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Rear End Swap

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I am new here and have a 1975 Datsun 620 with the stock rear and 4 speed tranny.

 

I am looking to swap the 4 speed with a 5 speed and have done it before on another truck.

 

I am also looking to swap the rear end for another out of a newer truck with taller gears so

I can do a better speed on the freeway. Right now I can do 50mph tops with somewhat larger tires

on 15 inch rims. I want to keep the 15's, but go lower profile tire so I can lower the truck.

 

The engine runs strong, so no problem with power to back these swaps up.

 

My questions are:

 

1. What year trucks can I get a direct fit 5 speed out of?

 

2. What trucks can I get a rear end out of that will fit that has taller gears (So I can go faster on the freeway at a lower rpm)? And is there a way to identify the rear end and the enclosed gears it has from the outside of the differential (Serial Number?)?

 

I had a chance to look at a 1986 Nissan Hardbody rear end and it looks greatly similar to mine. Minor difference in the brake lines, but couldnt notice anything else that would cause a problem swapping. Does this have a probable chance? Anyone done it? Results?

 

I would at least like to do 70 at around 2200 to 2500 RPM or maybe more.

 

I would be extremely greatful for any insight to this and any pointers.

I realize this may be discussed elsewhere also, so please point me in that direction too.

 

Sincerely,

John in Sac

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John, the rear swap is super easy but you dont want to swap the entire rearend. You only need to swap the third member out, many gears ratio's are available in the 720 trucks and some from the hardbody trucks. Most of these truck used the h190 rear differential and the third members will interchange. This way you dont have to mess around with the brakes lines being different either.

Use the search, this has been discussed many times before. I have 3.89 gears in my 76 620.

 

Sorry, i dont have all the answer for the trans though.

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I am also looking to swap the rear end for another out of a newer truck with taller gears so

I can do a better speed on the freeway. Right now I can do 50mph tops with somewhat larger tires

on 15 inch rims. I want to keep the 15's, but go lower profile tire so I can lower the truck.

 

The engine runs strong, so no problem with power to back these swaps up.

 

John, there is something amiss if your truck will only do 50 MPH. You should be able to do 80+ easily. Decreasing the ratio (dropping revs) when you are only making enough power to go 50 will likely lower engine output and slow you down. You need to solve the power problem you have and get your speed back up there.

 

Check that the secondary is working on your carb for a start. Engine off hold full throttle and try to lift the vacuum diaphragm on the secondary... is it free to move or stuck?

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What Mike said.I just picked up a center section from a 720 with 3.88 gears in it.With the 30%(or there abouts)increase in HP from the LZ-23,i can make the swap with no ill effects.Assuming you find your HP issue.you won't be able to frop below a 4.10 without out serious driveability issues.Having said that,it's not worth all the "hasselvation" to go from 4.37 to 4.10.Find your problem and put the OD box in.You'll like it.

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I think a lot of people don't understand that the L series redlines at 7000 rpm. 4000 rpm sounds like your pushing the motor hard when in reality your just getting into it.

 

That and every other car out there redlines around 5000 rpm....

 

I know I was suprised at the actual rpms I was running, when I installed my first tach. :)

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Ok, after doing some more searching on the third member subject, here is what I found and hope I got it right...

------------------------------------------------------------

All '72-'79 620 4.11, 4.375, 4.625 will fit.

All '80- '86.5 720 2wd 3.364, 3.70, 3.889, 4.11, 4.375 will fit.

 

For the 720 Look on the inner passenger side fender just below the hood hinge.

There is a credit card sized aluminum tag with the differential ratio stamped

on it.

Near the bottom is TRANS/AXLE...... FS5W71B............HF38

 

The 38 means 3.889, a 41 would be 4.11 and 43 is 4.375 and so on. Look yours

up first, so you know what you have to begin with.

 

Remember, this is what was installed at the factory, it could have been

changed so check the numbers stamped into the ring gear when you get it out.

Another method is to raise one wheel but make sure the other can't turn. Put

tranny in neutral and mark the raised tire and the drive shaft with some

chalk. Have someone turn the raised tire exactly 5 turns while you count the

drive shaft turns. Divide the drive shaft turns by 10 to get your ratio.

 

Example... 5 wheel turns produces 38.9 drives haft turns. 38.9/10 = 3.89 close

enough.

 

 

If your swapping diffs it's fairly easy. Unbolt the backing plates (4 nuts per

 

side), slide the axles out a bit - don't have to remove them. Unbolt the diff

 

(12 nut's or so), pull out the diff. Insert new diff, tighten bolts. Re insert

 

axles into diff, tighten bolts.

 

Brake lines have to come off for room to allow the axles to be removed. This can result in a snapped bleeder or brake line fitting if seized from age, so be prepared for this. You should bleed the rear brakes to remove any air that gets in.

 

Do not only pull far enough out to clear the spling and let them rest against the grease seal inside the axle tube, it will damage them. Pull them out carefully and set aside.

 

You will have io clean the mateing surfaces. I found that no one carries the gasket anymore. I used a clear blue gasket maker that is very thick and looks some what like tooth paste. I think it was called Permatex. DO NOT use that crappy silicone RTV stuff like used on valve covers. You can't afford to have this thing leak. Do it once.... do it right.

 

Check the pinion flange bolt pattern matches your drive shaft bolt pattern.

 

If you decide to use synthetic oil now, be sure to change the pinion seal now! That stuff is so slippery that it will start to leak past older worn seals.

 

Don't forget new fluid.

------------------------------------------------------------------------

 

Some of this, or most of it was copied and pasted. If this is for the most part all correct, I should have no problem locating a third member and doing the swap for the gears I want...

 

I will check back later for replies and go from there. Thanks for all the help so far, gotta love the wealth of info!!!

 

John

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You still need to find out why it won't go faster than 50 MPH. Once that's fixed you probably won't care to change the rear end.

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John, props to you for gathering so much info from the search function. It does sound like you found all the necessary info too. I do know that there is a version of the h190 third member that is longer, which requires a shorter driveline or a pinion flange change. But i think that it wasnt available until the later hardbody trucks.

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You still need to find out why it won't go faster than 50 MPH. Once that's fixed you probably won't care to change the rear end.

 

Its only going 50 because the engine is taching out at 5500 rpm. I know someone above said it can do 7000, but that is just a wee bit to high for me.

I had a 78 datsun 620 that was the same way, except it was 60 mph at around the 5500 mark, just cant remember exact rpm. I switched out the rear end on it a long time ago, but I couldnt remember what I got it out of because it was so long ago, but it did take care of my top speed issue. 80 at 3800 was a lot better.

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John, props to you for gathering so much info from the search function. It does sound like you found all the necessary info too. I do know that there is a version of the h190 third member that is longer, which requires a shorter driveline or a pinion flange change. But i think that it wasnt available until the later hardbody trucks.

 

Thanks. Im gonna go do a wrecking yard run sometime soon. I will make sure I got plenty of digital pics and measurements to take with me to make sure I get the right setup with minimal change. I might have to dig around and find a pic of my POS as it is now and use it as my avatar.

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Its only going 50 because the engine is taching out at 5500 rpm. I know someone above said it can do 7000, but that is just a wee bit to high for me.

I had a 78 datsun 620 that was the same way, except it was 60 mph at around the 5500 mark, just cant remember exact rpm. I switched out the rear end on it a long time ago, but I couldnt remember what I got it out of because it was so long ago, but it did take care of my top speed issue. 80 at 3800 was a lot better.

 

WTF???? 5,500 RPMs ????

 

The stock '75 rear end is 4.375 so no way it is running 5,500 RPMs at 50. To get that high you either are in third gear, clutch is slipping, you have 10" tires or a 5.12 rear end.

 

Oh, I suppose the tach could be wrong... this is the most likely. Was this the same tach used in the '78 that went 60 MPH @ 5,500???

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Tach is a Sun Supertach II set for the 4 banger (Same style tach, but different model as the 78). Clutch isnt slipping. Only thing I can think of is the original owner swapped for lowers. One reason I believe this is the fact that on the front lower portion of the bed you can see where decals were that shows a commercial rating "Max Gross 4000 lbs" like it was a company truck at one time. The truck is original to Woodland, CA and was used locally to deliver hay bails. It got rare use and only has 53,000 original miles, but the low gears and hauling hay to local dairies could explain the rears...

 

 

WTF???? 5,500 RPMs ????

 

The stock '75 rear end is 4.375 so no way it is running 5,500 RPMs at 50. To get that high you either are in third gear, clutch is slipping, you have 10" tires or a 5.12 rear end.

 

Oh, I suppose the tach could be wrong... this is the most likely. Was this the same tach used in the '78 that went 60 MPH @ 5,500???

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I dunnow. I did some figuring and 50 mph with stock tires you would need an 8.2 to one rear axle to get 5,500 rpms in 4th. The tach is wack .

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Believe me, the tach/dwell meter shows it accurate...

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I don't know what to say. It does not add up.

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No way it's doing 5500 at 50. I mean, my Mom's '73 with 4.88 rear gears and a 4-speed doesn't hit 4000 until you're doing over 60. The tallest rear gears I ever saw for the H190 were 5.3-something rock crawler gears. You go putting 3.889 rears with an L20B and a 5-speed and you'll wonder why clapped-out 45-year old school busses are out accelerating you.

 

A stock '75 620 (4.38 rears and a 4-speed) turns just UNDER 3500 rpms at 60MPH, and under 3000 @ 50. That's if you were running close-to-stock P205/70 R14 tires. Bigger tires, less RPM though the speedometer won't show it and the speedo will be off anyway.

 

Stray coil firings? What kind of dist does it have? I had a tach in a points truck and it jumped all over the place when the dist shaft developed a wobble (also ran like crap though, lots of backfiring trying to accelerate)

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It can't have rear gears doing that. How fast can you go in 1st?

 

Or heck, I have a speedo that went haywire. Odometer shows the right mileage, but the speedo only shows 40-ish when doing 60. The magnet has demagnetized so the needle doesn't climb properly.

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Right now I can do 50mph tops with somewhat larger tires

on 15 inch rims.

 

 

Your speedo is off for sure. Not sure what your 'somewhat' is, but these trucks ran a small tire on a 14" rim stock.

 

RIS (datsun dood from back in the day) was running 31"s on his stock 73 620 and could do 80...by my speedo with stock size tires.

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Stock 620 tires were 6:00 X 14 or 175/70R14. The tire is 23.65" tall and 74.29" around.

 

At 50MPH (4th gear) you are going 3,168,000 inches or divide by tire diameter 74.29" ... you are turning the rear wheels 42,643 times an hour... divide by 60 the rear wheels turn 710 times a minute. I say the axle is a 4.375 so wheel turns times 4.375 gives 3,109 RPMs at the motor.

 

5,500 RPMs on stock tires you would be traveling 88.44 MPH

 

 

 

Right now I can do 50mph tops with somewhat larger tires

on 15 inch rims. I want to keep the 15's, but go lower profile tire so I can lower the truck.

 

Larger tires would lower the RPMs at 50 or you would be going faster than the speedo says. So none of what you have described makes any sense. I realize you believe your experience in this but the figures do not add up.

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Tire size is 205/75-R15 on stock Pathfinder rims. Stock distributor (Rebuilt) with motorola ignition. I pulled the wires for the tach on my way home from work and hooked my Tach/Dwell alone and no change in readings. Going to try the Tire rotation versus the drive shaft rotation to see what I come up with as for gear ratio... Just got to find the jack again...

 

Currently at 5500 rpm and speedo showing 50, big rigs pass me by with ease.

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It can't have rear gears doing that. How fast can you go in 1st?

 

Or heck, I have a speedo that went haywire. Odometer shows the right mileage, but the speedo only shows 40-ish when doing 60. The magnet has demagnetized so the needle doesn't climb properly.

 

Top speed in first? Uhm, not sure. But I know it feels like I am in 4 low in my old toyota

 

Ok, I looked back in some of my old records about my old 78 620 and the rear end I got had 3.73 gear ratio and was shortened to match so the wheels wouldn't be sticking out. Got the rear end (6 Lug) from a Chevy Blazer, but it was free and we did the machine work at or shop. I was able to peg the speedo at 85 (After changing the gear for the speedo) and still climb quick with the stock L20B in 5th I just didnt push it much more than that. Suspension didnt feel right at the time.

 

Another question. My 78 620 had three leaf springs in the rear on each side. What is the norm, do all have 3?Reason I ask is, my friends 74 620 had three like my 78, but my 75 has 5....

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Is your ebrake on? Maybe you are geting drag from a bad bearing or brake cylinder...

 

 

Tire Size Comparison

 

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

175/70-14 4.8in 11.8in 23.6in 74.3in 853 0.0%

205/75-15 6.1in 13.6in 27.1in 85.2in 744 14.6%

 

Link to calculator, it's near the bottom

 

http://www.miata.net/garage/tirecalc.html

 

If your speedo reads 50 your either doing 57.3 or 42.7 mph...I'm gonna go with the 42.7. Anyone wanna check my math? :lol:

 

 

Edit: Different years ran different amounts of leafs. The early trucks had more than the later trucks.

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Tire size is 205/75-R15 on stock Pathfinder rims. Stock distributor (Rebuilt) with motorola ignition. I pulled the wires for the tach on my way home from work and hooked my Tach/Dwell alone and no change in readings. Going to try the Tire rotation versus the drive shaft rotation to see what I come up with as for gear ratio... Just got to find the jack again...

 

Currently at 5500 rpm and speedo showing 50, big rigs pass me by with ease.

 

 

Rigs can go 55. It's unlikely the tack and the speedo are both wrong. The tach is totally out to lunch.

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Ok, I just went and got some thin rope so I could put my rear slider window in and on the way, I tried to see how fast the truck would go in first gear. I will test using the 5500 rpm mark and at that mark my speedo would barely reach 10 mph. A friend of mine is volunteering his dyno next weekend to do some comparisons. His truck (77 620) and my truck side by side: mine is faster off the line, im in third while he is still in second. He has 4.375 gears and he said mine has 4.625. I kinda believe what he says as he is a tech at Maitia Nissan here in Sacramento. But only thing I kinda question is that I have fairly larger tires than he does (he has 185/60-R14's), so I thought I could do a bit better than what I was doing.

 

So I went for a ride with him in his truck and at 5500 rpm in 4th, he is around 60 mph (+/- a couple mph not sure how accurate his is either)... In 5th he can do 70 easy. As far as I can tell, 4th gear ratio are same in all 620 and 720 trannies, am I wrong?

 

He said there is no power loss like I explained earlier, just too low of a gear ratio in the rears... So I am going to try the 3.889's and see how I like, if too low then I might go 4.11...

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2eDeYe' date='10 July 2010 - 09:43 AM' timestamp='1278783837' post='323757']

Is your ebrake on? Maybe you are geting drag from a bad bearing or brake cylinder...

 

 

Tire Size Comparison

 

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

175/70-14 4.8in 11.8in 23.6in 74.3in 853 0.0%

205/75-15 6.1in 13.6in 27.1in 85.2in 744 14.6%

 

Link to calculator, it's near the bottom

 

http://www.miata.net...e/tirecalc.html

 

If your speedo reads 50 your either doing 57.3 or 42.7 mph...I'm gonna go with the 42.7. Anyone wanna check my math? :lol:

 

 

Edit: Different years ran different amounts of leafs. The early trucks had more than the later trucks.

 

Thanks for the info on the springs. I called my other friend and he told me he removed a couple of his springs. So that answers that. And I am willing to say my speedo may be off, but not the tach. But side by side comparrisons with another truck that has 4.375 gears suggest I got even lower... :-( I will post results in a week or so when I find the 3.889's and get them in...

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