I'm stuck figuring out my final drive and need help

[ConnortheWeller]

I've got a Datsun 520 that was mid L16 engine swap when I bought it. I've got it running now and it's fantastic but the gearing feels really off. First gear is unusable (far to low) and 3rd gear feels way too close to 2nd so I end up only using 2nd and 4th. 4th gear also feels light super low gearing because the engine revs really high at freeway speeds not to mention that with the speedo set up for the original engine it's showing well over 100 mph when I'm only going 60. I think the best fix is to swap out the current ring and pinion which I think is a 4.875 with one that's a 3.7 ratio. 

 

Some Important notes:

-To get what I have now to run I had to cut the drive shaft and shorten it to fit the new L16 engine+transmission. Ideally I'd like the new pinion and ring gear to not affect this spacing or change the 520 drive flange.

-I've got access to a machine shop with a Mill, engine lathe, and a vertical CNC in case I need to make any spacers or anything else that might be needed.

-Is what I'm asking for possible?

 

From my research the 520 has a h-190 with 4.875 stock, what exactly do I need to swap it to a 3.7 ratio and where/what should I look for?

 

 

 

Thank you so much for your help, I'm really sorry if I'm asking something that's already been answered.

[datzenmike]

All 520s and the first year '69 L521 were J13 powered with 4.875 differentials. All L16 powered PL521 were 4.375 differentials. Just for the record what year is your 520?

 

To swap an L16 in you would also have to swap the transmission also. Again, is the L16 from a 521, a 510 or perhaps from a 620?

 

520/521 510 wagons and the L16 powered 620 all used the same wide ratio gears in their four speeds. The 510 sedans were more of a mid range set.

 

First being almost useless and third too close to second is only the insanely low 4.875 ratios combined with the wide ratio four speeds. Run a 3.70 or a 3.889 or even a 4.11 differential and that will spread those gears wider apart.

 

Although you could swap just the R&P, the H-190 is a Salisbury type differential with the gears contained in a bolt in third member. Find a 3.889 H-190 third member, and just simply swap it into the axle. 

 

The '78-'80 810 wagons* manual and automatic, used a 3.70 but NOT the later '81-'84 Maxima

The '83-'86 720 2wd with automatic* used the H-190 and 3.70 gears.

 

*these were the only ones in 3.70 ratio I could find that also have the same side gear spline in them so that your axle splines will match up to.

 

The '81-'82 720 4 and 5 speeds** were 3.889

 

The '81-'82 720 2wd automatics** were 4.11

 

** different gears than you wanted but also having the correct truck side gears.

 

 

There are other H-190 options with 3.70 gearing but the side gears are for car axles and would need to be changed.

 

[ConnortheWeller]

5 minutes ago, datzenmike said:

All 520s and the first year '69 L521 were J13 powered with 4.875 differentials. All L16 powered PL521 were 4.375 differentials. Just for the record what year is your 520?

It's a 1968 520, so 4.875 then?

7 minutes ago, datzenmike said:

Find a 3.889 H-190 third member, and just simply swap it into the axle. 

is this the same for the 3.7 you mentioned as well? I'd like to set it up for highway driving and I understand 3.7 is that best for that, right?

[ConnortheWeller]

40 minutes ago, datzenmike said:

To swap an L16 in you would also have to swap the transmission also. Again, is the L16 from a 521, a 510 or perhaps from a 620?

oh missed this part, I'm honestly not really sure what it came out of, I want to say 510 but I'm really not sure, and yep transmission was swapped too. Dont all of the transmissions have a 1 ratio 4th gear? what would cause such a mismatch in rpm?

[datzenmike]

Everything I mentioned is for the H-190. Generally cars have a different side gear but not all. The 510 wagon, 610 wagon, Maxima wagon, '84 S12  use an H-190 but the side gears won't work with the 520 axles. If all you can find are the above you can swap your side gears in to convert them.

 

 

510, 521, '73 620 and 510 wagon 4 speed gear ratios.... 

 

1st...... 3.657

2nd.... 2.177

3rd.... 1.419

4th.... 1.000

 

510 sedan 4 speed ratios are closer together as the car is not going to need as much take off torque as a truck or wagon carrying a load.....

 

1st...... 3.382

2nd.... 2.013

3rd..... 1.312

4th..... 1.000

[Icehouse]

Tire size is important to when selecting gear ratios.  

[ConnortheWeller]

1 minute ago, Icehouse said:

Tire size is important to when selecting gear ratios.  

the truck has 24" diameter tires, dont know what that changes though

 

[Icehouse]

3 minutes ago, ConnortheWeller said:

the truck has 24" diameter tires, dont know what that changes though

 

 

4th gear or 5th is important to calculate.  I put 30" tires on my D21 4x4 because it kept 4th in the power for hills with my trailer.  31" fits but I would have been required to shift to 3rd for all hills.  All my other rigs are lowered.  Then you are lowering your gears with tires.  

[ConnortheWeller]

perfect, guess I just need make a new post seeing if anyone's got a third member out of either a '83-'86 720 2wd with automatic or '78-'80 810 wagons they'd sell me.

 

On a side not, I haven't delt with any part pulling yards before, anyone know how pick-n-pull works (can I just go pull a third member?) and if any of these would work? https://www.picknpull.com/check-inventory/vehicle-search?make=193&model=3774&distance=10&zip=97266&year=

^^^ those all seem like d21's but i have no idea how to tell the difference

[datzenmike]

1 hour ago, Icehouse said:

Tire size is important to when selecting gear ratios.  

 

And why I specified there were 3.889 and 4.11 also. 3.70 is a little low for an L16 but just fine for the highway if just commuting.

 

1 hour ago, ConnortheWeller said:

the truck has 24" diameter tires, dont know what that changes though

 

 

The 520/521 used 6:00 X 14 which I think is around 26" diameter. Anyone?

[Ooph!]

2 hours ago, datzenmike said:

 

The 520/521 used 6:00 X 14 which I think is around 26" diameter. Anyone?

 

Yes 26"

[datzenmike]

A 2" smaller diameter increases gear ratio about 8% so a 4.875 feels like 5.30 with a 24" tire.