Hey there, folks!

Definitely a newbie here, but I have tried my best to use the search feature. If you believe you see an article that I missed, please feel free to correct me - I apologize if I missed something! Hopefully this doesn't end up in the boneyard of repetitive threads.

I believe I know the dreaded culprit of my problems, but I would like a second opinion before I jump head-first into some shenanigans!

About my car: 1996 (GS) 420A (N/A), F5MC1, POS project car. This transmission, as I hope you would know does NOT have the 36mm nut to tighten (just horseshoe clips). It looked like many folks had trouble with these nuts...no innuendo intended.

Okay, now that we have that out of the way:
My car will pop out of 5th gear occassionally, when on the highway. It seems to only do this at high speeds (70-85mph), and only after a certain period of time (generally after about 5-10 minutes of maintaining that speed), especially going uphill. I have tested this at lower speeds/high pressure (such as going up a hill at lower speeds), and it will not pop out. Essentially, as long as I am not on the highway, it will not pop out. To me, this is a bit odd, as I would think that this higher torque/pressure on the tranny at lower speeds would make it pop out more often.

Okay, next piece: I was a newbie and put in gear oil (I believe it was 75w90?) after my first rebuild. I made the mistake of going to the parts store and asking for advice, instead of doing my own research. My mistake - I own that. Since then, I have put in ATF4. Although I stopped driving it very shortly after it began to pop out, I believe the damage has already been done, and my guess is that synchros will need to be replaced.

Bullet points:

• I have replaced all bearings and seals
• ATF4 flush (I am thinking I will do another flush just to ensure that the fluid has not been contaminated in any way
• new clutch and release (throwout) bearing
• Synchro/gear clearance is within specs (per rebuild manual)
• visual inspection of gears is good - All dogs, teeth, keys, and sleeves look brand new
• synchros are NOT brass
• only about 129k on the engine, and I believe fewer on the tranny
• shift forks/pads/sleeves were not worn/damaged
• cables/linkage do not appear to be damaged, but have also not been adjusted
• has NO problem shifting. Super, super smooth; no grinding, no funky noises. It just pops out occasionally.

Alright - knowing this (I hope I didn't overload y'all with info), do you believe that synchros are bad?
My follow-up question to this: why would synchros cause issues with staying in gear? I thought their purpose was to "match" shaft RPMS to ensure for a smooth and efficient shift?

Second question: anything I should try before replacing synchros? I have no qualms about tearing out the tranny...again (this will be the 4th time, so hey, I'm at least getting faster!), but wouldn't mind trying a cheaper/faster alternative first. Cable adjustment, perhaps?

Thank you all for reading this! I hope y'all have a great and safe 4th. 'Murica!

I don't think the syncros are bad, I think you have shaft play from bearing wear.
Now I am not the resident trans guru but have built alot of them and most of the time that symptom was due to excessive play between the gearsets. In most of what I did it was between the input shaft and the gearset on gm models. Shimming the input into the gearset or shimming the gearset into the back of the input fixed that 95% of the time. Those weren't dsm transmissions tho. But excessive space between the gears made them pop out of gear. I would spend 8 hrs tearing one down and reassembling to put a .25 cent spacer shim behind the input to fix the problem. That's when I was alot younger too .

Thanks for the response! That is an interesting concept. When I rebuilt it for the first time this summer, I questioned the bearings (thus the bearing replacements. haha). The more I read and the more I investigate, though, I'm beginning to think it may be the actual case that is "loose" and allowing a small movement in the shaft bearings. I will try to post pictures tomorrow, but I noticed "wear rings" on the backplate where it presses the two shaft bearings into the case. It almost looks like the bearings have spun within the case. I'm going to take a guess and say that's not good. If this does have lateral play between the bearing/case, I'm guessing that this could allow just enough movement at higher speeds to pop it out. I'm considering putting thin shims between the bearing and backplate (small enough to still allow a seal between backplate and case, though). Thoughts?

Or, should I try shimming the shaft, as you said, in the back of the input (I'm assuming you mean on the "inside" of the bearing between gears and bearings?)? Is there a clearance or spacing test to see where exactly I need to shim? I wouldn't know of any way to see the causality of the issue, aside from having a camera inside of the transmission and watching it pop out in real time. Ha!

I also have 2 other transmission cases/handful of spare parts, so I could replace the case fairly easily, if that's the route I need to go.
Thanks boss! I hope you didn't lose any fingers/toes/appendages with things that go boom yesterday.

I would expect the fiber ride synchro material may have been damaged by the gear oil, but if you aren't getting grinds on shifting you may be OK. On the actual gear itself is the other half of the synchro welded to the gear. Each tooth has a back cut that keeps the synchro engaged under load. If these are too worn you will have gears popping out.

It could also be a linkage issue. There really is no adjustment but just make sure nothing is binding or hung up. Check motor mounts.

silvreclips said:

I would expect the fiber ride synchro material may have been damaged by the gear oil, but if you aren't getting grinds on shifting you may be OK. On the actual gear itself is the other half of the synchro welded to the gear. Each tooth has a back cut that keeps the synchro engaged under load. If these are too worn you will have gears popping out.

It could also be a linkage issue. There really is no adjustment but just make sure nothing is binding or hung up. Check motor mounts.

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Thank you for the suggestion! Those dogs/teeth looked just fine - I was expecting a bit of tapering or wear on them, but there was none whatsoever (1st-5th honestly looked brand new).
Oh, never mind about the linkage adjustment, then. I know the front motor mount needs to be replaced - it's not in the best shape. I was just going to grab a new one when I order some of front suspension parts. Thanks guys! I appreciate your input!

I second the notion that it sounds like a shim problem. I had the same issue many years ago on a pretty much new transmission that I swapped a fancy differential into. It popped out of 4th and 5th due to the output shaft spacing being off. It's a pain to fix because you have to assemble, measure, shim, assemble, measure, shim... until you get it right but, atleast in my case, that's what the problem was and that's how I had to fix it.

Awesome! Thank you all for the suggestions. I'm glad I asked before I pulled the trigger with the synchros - I definitely appreciate y'all's input. I'm going to be wrapping up a few other projects first (need to work a bit on my pathfinder and motorcycle, and lay down a new coat of paint on my eclipse), but will update you with any progress that I make. The ladyfriend gets back from California in August, so my car-tinkering days for this summer are definitely numbered. Haha!

iceguru1114 said:

I second the notion that it sounds like a shim problem. I had the same issue many years ago on a pretty much new transmission that I swapped a fancy differential into. It popped out of 4th and 5th due to the output shaft spacing being off. It's a pain to fix because you have to assemble, measure, shim, assemble, measure, shim... until you get it right but, atleast in my case, that's what the problem was and that's how I had to fix it.

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What exactly are y'all talking about shimming?

The nvt350 does not require any shimming on the input or counter shaft. These are just roller bearings supporting each end of the shaft. There is nothing to shim. The differential has tapered bearings and needs to be shimmed to gain the correct rolling torque. But I don't see how this would cause 5th and 4th to pop out of gear.

I am saying to much distance between the gear sets, how ever that may be for this model of trans.

silvreclips said:

What exactly are y'all talking about shimming?

The nvt350 does not require any shimming on the input or counter shaft. These are just roller bearings supporting each end of the shaft. There is nothing to shim. The differential has tapered bearings and needs to be shimmed to gain the correct rolling torque. But I don't see how this would cause 5th and 4th to pop out of gear.

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The transmission that I was referencing was an 02J from VW which had shims under the roller bearing races (between bearings and housing). TSI1990 is right in citing gear spacing, however it is handled. It's done with shims under the shaft bearing races on most transmissions, I assumed it was the same for whatever mates to the 420A since shims are also used on the W5M.

Quick update, guys:
I think I have fixed the transmission. Fingers crossed!

From what I could see, it was indeed the backplate that was not properly holding the bearings in place. Just to clarify, the backplate that I am talking about on this 5FMC1 is the outer plate that covers the two bearings (outer input/output shaft bearings). As mentioned, the inside of this plate, where it meets the race of the bearings, was badly worn (there was a significant ring of wear where it met both bearings). 2 of the 13mm bolts were stripped inside the case, and this was allowing just enough lateral movement between gearsets to pop it out of gear at higher speeds. I ended up re-threading the case holes to a bit larger of a size, and instead using a stud (as opposed to a bolt) for the center bolt. This stud helped me over-torque by a few lbs. without worrying about stripping.

I played around with shimming that backplate to essentially "press" the bearings into the casing better. However, I could never get the shims the correct thickness. Either they were too thick and left a gap between the case and backplate, or they simply had no effect by being too thin. I ended up using a spare backplate that I had purchased last year. I forgot I had it, but happened upon it while rummaging through all my tranny parts. That's almost better than finding $20 in a pair of jeans.

At this point, transmission appears to be fixed. Now I get to focus on everything else that's wrong with the car.
Thanks again for all of your suggestions!

Glad you were able to easily fix it. Did both shafts have snap rings on them? That cover plate shouldn't be holding the bearings in.

Yes, both shafts had the snap rings on them, which held the shafts within the bearing. The inner input and output shaft bearings are just those roller bearings, so the shafts, feasibly, can have small amounts of lateral movements (as opposed to the cone bearings used in the differential housing). If the two outer bearings have just a bit of lateral movement in them after being pressed into the case, it will allow the shafts to move as well. Keep in mind, those two outer bearings are pressed in from the outside, so there is no lip on the case that would keep them from pressing out. If those two bearings are able to push outward under heavy load, the only thing really keeping the shaft in place would be the shift forks.

Definitely correct me if I'm wrong, but from what I found with the multiple rebuilds of this sucker, is that that plate keeps the bearing pressed into the case, and keeps the shafts from pressing the bearings out. I would post a picture, but now that I have that transmission working, I am afraid to touch anything on it or take that plate off. Haha!

A couple things to keep in mind as well......... The cable end busings (on both ends)(at the trans and the shifter)...... I personally use Amsoil Synchromesh, as its probably the closest we have to dealer OEM fluid, ATF+4 was just a cheap ass alternative and should never have been used..... But part of the problem is when people switch over to different fluids, they dont realize their trans may have already had negative effects on itm and by switching fluids just makes it worse. Thats at least my .02 from the many issues people have brought to my attention.