Fs5w71c input shaft WILL work in the older fs5w71b

[Mr.Disprosium]

So I was doing a pull in my built ka-et 720 with a stock trans and shredded the input shaft gear, I ended up swapping it out with a 91 300zx transmission for longer gears and stronger internals but it ended up being bad and I broke the center plate trying to fix it. so I decided to try to fix the stock trans. I took a new fs5w71c input shaft with the wider gear and 4th gear I had laying around and decided to see if it would work in the 720 b trans, to my surprise it did, and somehow there was no other damage to the trans other than an obliterated 4th gear set and it took out the input shaft bearing as well but that's not major. I saw a post about somebody asking this very question "https://ratsun.net/topic/76970-fs5w71b-input-shaft-interchange/" and it never got answered, well here's the answer. I can't insert my picture links or I would show yall the obliterated gears.

[ggzilla]

Good to know the 300ZX input shaft will swap

Which FSW71B do you have, the stock Z24 transmission?

 

Dunno if there are wide and close ratio versions but if there are then those two won't swap. For example ,with FSW71B for the 620 5-speed, there are two different input shafts

 

 

[datzenmike]

If the 720 is not a diesel or a 4x4 it's likely a mid ratio with a 22/31 input/counter gear set. If it fit, then the 71 C input must have the same 22 tooth gear

[Mr.Disprosium]

10 hours ago, ggzilla said:

Good to know the 300ZX input shaft will swap

Which FSW71B do you have, the stock Z24 transmission?

 

Dunno if there are wide and close ratio versions but if there are then those two won't swap. For example ,with FSW71B for the 620 5-speed, there are two different input shafts

 

 

Yeah it's a stock 83 2wd z24 transmission. Why would the input shaft gear set from a different ratio trans not work? Are the dimensions different?

[datzenmike]

The input shaft of a mid ration transmission has a 22 tooth gear engaging a 31 tooth gear on the counter shaft below it. 

 

A wide ratio transmission has a 21/32 gear set

A mid ratio transmission has a 22/31 gear set

A close ratio box uses a 23/29 gear set. 

 

As the teeth are all the same size, removing a tooth decreases the diameter of the input gear and this is compensated for by adding a tooth on the counter shaft which increases it's diameter to match. This is much the same as crown and pinion gears in a differential. A 4.11 pinion won't mesh with a 4.375 ring gear.

[ggzilla]

Excellent info, @datzenmike

 

I never understood wide vs close ratio until i did the math on the gears.

Wide-ratio and close ratio have the same gears 1-4, but the input gears differ

 

The math I did was for the Datsun 1200. 3rd gear does differ, but 1st, 2nd and 4th gears (the actual physical gears) are the same in a close-ratio box

as you can see for the close-ratio box, the overall ratios are closer together, and much higher gearing. First gear in a close ratio box is almost like taking off in 2nd gear

This higher gearing makes a close-ratio box not very good for a street car or a truck, but when paired with steep differential gears, it makes for quickest acceleration as the engine RPMs don't drop as much when you shift up to the next gear

 

 

Edited September 17, 2025 by ggzilla

[datzenmike]

My 710 has an '85 S12 CA18ET 5 speed. (with an L series front case swap) This is a wide ratio box with a 3.592 first gear compared to a mid ratio 3.321 first and a close ratio 3.062.  This 3.592 first brings the RPMs up much faster for spooling the turbo sooner in the S12. Added to this is a 4.11 to replace the 3.70 differential originally in the car. While there is more RPM drop between gears with a wide ratio it more than makes up for this by revving up into the power band much sooner and pulling away. 

[Mr.Disprosium]

On 9/17/2025 at 2:07 PM, datzenmike said:

The input shaft of a mid ration transmission has a 22 tooth gear engaging a 31 tooth gear on the counter shaft below it. 

 

A wide ratio transmission has a 21/32 gear set

A mid ratio transmission has a 22/31 gear set

A close ratio box uses a 23/29 gear set. 

 

As the teeth are all the same size, removing a tooth decreases the diameter of the input gear and this is compensated for by adding a tooth on the counter shaft which increases it's diameter to match. This is much the same as crown and pinion gears in a differential. A 4.11 pinion won't mesh with a 4.375 ring gear.

If you replaced both gears though they should fit on the shaft the same, and you could theoretically change the ratio if you replaced both gears, which I did. I was under the impression once a new gear has meshed and worn with its partner gear it cannot be meshed with another gear anyways. My truck has a 3.55 final drive and I believe the mid ratio trans at 80mph i sit at 3800rpms. The 91 300xz trans I temporarily swapped in made my ratios much nicer for modern speeds, I was cruising at 80mph at only 3100 rpms with a 195 65 14 tire.

[ggzilla]

The gear on the input shaft meshes with the gear on the cluster shaft, neither are replaceable gears. It's good to know the 1991 300ZX trans has the same input gears as the 1983 Z24 transmission

[datzenmike]

The counter shaft driven gear is removable. You could replace it but I think sold as a set. 

[ggzilla]

thanks for pointing that out. This makes it different than the smaller Nissan 5-speeds, and much more swappable

 

6 hours ago, Mr.Disprosium said:

you could theoretically change the ratio if you replaced both gears, which I did

 

58 minutes ago, datzenmike said:

The counter shaft driven gear is removable. You could replace it but I think sold as a set. 

I too see in the Nissan parts catalog it indicates the input counter gear is included when you buy a counter shaft ...

but also is sold as "Set Gear Drive"

 

For the 1983 720 there were two different sets:

* 32200-E9055 Set Gear Drive

* 32200-E9551 Set Gear Drive

Exactly what ratios these are, is not stated

 

But for counter gear assembly, there is only "14T"

* 32210-E9810 Counter Gear Assy

 

The 71C out of a D21 also has two sets

 

[Mr.Disprosium]

7 hours ago, ggzilla said:

The gear on the input shaft meshes with the gear on the cluster shaft, neither are replaceable gears. It's good to know the 1991 300ZX trans has the same input gears as the 1983 Z24 transmission

It is not the 300zx trans(rs5r30a) that has the same gears, it is the fs5w71c that has slightly wider input gears that will in the fs5w71b. The 91 300zx transmission is almost entirely different, much bigger gears and parts that definitely won't fit in the fs5w71b/c trannys, different case, different design. Bigger output shaft. I used an adapter plate and custom driveshaft to swap it onto my ka, also it has much longer gear ratios, 80mph at 3100 rpms vs 3800 rpms, but that trans ended up being bad, and I broke it further trying to fix it so I fixed my original transmission until I can afford another 300zx trans.

[Mr.Disprosium]

3 hours ago, datzenmike said:

The counter shaft driven gear is removable. You could replace it but I think sold as a set. 

Yes, and yeah I could only find gears as sets, no single gears available.