I was draining the fluid out of the automatic transmission of a '94 Sentra with a RL4F03A tranmission. While I was removing the magnets, one of them was holding onto a 10 mm hex nut. It was clean, not mangled, not stripped.

After getting back under the car to remove the transmission filter (it's bolted on), I looked all over the inside of the exposed part of the transmission (without taking anything else apart) and couldn't find any bolt that is missing a nut. In fact, I could not find any exposed nuts.

What did my mystery 10 mm hex nut fall off of?

  • Maybe somebody lost it during earlier repair and didn't care?
    – Mark
    Commented Dec 9, 2016 at 7:30
  • The last time the tranny was opened was for a routine fluid and filter change.
    – tlhIngan
    Commented Dec 9, 2016 at 7:38
  • 2
    Are you positive it wasn't attached to the magnet before you submerged it in the transmission fluid? Commented Dec 9, 2016 at 8:10
  • 1
    Is there ANY possibility the nut was in the drain pan before you drained the transmission?
    – CharlieRB
    Commented Dec 9, 2016 at 19:49
  • 1
    This tranny has a drain plug, so I drained the fluid 1st, then removed the pan. All I know for sure is that the nut wasn't in the pan the last time it got serviced.
    – tlhIngan
    Commented Dec 9, 2016 at 19:51

2 Answers 2


Okay. Lot's of research, long story, found the answer, don't like it.

  1. Overall design of the Nissan B13: one of the things that always impressed me about this vehicle is that it seems to have been designed with the DIY home mechanic in mind. Components are accessible and tasks can typically be accomplished using only 2 different wrench sizes, and maybe a screwdriver and pliers. When I got under there to flush the tranny, I found a drain plug for the tranny.
  2. Tranny filter vs tranny screen: What? Yes. Some trannies have a filter that you replace when you change the fluid, other trannies have a screen (metal mesh) that you clean when you change the fluid. Filters are usually clipped on, screens are usually bolted on. This RL04F03A tranny has a screen.
  3. Other people found nuts in their tranny pan: this post from a Miata forum talks about their tranny having a few bolts holding the screen onto the valve body via some threaded holes, but there's this 1 bolt that goes through the valve body and can only be tightened by a nut on the upper side of the valve body. It's just not feasible to get a finger in there to get the bolt started into the nut, you need to remove the valve body.
  4. But that's a Miata! This is a Nissan! Well, I was a bit surprised to find so many dadgum bolts holding a flimsy little filter-looking thing that turned out to be a screen. I went at it and removed the bolts, and, half way around, started noticing the bolts weren't all the same. Great. Just GREAT. There were 8 long ones, 2 short ones and 2 medium ones. The 2 medium ones flanked one of the short ones and the other short one is on the opposite side, I figured I could remember all that. Besides, it was too late to do the cardboard thing as most of the bolts were out. When it was time to reassemble, it turned out there were 2 kinds of long ones, 7 with threads all the way up, and 1 with threads near the end only. Aw crap. That's the one that needs the nut.
  5. Really? You need to remove the valve body? Well, if you want to take the screen off and put it back on, yes you should remove the valve body. Because of the risk of introducing dust, dirt and other particulates into the transmission by removing the valve body, many sources recommend leaving the screen in place and just changing the fluid. That's why there's a drain plug.
  6. Great. So how did the nut come lose and what do you do about it? Well, if you don't know there's a nut up there, you won't go looking for it. That last bolt, the one that needs the nut, won't thread tight, but you may not notice, since it goes through so much metal before getting to the other side and there's still plenty of tranny fluid dripping down the hole to make a "fluid lock" and make the bolt seem a bit tight. Fire up the car and the fluid will swish (that's a technical term) that nut around until the magnets grab it. That's what magnets are in the tranny pan for (metal shavings and things that come loose). That nut has been in there since the very first time the car was started up after the last tranny service. The tranny's been performing fine, it didn't miss that nut or it's associated bolt not being threaded in tight, it won't miss it going forward.

Long story short. On this tranny, don't even bother taking the pan off. Drain the fluid using the drain plug, then refill. I always wondered why my fancier other vehicles don't have a tranny drain plug. It's because they have filters, they WANT you to take the pan off so you may as well change the filter because it's just there. I don't want to generalize, but I would expect vehicles with a screen (not a filter) would have a tranny drain plug to discourage you from getting your dirty little fingers in there when you don't really need to.

And just for posterity, I'll link to 2 YouTube videos I've found about people tearing down this specific tranny. Not a single nut to be found, and the screen was installed off camera while the valve body was out of sight. Don't you just love it when you find a video with a long enough runtime to make you think it's complete, detailed and exhaustive only to find it skips right over the part you really need?

1997 Nissan Primeria SR20DE with RL04F03A

1997 Nissan Sentra GA16DE with RL04F03A

  • Great write-up! So you cannot put the nut back on the bolt without taking the transmission apart? Commented Dec 11, 2016 at 16:22
  • Good Article tlhIngan. I have a rl3f01a automatic transmission filter bolt won't tighten. I had been looking every where for a solution. It seems like this issue goes across a few different models . I haven't found the bolt up in there yet. I will reinstall everything less the bolt and leave the missing hex nut up in there somewhere. With some luck it will drop down into the pan and get captured by the magnets. Really frustrating ! I will report back on the issue after a test run Thanks for your information. :-)
    – eebaker69
    Commented Mar 15, 2018 at 21:45

I just did this job today on a 2005 Nissan Pulsar (Australia) Dropped the pan, dropped out the screen, was smart enough to do a diagram of the bolt sizes for each hole, but what hey!!!!!!! Why won’t this last filter screen bolt tighten up!!!!!!

Now I’ve found this post explaining that it’s an ongoing problem, I figure it’s one bolt holding the screen To the casing. I’m just going to change the gasket put the magnets back in the pan and put in the trans fluid.... I’m definitely not removing the valve body to tighten one bolt.

Fingers crossed nothing goes wrong 😖

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