'80 Datsun 720 - 5 speed manual transmission popping out of 5th gear

[zwebslinger]

Looks like the 4 speed and 5 speed of my '80 720 have identical ratios 1-4.  The FS5W71B that I have just has the additional 0.882 for 5th.

I'm so conflicted on what to do.  Anyone ever get the rebuild kit on rockauto?  Or would it be best to get parts from Nissan.  If that's even possible.

[Draker]

I've had luck buying form these guys:
https://cobratransmission.com/fs5-w71-synchro-bearing-gasket-and-seal-kit-50238005-1

 

Be sure to call or email to confirm compatibility, but this looks like the one. Not sure how the price compares to rockauto. 

[zwebslinger]

Looks like they only have parts for the 71C tranny, not the 71B that I have.

Price is a little more than double Rockauto, which would be a non issue if the quality is there.  Neither are likely from Japan tho.

[datzenmike]

All L series 4 and 5 speeds will interchange. Your '76 4 speed will bolt right into the '80 720.

 

The 280z 5 speed is what I call a mid ratio while the '80 2wd 720 had a wide ratio gear set (because truck) The first gears are...

 

76-'78 280z...... 3.321

'80 720 2wd..... 3.592

 

Fourth gear in both are 1.000 so you can see that the truck is lower geared for more torque in first but they both end up the same in 4th.

 

I still do not understand having to rev up through the gears in order to shift quietly. That's not right. If you are thinking of rebuilding it's probably the synchros that are buggered. Was there ever a clutch related problem in the past? Was it run low on oil in the past?

[zwebslinger]

1 hour ago, datzenmike said:

All L series 4 and 5 speeds will interchange. Your '76 4 speed will bolt right into the '80 720.

 

The 280z 5 speed is what I call a mid ratio while the '80 2wd 720 had a wide ratio gear set (because truck) The first gears are...

 

76-'78 280z...... 3.321

'80 720 2wd..... 3.592

 

Fourth gear in both are 1.000 so you can see that the truck is lower geared for more torque in first but they both end up the same in 4th.

 

I still do not understand having to rev up through the gears in order to shift quietly. That's not right. If you are thinking of rebuilding it's probably the synchros that are buggered. Was there ever a clutch related problem in the past? Was it run low on oil in the past?

I don't get it either.  Only thing is that the truck sat for about 15 years in a Florida garage before I got it.  He says that the tranny was kinda "loose", but always shifted perfectly.  There was no issue with 5th either. 

Never run low on GL4, and no clutch issues.   The clutch grabs very nicely even today.

[datzenmike]

The synchros are tiny clutches and the transmission must be disconnected from the engine for them to slow the spinning clutch disc, input shaft and counter gears to match the mainshaft speeds. It's the opposite when down shifting.

 

Could the counter gear be un-nessesseraly tight? 

If every gear affected, then all synchros are worn?

 

Place in neutral, lift one wheel and turn the driveshaft. You should be able to spin it by hand easily enough.   

[RLJ]

Not trying to hijack thread. Just curious. 

Posts have said that growling noise indicates countershaft bearings going out. Meaning growling all the time in all gears?

Growling in neutral with clutch out, growling in 1st and 2nd but quiet in 3rd, 4th and fifth. Does that mean one countershaft end bearing worn out but not the other?

[bottomwatcher]

Usually all gears except 4th is a bad countershaft bearing.

[datzenmike]

More noticeable under load in all but 4th. Fourth isn't a gear really but a direct coupling through the transmission from clutch to driveshaft. All other gears the power is diverted down onto the counter shaft then up onto the gear selected on the mainshaft and out. When power is applied the force tries to push the countershaft and mainshaft apart and this support is taken up by bearings. When damaged they growl.

 

5 hours ago, RLJ said:

Not trying to hijack thread. Just curious. 

Posts have said that growling noise indicates countershaft bearings going out. Meaning growling all the time in all gears?

Growling in neutral with clutch out, growling in 1st and 2nd but quiet in 3rd, 4th and fifth. Does that mean one countershaft end bearing worn out but not the other?

 

Torque multiplication is highest id first then second. Given time 3 and 5 will probably start to growl.

[RLJ]

Pulled the trans apart today. Countershaft rear ball bearing is shot. Cage is gone and some balls also. Looks like I caught it before mass destruction. Thinking because it’s the lowest bearing in the case debris just dropped down and magnet in drain plug probably caught most of it. Another thing PO never mentioned when he said he had just changed the trans oil. 

 

Everything else looks pretty good. Going to replace all bearings, seals, and synchros. Need to order parts. 

[RLJ]

Actually it’s the center ball bearing that’s in the intermediate steel plate. 

 

Datzenmike, I think You called it correctly as far as highest torque multiplication in first and second. Made noise in neutral with clutch out and driving in first and second. 

[datzenmike]

Usually, and I say again.... usually, it's the large front counter bearing that fails. The rear one that is supported in the rear case can grind a hole in it.

[zwebslinger]

On 3/24/2021 at 7:45 PM, datzenmike said:

The synchros are tiny clutches and the transmission must be disconnected from the engine for them to slow the spinning clutch disc, input shaft and counter gears to match the mainshaft speeds. It's the opposite when down shifting.

 

Could the counter gear be un-nessesseraly tight? 

If every gear affected, then all synchros are worn?

 

Place in neutral, lift one wheel and turn the driveshaft. You should be able to spin it by hand easily enough.   

Did that just now.  Ridiculously easy and smooth to turn by hand.  No noise whatsoever.

[yenpit]

On 3/24/2021 at 10:14 AM, zwebslinger said:

Looks like the 4 speed and 5 speed of my '80 720 have identical ratios 1-4.  The FS5W71B that I have just has the additional 0.882 for 5th.

I'm so conflicted on what to do.  Anyone ever get the rebuild kit on rockauto?  Or would it be best to get parts from Nissan.  If that's even possible.

 

This is all you ever get in the rebuild kits.  I just pulled a 5speed out of my 1980 720 parts truck, looks identical externally to the last of the 280ZX 5speeds that we have rebuilt before..............I have not opened mine up yet.  That said, I think all these parts are the same regardless.  We use these kits in vintage race boxes, never had a part failure after rebuild, so quality seems just fine.  Nissan still has many if not all, but would be stupid expensive.  The only thing that these kits DO NOT HAVE are the upper shift shaft oring & lip seal, only available from Nissan.  You need to remove the upper shift shaft from the tail housing, which is a biotch to do on the earlier 5speeds...........if your 5speed is the same as mine, you should have an oval plate with 2 bolts on the drivers side of the tail housing.  This plate allows access to remove the biotch taper pin on the upper shift shaft, MUCH easier than the older 5speeds!  Rebuilding is pretty straight forward, just be methodical & patient!  PS every single USED reverse gear shows wear on the gear teeth, cuz dumbasses jamb in reverse while still rolling (no synchro) or without running thru a forward gear first, then to reverse.  If no broken teeth, just re-use it!

[datzenmike]

Striking rod. That little finger that engages the shift forks has a cotter pin like on the old style bicycles and impossible to easily get at to pound out.

 

The 'oval plate' is the reverse check sleeve. Makes it impossible to accidentally down shift from 5th directly into 4th. All '80 and up 71B and C have this feature.

 

You might be able to slip the reverse idler on backwards so the wear and chipping is facing the other way.

[bottomwatcher]

I think you meant 5th to reverse Mike.

[yenpit]

3 hours ago, datzenmike said:

Striking rod. That little finger that engages the shift forks has a cotter pin like on the old style bicycles and impossible to easily get at to pound out.

 

The 'oval plate' is the reverse check sleeve. Makes it impossible to accidentally down shift from 5th directly into 4th. All '80 and up 71B and C have this feature.

 

You might be able to slip the reverse idler on backwards so the wear and chipping is facing the other way.

 

Yes, we call it a taper pin!  Yes, it is an access hole for the reverse lockout, but it also allows access to that stupid taper pin, from the outside, in the direction to knock out that taper pin!  Still be sure to shove a small block of wood on the inside, to support the shaft while you knock out that taper pin............first one, we bent the shift rod!😫

 

That oval access plate was only on the last (3rd gen) of the 280ZX 5speeds.  The 1st & 2nd gen of the 280ZX 5speeds did not have that access plate.

[datzenmike]

Block of wood!! genius! never thought of that.

[RLJ]

Regarding removing  the tapered pin.

 

After inspecting came up with this idea. In the earlier style case, measure and locate where the pin is. Drill a hole in the case that allows a straight shot to tap out the tapered pin. Thread the hole you drilled for a tapered plug to thread in. 

 

This only needs disassembly to replace the o ring and seal in the shift tube. Once out believe you need to remove the plug at the end of the shift tube so the shift rod can be removed and oil seal replaced. 

 

Since mine wasn’t leaking and they’re at the top of the transmission minimizing oil cumulation, I’m electing not to replace them. 

[datzenmike]

Striking rod. Good idea using a threaded hole. If not leaking leave alone.

[zwebslinger]

So Mike,

Since my 4 speed '76 Z tranny will bolt right up to my '80 720, stupid question but I still use the 720 part numbers for clutch parts, right?

Just the tranny seals and parts would be different I'm guessing.

 

Think I'm going to stick the 4 speed in the truck so that it's back in operation while I rebuild the 5 speed (eventually).

 

Trying to come up with a list of things I should change since I'm pulling the trans.

Looks like I need a new withdrawl lever dust boot, front and rear oil seals, front cover gasket, shifter bushings, reverse and top gear switches and engine rear main seal, .

Anything I'm missing?

 

What exactly does the top gear switch even do?

[datzenmike]

Just keep the release bearing and collar with the truck. The collar length is intimately linked to the pressure plate you have. If there is a collar on the Z car 4 speed, do NOT use it.

 

Front and rear seal. Cover gasket may be hard to find. If so small amount of RTV will work. Too much will squish into the inside and you dont want that in the counter bearing.

 

For the two case halves to adapter plate RTV will work but I like gray Permatex sealant. Use sparingly, join parts, finger tight bolts wait 10? min. then final tightening.

 

GL-4 gear oil about 2 liters (basically 2 US Qts.) With new seals you might like the shifting improvement of a full synthetic. Vastly improved first few cold shifts or use in cold weather. Down shifts without grinding are also improved. With the same effort using the 90w GL-4 oil, the synthetic is surprisingly faster shifting. 

 

 

Top Gear (4th) Switch. (also the 5th gear switch does the same thing so 4th and 5th and considered top gear)

 

Tells the emissions equipment that you are out on the highway.

[zwebslinger]

I don't need to split the Z case tho, do I?

[datzenmike]

Hell no. I meant when doing a rebuild on the 5 speed.