Differential Swap

[hardhead320]

Recently finished up a total resto on my 65 320 drove it down the highway and could not believe it only does 50 mph, the J15 engine sounded like it was rev'n about 5000 rpm long story short 50 mph is max! I heard roadster rear end will bolt right in, is this true? and if it is what year and model will go into my L320? Brakes, drive shaft, E brake cable, shock mounts, diff width? Through me your opion and any information that could help me. I am thinking of buying a cheap broken down roadster that is trashed but before I do I need to know what year and model will fit my L320. 50 mph is just not going to cut it!

 

[jefe de jefes]

Nice job!

[hardhead320]

thanks I am still recovering from it ! I did everything you see except upholstery.

[datzenmike]

This is the 13 spline side gear in the differential from a 320. The axle spline goes in here and mates with it.

 

 

This is the 23 spline side gear from a 510 wagon differential, exactly the same as a Roadster differential

 

 

All three uses the same H-190 rear end but there are differences in the side gears. Yes the Roadster diff or even a wagon diff will bolt in but the side gears must be swapped. Roadsters are somewhat rare and command high prices for their parts. A 510 wagon would be easier to locate but you still have to swap the side gears. A much more common vehicle would be the 521/620 720 truck. There were millions made and they are common. The ratios range from 4.88, 4.625, 4.375, 4.11, 3.889. 3.70, 3.545 and even 3.364 so lots of ratios to choose from.

 

Now having said all that, your tiny J motor? probably has a 4.88 (or higher ratio) and although it sounds like it's going to blow up... it won't. You could go down to 4.625 and maybe 4.375. There's a reason Nissan used such high number gears in your 320. The motor is very small and with little power. In order to carry a load at highway speeds some top end speed must be sacrificed to do this. The limiting factor to any vehicle's top speed is wind resistance or the amount of power the motor has. A lower ratio can allow you to go faster but it also drops the engine RPMs. Lower RPMs at the same speed means less power to push through the air. It's a vicious circle and can result in a vehicle that can go faster in 3rd gear than 4th. This is called power limiting.

 

Who knows? You may be able to drive with a 4.11 in the back but windy days or hills will require constant down shifting for more power.

 

 

 

 

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[Jayden71]

I like that truck!! :D

[wayno]

Hi there Hardhead320, you probibly have 488 gears in your truck, you can change the gearing by putting a 521 pumpkin(the center section that unbolts from the rear casing)in it with 437 gears, you will have to lift the rear of the truck and remove the tires, drain the fluid, disconnect the brake lines and park brake cables, unbolt the axles and pull out the axles, unbolt the driveshaft, unbolt the third member and remove, now you will have to change the spider gears out so that you can put you 320 axle back in, because the 521 axle is differant. It has been a long time since I did this, I know you have to remove the ring gear and then you can get the shaft out that holds the spider gears in, then you put the 320 spiders in the 521 pumpkin(437 gears), put the shaft back in that holds the spiders, put the ring gear back on and reassemble the rearend, there is a little more though to it than this, but it is an outline. You can do this with most the rear axles all the way to maybe 1984, there are a lot of differant ratios such as 488, 437, 410, 388, 370, and I even seen once a 340? It is not as hard as it sounds, it only took me about an hour to change out the spiders themselves the first time. When you take off the ring gear you should mark it, make sure you put it back in the same exact place. You should check the gearing in your truck now to make sure that you do in fact have 488 gears, you do not want to make to big of a jump, I have a 1964 NL320 that has 388 gears I beleave. I hope this information helps you. If you need any help, let me know, I will try to help. wayno

[wayno]

I see I wasn't fast enough, I type slowwwww. wayno

[hardhead320]

Wayno and Mike thanks for the reply the vehicles you guys listed I will be looking for to possibly remove the rear end out of or worst case scenario buy the whole car outright to use the rear end. I guess you guys are leaning towards just changing spider gears out to new ratio vs. replacing whole rear end. You made it sound easy when you said it took you about an hour to make the spider gear swap out. Mike great pictures as far as the number of teeth in the carrier. And now I have to determine what rear end I have and what gear ratio it is, got any suggestions on what I have? (gear ratio) Does Datsun mark the housing with a tag or a stamp in the cover? Or a web site with a list? I will be looking next week for a new rear end out of a 510,521,620 or 720. So I will need to know how to identify correct gear ratio on perspective new rear end. You guys have been a big help. Like I said I was a little shocked when I only reached 50 mph. I want to be able to drive this truck to shows and in California you will get ran over going 50 mph when everyone else is doing 75 mph once again thanks.

[datzenmike]

The ratio is stamped into the outer edge of the ring gear like this:

 

 

Divide 37 by 8 and you get a 4.625 ratio.

 

All 521s were 4.375

Early '72-'73 620 automatics were 4.625 as was the '74. All other years were 4.375 except the 79 standard.

All 720s have an engine tag under the pass side hood hinge with the gear ratio stamped into it.

 

Bottom line far right "HF38" means 3.889 ratio an HF41 would be 4.11 ratio, 43 would be 4.375 and so on. Avoid 4X4s as some had different drive shaft yokes and later ones were C-200 diffs.

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[hardhead320]

Mike thanks for the quick response. Ok so bottom line any of the cars you mentioned above, the spider gears will interchange with my 320 axels. Exception being the 4x4 cars, plus if I want to identify any rear end ratios on any cars other than the 720 I have to remove the dif cover to see the ratio stamped on the ring gear right?

[Dime Dave]

Mike thanks for the quick response. Ok so bottom line any of the cars you mentioned above, the spider gears will interchange with my 320 axels. Exception being the 4x4 cars, plus if I want to identify any rear end ratios on any cars other than the 720 I have to remove the dif cover to see the ratio stamped on the ring gear right?

 

Mike Klotz had a video he did showing the whole swap of the side gears. I tried to find it but had no luck. It was a great video and should be reposted if it has been lost.

 

As I remember from a friend who tried to do the side gear swap using a 720 (cast iron center)side gears, tha they could not be swapped into the 510 Wagon alloy center. He did create a 4.375 R&P by taking the ring gear carrier (complete internal differential section) from the 510 alloy center and intalling it into the 720 cast iron center. It may be possible to do the same with the 320 internal differential and put it into a cast iron center with the gears you want.

 

Here is a link to the swap that I posted a little while back:

320/Wagon Side Gear Swap

[datzenmike]

Ah yes, Mr Klotz. Well I knew exactly where to look for this gem. Here it is>>>>

 

http://community.rat...ider-gear-swap/

 

 

More info

[wayno]

Mike thanks for the quick response. Ok so bottom line any of the cars you mentioned above, the spider gears will interchange with my 320 axels. Exception being the 4x4 cars, plus if I want to identify any rear end ratios on any cars other than the 720 I have to remove the dif cover to see the ratio stamped on the ring gear right?

 

Hi there, you do not have to remove the cover to get an aproximate idea of the gear ratio, you need the rear of the vehicle to be in the air(or at least one rear wheel), you need to pin one of the tires/drums so it won't move,or have someone hold it so it doesn't move, put the transmission in neutral, or remove the driveline, make a visible mark on the rearend flange(what the driveline connected to) so that you can count how many times it turns with some kind of accuracy on the side that is turned, make a mark on the tire/brakedrum, I mark strait up most the time. Then turn the tire/brakedrum two complete times around counting how many times the driveline/flange turns. If the driveline/flange turned 3 1/2 times that would be 350 gears, if it turned 4 1/3 times approximatly that would be 437 gears, a little more than 3 3/4 times around would be 388 gears, understand? I would stick with the two wheel drive 720 truck gears before 1985. I was able to swap out the gears without removing the cage(what the ring gear connects to), but I did remove the ring gear from the cage. The point is to put your spider gears in the 720 pumpkin with the better gearing, so your axles will work. Most of the rearends(datsun/nissan truck 1966 to the 80s) out there are at least 4" wider than your rearend. wayno

[datzenmike]

The point is to put your spider gears in the 720 pumpkin with the better gearing, so your axles will work. Most of the rearends(datsun/nissan truck 1966 to the 80s) out there are at least 4" wider than your rearend. wayno

 

I think you meant to say side gears but this will be obvious once it is apart. Also If one axle is locked and you turn the other wheel once the ring gear will turn once and turn the pinion what ever the ratio is. If you turn twice you have to divide the answer by two to get the ratio. I think this is correct.

 

[wayno]

Hi there Datzenmike, I went out and did the ratio check a few times, when one drum is locked, you have to turn the other drum two rotations and count how many times the driveline turns, and that is the gear ratio, 3 1/2 turns would be 350 gearing. When I am at the u-pull wrecking yards I always find some way of locking one side because I can't see it from the side I am on. If it was a LSD and both drums turned the same way at the same time, then one rotation gives the gear ratio. wayno

[datzenmike]

Hi there Datzenmike, I went out and did the ratio check a few times, when one drum is locked, you have to turn the other drum two rotations and count how many times the driveline turns, and that is the gear ratio, 3 1/2 turns would be 350 gearing. When I am at the u-pull wrecking yards I always find some way of locking one side because I can't see it from the side I am on. If it was a LSD and both drums turned the same way at the same time, then one rotation gives the gear ratio. wayno

 

:lol: Having a hell of a time wrapping my head around this....

[bananahamuck]

First the truck WOW! just,,, WOW!! Second Is this tread info also applicable to a 411 sedan?

[mklotz70]

Very sweet truck. As long as you don't care about the rig being a bit sluggish off the line, you'll like the 3.89's in it. I put them in my NL and could do 60-65 no prob...I was passing people on the highway, so most likely I was doing 70+. With a J15(I had an E1), you won't have any prob at all off the line. In fact, you'll be able to get across an intersection without having to shift :)