Hi all, new to the forum so hopefully I am posting in the correct location but hoping someone can help with a transmission issue. I am nearing the end of a complete teardown and rebuild of my 5/71 240Z with the Monkey motion F4W71A trans. Finally got the car mostly back together and running, but I am getting an awful clunking sound from what sounds like the transmission that only happens in 3rd gear. 1st, 2nd and 4th all seem relatively quiet, I have maybe noticed a slight noise in reverse. Maybe not surprising considering the car has been sitting for the better part of 15 years prior to the rebuild. Anyway, I do not notice any grinding or trouble shifting for any of the gears, or any noise or vibration in neutral. Assume this is maybe a bad bearing or potentially a problem with the gear itself? Anyone experienced a similar issue?

I was not able to get a good sound clip, however I did pull the transmission yesterday and plan on rebuilding it myself, which brings me to my other question on the rebuild kits. It sounds like the parts are expensive/hard to come by for the A type boxes, I did find a bearing kit (https://www.datsunparts.com/1049)  that says it is for both the F4W71A AND FS5W71A that was apparently the 5 speed used in the 2000 Roadster? Is this the same FS5W71A used in the 280Z, and if so would a kit like the one here https://www.drivetrain.com/nissan-fs5w71-synchro-kit-07-nissan-fs5w71-synchro-kit-dts-bk104 work for the F4W71A? If that is the case the parts seem readily available and fairly cheap from drivetrain.com, but maybe I am missing something. 

Be sure and check the shift forks and sliders for wear. The 3rd gear maybe snapping in with slop from the slider

I will check that, thanks for the suggestion. Would that manifest more when shifting into 3rd or once the transmission is already in gear? Or both? To add some clarification that clunking sound is continual while in third regardless of speed from what I can tell. Actually shifting into 3rd seems pretty smooth, just as soon as I let out the clutch it starts clunking.   

3 hours ago, AnvilZ said:

awful clunking sound

 

36 minutes ago, AnvilZ said:

To add some clarification that clunking sound is continual while in third regardless of speed from what I can tell.

Can you add some detail?  Are you saying that the clunking rate is constant despite RPM?  How can that be?  Does it continue to clunk if you're in 3rd with the clutch pedal down, car not moving?  Or does it only happen when the car is rolling?  Clunk is not very descriptive, even with the awful added.

Sorry I see how that is confusing, I meant to say that it is continual throughout 3rd gear at any speed. The frequency is definitely speed dependent though, it would increase with RPM. It does not clunk with the clutch pedal down in 3rd. I don't know how to describe it other than a clunk, maybe there is a bit of grinding going on as well that I cannot hear over the engine noise. Definitely not a whining sound though. 

 Is the clunk there during acceleration and deceleration and is it louder under either condition?

From what I can tell it is about the same under acceleration and deceleration. I had also considered that maybe it was something to do with the harmonics of the drivetrain, maybe something in the propeller shaft/diff/half shafts but it stops as soon as I shift to second or fourth.

Sounds like something on/in the coupling sleeve and hub gears.  Could be a chunk stuck in the teeth or a broken tooth.  The shafts and main bearings are all shared between and the gears themselves are constantly meshed so a gear problem should be heard all the time.  Only the coupling sleeve and hub meshes when you select a new gear.  Although, there is a needle bearing under each gear that might deflect under load.

Others have more transmission experience.  Seems like you'll need to take it out and apart though to know for sure.

On further thought I'm not sure about my post above.  The sleeve and hub mate around the full circumference, the gear teeth mesh at a single point around the circumference.  So a noise that changes with RPM would have to be related to the mesh point of the gears.  I think.

Anyway.  Another quick way to get a look is to drain the fluid.  The stuff that comes out will tell a story.  Looks like you must have drained it, was there anything there?

Another question. Does the clunk volume increase during hard acceleration or deceleration?

@Zed Head I drained the original fluid a while back and didn't pay much attention to it, now I wish I had. I did replace it with Redline MT-90 during the rebuild and when I drained that yesterday I didnt see any metal shavings or chunks in that or on the magnetic drain plug. Although I have only put a few miles on the car due to the issue with the transmission since I got it running again. 

@Mark Maras good question, I did not check that before pulling the transmission.

Just got it torn down though, here is a video with it in 3rd gear. No chipped teeth or issues with the synchros that I can see on visual inspection but I am no expert. 

I think that one of your adapter plate bearings is bad.  I don't think that the shaft should move back and forth like that.

@EuroDat @zKars @Dave WM