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This subject has been covered before but i cant remember the thread name. This is not for 620's but Z cars & i found it informative...

http://www.geocities.com/z_design_studio/transmission.html

Id say PM Datsunaholic or Datzenmike...i bet they have the info.

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So all the 4 speed 620s came with the 4.375? I am thinking ahead here, but want to lower the gear ratio, with the 215-40-18 tires and 4 speed the truck winds up pretty high on the freeway. I am doing an engine swap for something with a LOT more torque beginning of next year so a lower gear set will be beneficial later also.

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All '74-'79 4spds had the 4.375 rear ends.

 

Lowering the rear end will cut performance. Here's an idea... replace the 4 spd with an over drive 5 spd. The 5th gear lowers engine RPMs like a rear end change but only in 5th. You get to keep your performance and good mileage.... cake and eat it too!

 

The '77-'79 620 5spd

The '80 720 (2wd only) 5spd

The '77-'83 280z

 

All the above will bolt into a '75 620 and have varying amounts of over drive.

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All '74-'79 4spds had the 4.375 rear ends.

 

Lowering the rear end will cut performance. Here's an idea... replace the 4 spd with an over drive 5 spd. The 5th gear lowers engine RPMs like a rear end change but only in 5th. You get to keep your performance and good mileage.... cake and eat it too!

 

 

.

 

I recently installed 3.70 gears into my 76 620. I just bolted in a third member from an 85 720. Took me about two hours. Need a few more tanks of gas on them to give a better idea of gas mileage. Dont think i am getting the mileage improvement that i wanted around town though. Really want to make a road trip. I am still running the stock 4 speed too. Will change it to a 5 speed when i find one cheap.

 

I have to agree and disagree with Datzenmike about the performance. In theory mike is completely correct. And this would be a problem if we were running 33" tires like mike is, BUT we are not. We are running close to stock diameter tires. My L20b has plenty of torque to compensate for the gear change. Average daily driving around town i do not feel a power loss, but rather a quieter truck that isnt shifting through three gears to cross an intersection at a stop light. Absolutely no problem keeping up with traffic, In a certain respect it actually feels faster than before because each gear takes you so much further.

 

If i find a set of 3.90 gears cheap, i will buy those also. Just to try them. I have ran the stock 4.37 gears, 4.11 gears, and now the 3.70 gears in the same truck, same motor, same transmission. The 4.11 was a waste of time, not even close to enough change in RPM. The 3.70 are the very best so far, but i wouldn't mind trying the 3.90 gears.

 

Jason

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  • 1 month later...
1600 roadsters come stock with 3.90 rear end, 2000's with 3.70. Dunno if they're wide enough to fit a truck.

 

Shit, I had a long hard day to-day in the bush. It was too far and too hard to walk out for lunch, so I'm kinda fucked. You were talking about rear ends not trannys, sorry. I think the Roadster gears will swap, they and the 620 both used H-190 carriers. Forget the whole thing as the axles will be 4 bolt anyway. They did use an aluminum housing though. Doubt if you'll find anyone willing to part with them anyway.

 

If you are looking for a 3.70 or 3.90 (3.889) look at the 720 trucks, both were used on the 2wd. The '84 2wd Z24 standard tranny used 3.70 rear gears. The auto tranny used 3.889... forget the 4X4s.

 

To help in your search... look only at '83.5 and up with Z24 engines. Look on the inner passenger side fender just below the hood hinge. There is a small aluminum tag and the last two lines near the bottom will say something like...

 

ENGINE motor Z24(S)........2389cc

 

TRANS/AXLE FS5W71B....... HF38

 

HF38 is the axle ratio installed at the factory, 38 means 3.889, 37 would be 3.70, 41 is 4.11 and so on..... Nice of Nissan to do this, makes shopping much easier.

 

Oh, and thanks for the photo... I don't have a Maxima tranny pix only a long tail dogleg. All I can tell is it's a post '80 L series.. maybe from a Z car?

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limited slip diff's in 2wd 720's? Were they even available? I don't think I've ever run across one in a wrecking yard .............

 

Next visit to P&P, a 3.70 or 3.90 will definately be on the agenda for the KC - it's geared a bit low already, and going to smaller diameter rear tires is just going to make the problem worse. Will have to decide whether to take the whole rear axle, or just pull the diff carrier.

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how hard is it to do the rear end gear swap?

 

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.

 

Don't forget new fluid.

 

If your just replacing the ring and pinion...well, that's a bit more work. I also recommend picking up that new LSD before you start if your doing the ring and pinion. ;)

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2eDeYe;79617']If your swapping diffs it's fairly easy. Unbolt the backing plates (4 nuts per side)' date=' 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.

 

Don't forget new fluid.

 

If your just replacing the ring and pinion...well, that's a bit more work. I also recommend picking up that new LSD before you start if your doing the ring and pinion. ;)[/quote']

 

This is encouraging to hear, doesnt sound very difficult at all. Are there special things one should look out for when doing this? Problems that may arise?

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limited slip diff's in 2wd 720's? Were they even available? I don't think I've ever run across one in a wrecking yard .............

 

And you won't.

 

Factory LSDs weren't available untill the D-21 Hardbody or WD-21 Pathfinder, and then only on the Sport Package option with VG motor. And then it was on the H-233 rear end and that won't fit unless you swap the whole axle. The Pathfinder also had disc brakes but you would have to convert from coil to leaf springs.

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This is encouraging to hear, doesnt sound very difficult at all. Are there special things one should look out for when doing this? Problems that may arise?

 

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.

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