Rear End Swap

[]2eDeYe]

It's entirely posible you have 5:38's swapped in there...won't know til you get into it.

[cobra269]

Ok, is there something left out of this or did I do it right?

 

This is the formula I followed:

"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. "

 

I did this exact formula three times on my truck just to insure my findings were right...

for 5 wheel turns, my driveshaft rotated 11 times. So according to the formula I have a 1.1 gear ratio???

 

I blocked the left wheel so it wouldnt roll out the driveway and had the right leg lifted. I marked the driveshaft to third member with chalk.

marked the tire to ground with chalk...

[datsunaholic]

Well, if your rear axle was really showing 11 turns to 5 wheel turns, you'd have a 1.1:1 rear ratio, and you could do 200MPH in 4th. That of course isn't realistic.

 

I'm sorry, but so far none of your numbers make sense.

 

 

Here's what a bone-stock 1975 620 should have:

 

L20B engine

4-speed trans with gear ratios of:

1st 3.592:1

2nd 2.246:1

3rd: 1.415:1

4th: 1:1

 

Rear axle with 4.375 ratio (35/8 tooth ring/pinion)

 

 

If all that is installed, and the speedometer cog hasn't been changed, the engine should run at ~3200 RPM at 50MPH on the speedometer, irregardless of tire size. Different tire sizes would throw the speedo off, but only incrementally. The RPM to MPH as read on the gauges would still be the same, only the actual speed would be different.

 

So, to get 5500 RPM on a working tach, and have a reading of 50MPH on the speedo, and be in 4th gear, the speedo cog would have to be very, very different. To get a true speed of 5500@ 50MPH, you'd also need 7.52:1 rear gears, which I've never heard of. Ranges in the 5.xx:1, yes.

 

 

So, I got your 1st gear (5500 @ 10mph) numbers.

 

A stock truck would read about 2300 RPM at 10MPH in 1st gear.

 

For a truck really turning 5500 rpm @ 50MPH in 4th gear, in 1st gear it would turn 3950 RPM @ 10MPH. Not 5500. Of course, 5500 would only be 13MPH. But, at that low speed the speedo is terribly inaccurate. SO that's not a good test.

 

 

So, honestly, the only thing I can think of is:

 

1) The speedo and tach are both whacked. The semis passing you are doing 70.

 

2) Someone stuck rock-crawler gears in the back (is it even a Datsun rear axle?)

 

3) There's a compound-low in it (I'd think that'd be hard to miss, having a second gearbox in a 2wd truck)

 

4) You're driving in 3rd thinking it's 4th (though a stock trans, it'd only turn 4500 RPM @ 50 in 3rd)

 

5) Someone stuck a Nissan-Competition 5-speed in it with a 1:1 5th gear, and 4th is 1.7:1- you just haven't noticed the extra gear... Though that's totally unlikely, as 1.7:1 is a more likely a 3rd gear in a comp box.

 

 

 

 

I just don't know. Too bad you aren't closer, because I'd really want to look at this mystery truck. I've never had a 620 pickup that couldn't pull 70MPH with ease, though it's deafening.

[Z-train]

Cheap way to verify tach-borrow someone's timing gun that has a tach function in it.

[datzenmike]

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 .

 

 

To get a true speed of 5500@ 50MPH, you'd also need 7.52:1 rear gears, which I've never heard of. Ranges in the 5.xx:1, yes.

 

Thanks Doug, I was somewhere in the ball park or just over the fence.

[cobra269]

Hey, I really do appreciate all of your inputs. It makes no sense to me either. When I get that sucker pulled out, will count teeth and post here. Hopefully after that we can be at ease knowing what the hell it was. I kinda wish one of you guys was close too so I know I am not the only one dealing with a mystery... :-(

[datzenmike]

The ratio is stamped on the outer edge of the ring gear, if Nissan. Should be marked 35/8 for a 4.375 ratio.

 

I just raise one wheel and place in neutral. Mark drive shaft and top of tire. Turn tire 10 time count the drive shaft turns. About 40 turns would be 4.11, just under 44 would be 4.375.

[Paul G.]

Uh... wouldn't that be 20.5 turns for a 4.11 and just shy of 22 for 4.375? After all, if you turn one tire 10 turns with the other tire on the ground the ring gear will only turn 5 times (a differential has to be good for something.)

[datzenmike]

I think the axle is splined to the diff case assy. and if it turns then the ring gear turns the same. But only if the other wheel is on the ground. Easy to check on your car or truck if you know your ratio. I think you will find the the drive shaft will turn about 4 1/3 turns for every one turn of the tire on a 4.375.

[cobra269]

Picked this up from an 82 720. Looks to be in great shape. Only thing is, I couldnt find any in 3.889. I found one with 3.55's and two with the 3.70's so I grabbed the first easiest one that allready had an axle pulled and other one was allready partially apart. Drive shaft was down so when I got the 10 or so nuts off the diff, I tugged on the drive shaft and it popped right out. Picked up a few other things also, but thats in another thread....

 

[cobra269]

As updated in my build post. Swapped out the gears and all is perfect now. Except the reading of the speedo, but my GPS gives me a great idea what I am actually doing now. 50 mph in third is easily obtainable with plenty of power to take off the line. 4th I can do 70 with ease and even have plenty power left to go more if I wanted. I extremely appreciate everyones help here and just so everone knows, the stamp on the ring gear that came out was 35:8 wich I think someone mentioned in an earlier post as being 4.375's. still not sure why I could only do 50 at 5500, but in 4th now at 2500 or so, I am doing round 65, not exact as my tach is unhooked and speedo is off, just a close guess... Now to get that 5 speed in in a few weeks. Got to go make a parts wanted list and post to the wanted section.... :-)

[]2eDeYe]

Weird... :blink:

 

:D

[datzenmike]

I think if you unhooked your tach you could have gone faster. The tach is lying to you.

 

 

I'll bet when you do hook it back up with this new rear end it will say you are running 4,800 rpms at 50. Don't believe it!

[Dawgz83948]

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

 

Sorry to dredge up such an old thread, but is this above list correct?

[cobra269]

Everything is still the same...

[Scgreen620]

I think it might be your speedo my speedo says it's going 30 but my gps says it's running 40 ... Check it out you might see a difference

[superbee620]

I was wondering what year 620s have the 4.637 rear. I put a 280zx 5 speed in my 73 620 and it will run 2000 mph downhill lol. But seriously i need a lower rear gear for pulling. Thanks.

PS king rat warned me this would happen before I installed it.

[datzenmike]

It's 4.625 and '73 L16 automatics had them. Maybe the '74 L18 but my info says no.

 

Lower gear for pulling?????? 4.625 is already insanely low. You want lower look at the J13 powered 520 they were 4.875.

[G-Duax]

Also keep in mind if going to a deeper gear ratio, you may need a ring gear spacer.

You used to be able to buy them from Nismo, in 3mm, 4mm, and 7mm, but those are long gone.

I had to make mine when I went to the 4.62 gears going on an LSD meant for 3.7 gears, but I have machine equipment in my garage.......

[scooter]

Automatic 73 620's had 4.88's I believe. Always up between 4500 and 5000 when I had that one on the highway

[cobra269]

After 7 years away from this topic, I am still running the 3.70 rear gears with the 5 speed. Only difference is, I am running 185/60-R15's now... If I had to do it over again, which may be someday soon, I would go with the 3.55's in the rear with my old tires pictured in my profile... With current setup, my speedo is 2 mph faster than my GPS at 55 mph currently. I pull a 600 pound unloaded trailer with ease (Stopping is a different story) and with the trailer and truck both loaded, I have only minimal issues with speed... I can still do 65 mph easy with 1.25 tons added via loaded material with the combination of truck and trailer. So, when it comes down to it, my only issues are hampered by load!

No! I would not pull 1.25 extra tons with my prior tires! LOL...

FYI: 2,000 in trailer and 500 lbs in truck...

Look for me sometime soon, driving south down I-5, heading to Stockton, with an extended Datsun Truck Bed Trailer, loaded with crates of power supplies, laptops, cable boxes, computer motherboards, etc... Most likely in the Fall!

 

[banzai510(hainz)]

Be honest I think that gear ratio is perfect. Can hit Hwyway cruising speeds.

I went with a 370 in my 510 and can tell the lost torq and really get it wind up to get the carbs flowing(mikunis)

 

3.55 you need a Real torq motor like a 280z motor

 

355 will kill your power going up hill!!!!!!!!!and with a load. Clutch killer

Edited June 21, 2019 by banzai510(hainz)

[datzenmike]

This s a '75 with a small port L16 intake and weber turning a 3.70 with 500 pounds in the box plus a 600 pound trailer with  2,000 pounds in it. Basically the weight of a second 620. On the level you are limited by wind resistance and eventually you will get the speed limit. Realistically, on hills, the truck pulling itself and the weight of a second truck, must lift this weight vertically. You won't be able to cruise up hills. .  

[cobra269]

3 hours ago, banzai510(hainz) said:

Be honest I think that gear ratio is perfect. Can hit Hwyway cruising speeds.

I went with a 370 in my 510 and can tell the lost torq and really get it wind up to get the carbs flowing(mikunis)

 

3.55 you need a Real torq motor like a 280z motor

 

355 will kill your power going up hill!!!!!!!!!and with a load. Clutch killer

Don't get me wrong, I love these 3.70's with the current tires, but when I go back to my rim huggers, I'll essentially be creeping up to theoretically a 3.88. So With my tire huggers and a 3.55, I should be close to what I have now, when it comes to RPM vs MPH vs HP...

Right now, truck alone, I can cruise up a 12% grade in 5th and still gain speed (Slowly). With trailer hooked and loaded, I can pull that same grade in 3rd easily and still gain speed, but 4th has no chance. Unloaded trailer, I can maintain speed in 4th on the same grade.

And I have to correct/clarify myself from earlier. The 1.25 tons is with the weight of the trailer included.

[cobra269]

2 hours ago, datzenmike said:

This s a '75 with a small port L16 intake and weber turning a 3.70 with 500 pounds in the box plus a 600 pound trailer with  2,000 pounds in it. Basically the weight of a second 620. On the level you are limited by wind resistance and eventually you will get the speed limit. Realistically, on hills, the truck pulling itself and the weight of a second truck, must lift this weight vertically. You won't be able to cruise up hills. .  

I don't typically pull a loaded trailer with a loaded truck up hills for very long for enjoyment. LOL. And I never go above cab height when loaded, so I don't really get much extra wind drag.

If I have a Refrigerator standing upright in the bed of the truck, with other appliances and the trailer, then I really notice a difference, but downshifting into 4th takes care of it.

But as a daily, with no load and no trailer, no problem...

And my fuel mileage? WHOOPPIE!!!! When I had the 4.37's, from Redding to Stockton, it cost me $60 one-way (Loaded). That same trip (Loaded) with the 3.70, $30. And that compensating for the difference in price of fuel.

But I got one question? How do I keep people from asking me if I want to sell the truck? I get one person a week on average! Hahaa!