NM 521

[datzenmike]

On the face of it you have increased your 4.375 about 9% to 4.77 by running tires that are 2" shorter diameter, so yes that's like running 4.88 gears from a 520 in it.

 

Just to get back to stock 'feel' you need a gear ratio 9% lower than the 4.375 that you now have to... around 4.00 but this only returns you to stock and an accurate speedometer. Probably this won't be hauling heavy loads so you can go even lower.  

 

A 3.889 will drop RPMs/speed by 12%

A 3.70 will drop RPMs/speed by 18%

 

You will have to drive at highway speeds. Use GPS or something other than your speedometer. Note the speed and RPMs and multiply by...

0.12 for the 3.889 differential to get your new RPMs at that same speed.

0.18 for the 3.70 differential.

 

For example if 3,500 @ 70 then 3,500 X 0.18 = 630 less or 3,500-630 = 2,870 for the 3.7

 

Keep in mind that you can't keep lowering engine RPM indefinitely as the engine has to be able to make enough HP @ that RPM to push through the air at 70 MPH.

[datsunfreak]

40 minutes ago, datzenmike said:

Keep in mind that you can't keep lowering engine RPM indefinitely as the engine has to be able to make enough HP @ that RPM to push through the air at 70 MPH.

 

3.90s will do that. Since he is running 510 sized tires, might as well swap to 510 gears.  😁

[d.p]

2 hours ago, datsunfreak said:

 

3.90s will do that. Since he is running 510 sized tires, might as well swap to 510 gears.  😁

 Direct swap or what? And I don’t really care about MPH because right now is a good 10-15MPH off already. 

[datzenmike]

3 hours ago, datsunfreak said:

 

3.90s will do that. Since he is running 510 sized tires, might as well swap to 510 gears.  😁

 

Well there is a weight difference. Think of a 510 but with 4-500 extra pounds in it OR two fat chicks in the trunk.

 

3.90s is what I would run with L16 or L20B and 3.70 or 3.90 if L20B.

[Oilspot]

33 minutes ago, d.p said:

 Direct swap or what? And I don’t really care about MPH because right now is a good 10-15MPH off already. 

From what I understand... If you get the right unit (correct splines etc) it's a direct swap. Exactly the reason I was looking at a  third member swap in the first place. 

I know on other vehicles third member swaps are super easy. People more versed in Datsun here can confirm if there's any extra work/ catches.  

Edited February 2, 2022 by Oilspot

[datzenmike]

Early, up into the '70s the housing was aluminum. Lighter but not as strong. After were cast iron or steel. Early '80s they were upgraded again. There were 13, 23 and 29 spline H-190s. 13 spline were very early trucks. Other than the '82-'83 S110 and the '84 S12 all car H-190s were 23 side gear spline. All other trucks are 29.  

[datsunfreak]

2 hours ago, datzenmike said:

Well there is a weight difference. Think of a 510 but with 4-500 extra pounds in it OR two fat chicks in the trunk.

 

My 521 (L18/5spd) weighed 2280. That's only about 200ish over the average 510. 

 

3 hours ago, d.p said:

 Direct swap or what? And I don’t really care about MPH because right now is a good 10-15MPH off already. 

 

As Mike pointed out, direct swapping a third member only works if your donor has the same spline axles.

 

You should be able to pull out your diff and swap the gears from the donor onto your carrier, though. You'll need to reset the gear lash, but that's not too tough.

 

Best option (IMO), if you can afford it, is drop off both diffs at a shop that specialize in this sort of thing and have them swap the gears. 

[wayno]

I used to sell gearsets every once in a while to 320 truck owners that wanted something other than 4.88 gears, one time I sold a set of 3.5 gears to a 320 guy with an LD20 diesel engine in it as it had the torque to turn them gears, all I ever did was remove the ring gear, remove the pin holding the shaft that held the spider gears in, I took the 720 side gears out, put the 320 side gears in, put the shaft back in and pounded the roll pin back in holding the shaft in, put the ring gear back on exactly the same as it was before(I marked them), and it was ready for the new owner, it did not need anything, it was a 720 gearset with 320 side gears.

 

That pin in the 320 gearset was solid, while the 720 pin was a roll pin if I am calling by the right name, that 320 pin could be a PITA to get out sometimes and it would come out shaped like a banana sometimes, I would always put the 320 gearset back together with the 720 side gears as the drag and circle track racers wanted them gears, it was a win/win as no parts were wasted.

Edited February 2, 2022 by wayno

[datzenmike]

1 hour ago, datsunfreak said:

 

My 521 (L18/5spd) weighed 2280. That's only about 200ish over the average 510. 

 

 

 

Assumed it was the same as the 620. Good to know.

 

 

[mainer311]

Just remember folks, changing a rear end is going to make the truck more sluggish in every gear. With a stock L16, it’s going to be a dog.

[Oilspot]

2 minutes ago, mainer311 said:

Just remember folks, changing a rear end is going to make the truck more sluggish in every gear. With a stock L16, it’s going to be a dog.

Unless your correcting for a gear reduction at the tire.... Like in DP's case. 

[d.p]

3 hours ago, datsunfreak said:

 

My 521 (L18/5spd) weighed 2280. That's only about 200ish over the average 510. 

 

 

As Mike pointed out, direct swapping a third member only works if your donor has the same spline axles.

 

You should be able to pull out your diff and swap the gears from the donor onto your carrier, though. You'll need to reset the gear lash, but that's not too tough.

 

Best option (IMO), if you can afford it, is drop off both diffs at a shop that specialize in this sort of thing and have them swap the gears. 

 
ok but I only got 1 diff. I need to know what diff I need to buy before I came take two to the diff shop. 
 

[datzenmike]

 

 

DP your smaller tires are throwing your speedometer way off. You won't be going 70MPH but a lot less. Go out and use a GPS or follow someone at 50 MPH and get an RPM reading in 4th which is one to one. Post this number and we'll go from there.

 

5th is no good because we don't know for absolutely sure what transmission you have. 4th @ 50 will be easier math.

 

 

 

 

So 3 things. You can be thankful

[wayno]

Hi there D.P, all you need is a 390(3.89) or a 370 720 truck gearset, I do not remember what engine you have in your truck, that would determine what you likely would want, if it is a built L16 I would likely go with the 3.90(3.89), if it was a built L20b and you're not hauling a lot of weight maybe the 3.70 gears, but you need torque to get it moving, but with the tires you appear to be using(comments from others and what little I have read) if you really are running the equivalent of 488 gears going to 3.70 is going to be more than one gear difference, if you can start out in 2nd gear right now and not feel that it is sluggish you MIGHT be able to use the 3.70 gears.

 

OK, you brought up a subject that I will address, why will you be taking the gearset to the shop, if it is just to change out the gearset you can likely do that yourself, I can get one out in the wrecking yard in 30 minutes if I cut the brake line tubing and the e-brake cable, less than an hour if I remove the brake lines from the brake cylinders and disconnect the e-brake cable at the center, to remove your gearset you disconnect the brake lines from the wheel cylinders, disconnect your e-brake cable from the center by the cross member, drain the rearend, disconnect the driveline from the gearset, take the 4 locknuts off the axle ends and pull the drum/axle out about 4" on both sides, take all the nuts/lock washers off the studs holding the gearset in the axle housing and pull the gearset out, clean the inside of the center of the axle the best you can with rags, sometimes the gearset does not want to come out, if this is the case you need to use a hammer or heavy rubber mallet and hit the sides of the gearset near the driveline mount flange until it is loose then pull it straight out, once out look to see if the gasket on the rear axle stayed on the rear axle housing and if it is smooth or if it was destroyed getting the gearset out, if the gearset came out clean and the gasket is still on the rear axle housing then make sure your new to your truck gearset is clean of gasket material, then install it in your rear axle housing and put it back together the way you took it out, and don't forget to fill it with gear oil, I just use cheap G5, but others may have a better higher quality gear oil to suggest, bleed the rear brakes making sure the reservoir does not go dry, remember to connect the e-brake cable and your good to go, again remember, going from what your gearing is to a 3.70 I would put it in 3rd gear and try to get it moving min 3rd gear, if it is easy to do then I would likely go with the 3.70 gears.

 

I can change everything out by myself in likely 2 hours including bleeding the brakes, at least I used to, the last time it was done at my house it took my friend and I just over 2 hours, but he was very picky about cleaning everything.

 

I have made rearend gaskets out of large cereal boxes or better yet oil pan gasket boxes as they are large enough and have no folds in them, that is why you need a large cereal box, do not use anything with a fold in it, but I do not use sealer unless it is a C200 with a rear cover.

 

5 hours ago, d.p said:

 
ok but I only got 1 diff. I need to know what diff I need to buy before I came take two to the diff shop. 
 

 

Edited February 3, 2022 by wayno

[d.p]

I know what my speed is because I use GPS…of course I don’t rely on my Speedo. How else would I know it’s off? 
 

I’ll go 50 in 4th and report back. And @waynoi have a built L20b  I’ll look around for a rear end and I don’t haul a lot of weight but I do at times. Probably the heaviest load I do is a yard of mulch but nothing heavier than the for the most part.  But if I solve this issue ill just pay to have the mulch delivered.  

[wayno]

I had a 390(3.89) gearset in my work truck once(it now has 4.625 gears), it was a bitch to get it started when at a stop sign/light that was on an uphill grade, I had P195 tires and an L20b at the time with a race head/stock L20b cam and dual SUs, the truck itself weighed 3400lbs, the trailer with my equipment weighed around 2000lbs, that is 5400lbs, it takes grunt to get that type of weight moving, your truck weighs a lot less, but if you have a high performance cam it might not have the torque, HP cams have less torque and more HP at higher RPMs, it really depends on how much grunt your engine has with the wheels/tires you have, if you can get it going fairly easy in 3rd gear you likely could use 3.70 gears.

 

Torque(grunt) is everything, I have 3.3 gears in my 521 kingcab, but I have an SD25 diesel engine with a turbocharger, it has a lot of torque, and the turbo woke up the HP, I could not even turn 3.50(3.5) gears without the turbo when I got to a hill on the freeway, the engine is rated to have 71hp stock and I was down shifting and floored when I came to a hill, and the hill was not even that steep, now it goes right up the hill, now I have to worry about EGTs.

 

Edit 

 

I suppose this subject should be talked about in your thread. lol

Edited February 3, 2022 by wayno

[datsunfreak]

11 hours ago, d.p said:

 
ok but I only got 1 diff. I need to know what diff I need to buy before I came take two to the diff shop. 
 

 

Sorry, I keep bouncing back and forth between what's on your truck and what is on Oilspot's truck. 😄

 

But to me, 3.90 is always the answer if it's a 4cyl. 👍

[d.p]

30 minutes ago, datsunfreak said:

 

Sorry, I keep bouncing back and forth between what's on your truck and what is on Oilspot's truck. 😄

 

But to me, 3.90 is always the answer if it's a 4cyl. 👍

Thanks @datsunfreak is there a specific car.truck I should be looking at to grab from? Or any 3.90 would work?

 

like would this work? 
 

https://whiteheadperformance.com/product/nissan-oem-3-90-r180-diff-differential-used-datsun-240z-260z-280z-280zx/

Edited February 3, 2022 by d.p

[datzenmike]

That's for an IRS rear end and won't work in a truck. Look at your rear axle and you'll see.

 

Best bet...

The 3.889 was common in 2wd 5 speed trucks. (and already has the correct side gears) There is an engine tag on the inner passenger side fender just below the hood hinge that lists the differential ratio. Lower right corner.... HF 38 is 3.889. HF41 would be 4.11, HF 37 would be 3.70 and so on. Has to be 2wd as 4x4  all had the wrong gears.

 

 

 

 

[thisismatt]

1 hour ago, d.p said:

Thanks @datsunfreak is there a specific car.truck I should be looking at to grab from? Or any 3.90 would work?

 

like would this work? 
 

https://whiteheadperformance.com/product/nissan-oem-3-90-r180-diff-differential-used-datsun-240z-260z-280z-280zx/

 

[d.p]

Lol thanks @datzenmike and I’d wish ill will upon @thisismattbut he already living in BFE hell.  
 

and I just searched r190 Datsun and that was the first link lol.   And what the difference between r180 and r190 if they’re both 3.9 ?
 

 

Edited February 3, 2022 by d.p

[Crashtd420]

27 minutes ago, d.p said:

Lol thanks @datzenmike and I’d wish ill will upon @thisismattbut he already living in BFE hell.  
 

and I just searched r190 Datsun and that was the first link lol.   And what the difference between r180 and r190 if they’re both 3.9 ?
 

 

You have a h190 differential....

[d.p]

Fuck me, so I need a 3.9 H190?  If so what year model truck should I be looking for? 

[mainer311]

Mike already said 5 speed 4x2. My guess is 620’s and 720’s from mid 70’s onwards. Pretty sure even D21’s have the H190.

 

A bunch of good shit found here:

http://datsun1200.com/modules/mediawiki/index.php?title=H190

Edited February 3, 2022 by mainer311

[thisismatt]

The 160, 180, 190, 200, 233, etc is the ring/crown gear diameter in millimeters. R is an independent rear/front diff, H is solid/live axle. As touched on before, there are also year, & truck vs car differences.