4 compatible.

 

Excellent cold shifting, faster or less effort up shifting and now down shifting with out grinding ... I am impressed with this stuff. It's about $17 CDN a liter at Lordco so about $35 to fill a 71B, an F4W63 should be about the same, about 2 US quarts.  

 

It's main ingredient is made by Shell, called Shell MTX fluid. I doubt every transmission oil is made separately... why re-invent the wheel? With a different coloring additive it might be Red or Purple and called something else, if you know what I mean.

 

 

[Stoffregen Motorsports]

I have a long history with Redline. I knew the proprietor of Redline and I have friends that work for Redline. I also know that their products are marketed to fill a gap or for a specific motorsports purpose.

 

I agree with you on the marketing aspect of the ingredients though. It is all probably made from the same stuff.

[Digger48]

Thanks for the info.  In checking with Transmission Exchange in Portland I am unable to get a kit with baulk rings, seals, gaskets, etc. for this 4-speed.  The Datsun dealer said most of the parts are available, but their prices are pretty high (quoted me $56 per baulk ring -- ouch), so I have decided to put this 4-speed back together as-is (with the very-likely worn baulk rings) and look at the 71B 5-speed.  I will give this 4-speed (F4W63) to anyone who wants it. 

[Stoffregen Motorsports]

Yeah, OEM synchros are not cheap.

 

One trick is to change the synchros around,  in other words, take the fourth gear synchro (which almost never gets worn out) and swap it onto first gear, etc. You can tell how worn they are by how far down the cone they sit.

[Digger48]

Thanks for the suggestion.  I did check the baulk ring to gear clearance with baulk ring pushed fully toward gear.  The service manual recommended space of 1.2 to 1.6 mm was more than double the approx. 0.6 mm I had with 4th and 3rd gears.