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FS5W71B vs. FS5w71C


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This idea has been lingering the back of my head for a while, but I want to get some more definitive information on the subject. I have found that there have been a few people that have swapped their "B" transmission to a "C" transmission, and I have also discovered that rebuild kits for the B series transmission are difficult to source. I have been thinking about this because my truck ('86 720 4x4 with a "short" transmission) really hates going into reverse, like, really hates reverse. I come to a complete stop, put it in neutral, clutch in, clutch out, clutch in, then slowly, and I mean SLOWLY feed it into reverse. I feel the gears catch, and eventually it goes in, and that situation certainly doesn't give me any warm fuzzies. My worry is that eventually there will be some sort of catastrophic failure and my transmission will be toast. My questions are as follows:


Should I be concerned about my reverse gear? To my knowledge it is an unsynchronized reverse gear, so maybe this is just normal operating procedure.  

If I need a new transmission, and elect to change from a "B" to a "C", which vehicles should I source my transmission from? It seems a lot easier to source a rebuild C than any condition B. 

The B and C are two different lengths, so some driveshaft modifications will need to be made. Does anyone know any details on that?


Side Note:

I haven't driven my truck for a few months because I have been in Germany studying abroad, but this is just something that was in the back of my mind. I haven't seen a Datsun anything since I've been out here, so I am on the forums almost every day because I miss driving my 720 so much. You guys are giving me my fix until I get home at the end of December, so much appreciation for that.  :rofl:


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If you mean you can hear the teeth hit and rub together (light grinding sound) then this is a clutch problem. When the clutch pedal is depressed the connection between spinning engine and the transmission is severed. The parts that were spinning in the transmission very quickly stop because they are in an oil bath. If they keep spinning then they are still being turned through clutch contact.


These checks can be done easily enough...


Check your clutch master fluid level. Top it up

Air might have got into the hydraulics. Can be bled.

Check with fingers, that the clutch pedal has about 1/16" of play before it becomes firm to the touch. Can be adjusted.

Transmission fluid low empty? Top it up. GL-4 only.

Added carpet or floor mat too thick under the clutch pedal. Remove



These need the transmission removed...


Flywheel or pressure plate surface damaged and rubbing together.

Clutch disc damaged and rubbing flywheel or pressure plate.

Pressure plate diaphragm spring fingers broken.

Foreign object (torsional spring from clutch disc or diaphragm finger) stuck in clutch.

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What Mike said...

This is because reverse is non-syncro, and if the main shaft is turning at all, there is nothing helping those gear teeth line up.


May also have something to due with what gear oil is in your trans.

When was the last time you changed it ?

Or have you never changed it, and it's turning to glue ?

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The clutch was changed at around... ah, I don't have my book of records on me but I want to say that it was changed at 135,000, and there are a little over 141,000 miles on the truck. When we changed the clutch we changed the transmission oil, and I believe we used Redline 50305 MT-90 75W90 GL-4 (according to my Amazon account). After we changed the clutch we put a new master and slave in, bleed it properly, ect... I may have to adjust the pedal play because I like the clutch to operate right at the bottom of the pedal actuation so I do not have to move my foot so much; maybe that is what is causing the issue. When I get back to the US I will double check all of the fluids (even though I do it every time I fill up for gas). 

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As for other options of trans, you are stuck with the B shorty unless you are willing to do some modifying.


A 2wd 71c trans can be stuffed in there If it's from a d21. You will have to shorten your intermediate driveshaft and possibly modify your transmission mount. This will be a tiny shaft and possibly prone too vibration.


You cannot use a 4wd type with your tcase. A d21 4wd w/tcase will require new driveshafts, modifying your shifter opening in the body, custom mounts, and will set your front drive shaft at an odd angle.


Best bet might be to buy another trans and gut it for parts. If it turns out you do need work on trans internals.

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The b series fails from an undersized counter shaft bearing. Mike will correct me if I am wrong but I believe your year b uses the larger bearing. That being said the c series although stronger has had countershaft bearing issues in the hardbody. A heavier truck but probably equal to a lifted 720. The reverse gear is next to 5th on the countershaft so this may be a warning the countershaft bearings are having issues. Rebuild now before blowout will get you another 100 miles. Wait and you will have scrap. I am running a 88 2 wd hardbody c in my 82 4x4 14 years, no issues.

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The larger 62mm bearing was introduced on the '85 transmissions, so the 720 and the S12 got them. These were the last 71B vehicles to use them. I just happen to have an '85 720 diesel and an '85 CA18ET S12 5 peed... they both have it.


Just recently was talking to Jeff and were discussing the CA18ET box I have. It also has different shift dogs and the input main and counter gears are longer just like the 71C. And look at the size of the shift forks!



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The front counter bearing is usually the first one to begin failure and why it was increased in size from 56mm to 62mm. ALL power is transmitted through it except when in fourth 'gear'. Which isn't a gear at all really but a solid connection through the transmission from clutch to driveshaft and why it should be used for hill climbing, passing or hauling heavy loads on the highway. When power is applied, the main and counter shaft try to move apart and the bearings hold it in place. They are cooled and lubricated by the transmission oil. Any time the clutch is engaged, even if in neutral, the countershaft is spinning in an oil bath splashing oil on all the mainshaft gears.


These trucks are getting old... are old. By the third owner they are a cheap investment and driven into the ground and ignored rather than keeping up the maintenance. Transmission oil should be changed every 30,000 miles. There will be trucks out there with maybe 2 or 3 widely spaced oil changes on them and in some cases very likely the original oil from the factory just topped up. $10 every 5 years and an hour work.


The counter bearing does not go quietly into the night. It will make noise first, if you have a muffler you will hear it. It's a soft growling sound under load like driving on a gravel road and conspicuously absent in forth gear. Low oil will also do this, but not for long. The bearing is about $12 and the average owner can change it with a few tools and a borrowed gear puller.

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I have a long 31.5" dogleg. (without the front and rear cases)

A shorty 71B 4x4 box

An '82 or '83 280zx box (non turbo)

'85 720 diesel (all apart)

'79 290zx box in my 710. (first year zx and the only year mid ratio)

'85 S12 CA18ET (front case traded for a KA head)

I think there is a Z series 5 speed there somewhere and a spare L series front case..


Two 620 and one car 3 speed automatics, plus a 280 zx turbo automatic in pieces


That's all I can remember..

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So much great stuff in here. Here's what I am thinking: When I get home, I will adjust the pedal play, take a second look at the master cylinder level and make sure we have enough fluid; this should hopefully fix my reverse issue (based on what Mike and Wayno have said, it seems to be that the clutch just isn't quite disengaging all the way).The transmission doesn't make any of the stated growling or whining noises, but when it does, it will most likely be the front countershaft bearing, which in my B series is 62 mm because it is a later one. Does anyone have a source for bearings for the B transmission? It seems as though every transmission rebuild website I go to thinks that my truck has a 71C transmission.

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The B got some up grades slowly transforming it into a C. There were no Bs after '86.5 and there were only the S12 and 720 that were RWD then anyway. It's possible that '85 or '86 were Bs mostly in name only.


The '85 one that I pictured has that funky 1st/2nd synchro, the C type shift forks, the wide input gear and 62mm counter bearing. 

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Is a good place to start if only for part numbers.


My transmission guy took 3 months to rebuild my tranny. Not because of a poor work ethic but finding the right parts is a major PIA.

The races on the countershaft had welded to it from years of running with little lubrication. Somehow he found a replacement countershaft and now it's happy wheelin'.




Deschutes Brewery, Bend, OR. 11/5/17

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