Little Trucks, Big World

Hi guys, hopefully I can get a little insight as to what is going on with my transmission.

Here is the background.   I just had this transmission installed about two years ago when my original locked up when i tried to grab a downshift for an exit ramp.

This one was the only one I could locate and it was out in California, supposedly it was professionally rebuilt and I had it shipped to my installer.   

After the install I drove it for a couple months and then decided to rebuild my truck and give it a longer life so it was down for two years or so while I worked on it.   I live in a townhouse with a homeowners association that likes to get in peoples business so it was fun trying to hide all the work I was doing on it from head gasket to cutting out bad sections and welding in new ones and replacing the tank, bed, etc.

I finished it up and it looks great and drives really well also but for whatever reason, the other day when i was driving home (actually turning into the entrance for my development) I was cruising down the road in 5th gear and grabbed a downshift to 4th gear and when I went to go into 3rd the stick would not move.

When I let the clutch out it would still pull in 4th gear so I pulled into a parking lot to get out of traffic

and see what was going on.   If I pushed in the clutch it would roll backwards without any issue and if I let it out the truck would try to pull.   I checked the clutch reservoir and it is full, I also checked the slave cylinder and it is fine (and new) and the transmission has fluid.

I just took out the console to get a look but their really isn't anything to see and the small plate on the side is so hidden up in the tunnel I don't think anything of merit will be seen that way.   

I tried to pump the clutch and work the shifter and it just will not budge at all.

Have any of you had an issue like this?   If so, what did you do to fix it?

I am bouncing around what to do if this has to come out of the truck again, I would like to put in something much stronger and less prone to failure, it seems like our box is made out of toothpicks and rubber cement.

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Yeah I went through 4 b model trans. Went to a c model no issues 14 plus years later.

Thanks Channing

I'm not familiar with the "b" and "c" designations.   was the "c" from the hardbody years?   Is it the same length as ours that I can get one and install it without fitment issues?   Or will I need to have the driveshaft changed?

A little short. I have 4x4 so the primary driveshaft needed lengthening $100 15 years ago.trim shifter hole simple plate to adapt trans mount and two spacers. A c series from 1988 hardbody.
So is the flywheel and clutch usable from the b series?

Does the bellhousing bolt right up or do I need to combine the "b" bellhousing with the "c" trans.?

I was starting to read up on it last night and ran across a couple posts you made and others you chimed in on from years ago but did not see a definitive "how-to" on the c-series trans. installation into a 720.

Does such a thing even exist??

Thank you!
It bolts right up. The plate I am talking about the is a simple piece half inch steel with 4 holes to bolt the trans mount to the trans because the rear holes on the trans are a couple inches short of the original location. This raises the rear of the trans a half inch. To bring it back stock height I put half inch spacer between frame and crossmember. A slight trim around shifter hole and I remember removing a collar on the driveshaft to slide it in but that is alll.

Two transmissions locking up? This would indicate a usage problem.

How are you down shifting? Big rev and down shift into the next lowest gear, then 3rd then 2nd, using the engine compression to slow the truck long before it's needed.... or, low speed slip it in as you enter the driveway? The former is not needed and just accelerates wear on the synchros and snynchro hubs and clutch. When slowing down, the later is preferable, as brake shoes are cheaper and easier to replace.

The KA series 4 cylinder 71C in the D21 Hardbody is stronger than the 71B and will handle a stronger engine and more load, but synchros are synchros and wear is inevitable. It also shares the exact same bolt pattern as the Z series, so bolt up is perfect. You can use your clutch and pressure plate, but also use your release bearing collar. That's the housing that the release bearing is held by. As mentioned the 71C is a different length so the drive shaft will need a slight lengthening.

The usage problem is bigger tires on a heavier 4x4 truck. The b series is a car trans used in the 280z. Slap bigger tires on a truck with the aerodynamics of a brick tow something and the undersized countershaft bearings blow out. That being said the c series have countershaft bearing issues in the hardbody. An even heavier truck than the 720. They seem to live ok in the 720.
First transmission was a case of trying to grab fourth gear and it just wouldn't go in and the more speed that was lost it should have gone into a lower gear, but wouldn't. It just would grind at all times.

Finally after a little work it went into first and i crawled the final mile to the apartment where i could not get reverse but let it roll in nuetral back into a parking space and that is where it sat for months before I could get the current transmission put in.

I installed a new slave, line and inspected the reservoir for any kind of leaks but all was good.
It had just given up internally.

This one just got stuck between gears.

But to answer your question, yes i do actually raise the rpm's to drop a gear, it's something i have always done.
I heel/toe downshift in my car and since i drive an 18 wheeler it is an everyday occurrence when i shift.

I do run a little bit bigger tire and acceleration in 5 th gear is usually only done on the freeway since these trucks have so little power you always have to grab a lower gear.

I never tow anything.

The same 71B was used in the six cylinder non turbo 280zx with at least 50% more HP and torque and in a 3,000 pound car.

In '85 the 720 got the same larger 62mm counter bearing upgrade as the 71C in the Hardbody.

Will i need to get a different slave cylinder for the "C"?

I want to make sure i have all the parts i need.
I don't know much about 280Zs. Do they blow up the b's as much as the trucks do? I know even the 350Zs have 3rd gear crunch issues. There is 2 companies that make a ka to nissan 30A adapters because the ka turbo guys keep blowing up ka trans. I am running a kae on my c trans and all well so far. I don't rag on it but if it gives up I may try the 30A swap.

What is the 30A from?

I have been driving manual since I was 17.

I had an '80 Scirocco, '86 Pulsar, '85 Mustang GT, '90 Eagle Talon Tsi awd, '95 Jeep Wrangler, '88 Sentra, '03 Sentra SE-R SpecV. '06Mazdaspeed 6, '96 Maxima, '07 Mazda Miata...every single one was a manual and while I don't beat on them, slam gears, and drop the clutch like I did as a younger man I have never had one fail except one....my '84 Nissan 4x4 Pickup (twice)


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