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Matching Speedo Gears after Rear End Swap

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Hello Ratsun Experts, Well I followed your fine advice today that I had read on Ratsun, and managed to remove a 'third member' from a scrap yard 1983 two wheel drive 720 ( 3.889 ratio) to swap into a 76 Datsun 620 I have.(It was surprisingly easy to do!)

I forgot to remove the speedometer gear,and now at home, I am wondering if I need to remove both the speedo gear AND the speedometer drive gear , which sounds to me , like its buried deep inside the tranny, and hard to get out? Just what do I need to keep the speedo accurate, and is it a major pain to pull the drive gear out? Or do I even have to? Thanks in advance for your kind help! Kev

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Guest kamakazi620

Hello Ratsun Experts, Well I followed your fine advice today that I had read on Ratsun, and managed to remove a 'third member' from a scrap yard 1983 two wheel drive 720 ( 3.889 ratio) to swap into a 76 Datsun 620 I have.(It was surprisingly easy to do!)

I forgot to remove the speedometer gear,and now at home, I am wondering if I need to remove both the speedo gear AND the speedometer drive gear , which sounds to me , like its buried deep inside the tranny, and hard to get out? Just what do I need to keep the speedo accurate, and is it a major pain to pull the drive gear out? Or do I even have to? Thanks in advance for your kind help! Kev

Just the thingy that goes in the trans the gears inside are all the same

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Unfortunately the speedometer pinion in the 720 tranny won't fit your 620? Well it will fit, but will be upside down because the way it is mounted in the tranny changed in '80. Here's why...

 

Your tranny

280zzxFS5W71Btail003Large.jpg

 

Most post '80s trannys

post80sFS5W71BtailLarge-2.jpg

 

See the 10mm head bolt? Pre '80 trannys are at 12:00 and after are at 6:00. Here are pre and post '80s pinions. They are similar but are offset 180 to each other

 

transcogs.jpg

 

 

If you have a steady hand and an angle grinder with a cutting disc you could do this yourself. This would allow the 720 pinion to be installed upside down. Or you could maybe have a shop cut the retainer slot 180 around the pinion like this...

 

pre80pinionint0post80ransmods001.jpg

 

 

 

 

 

 

.

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Thanks a lot, Kamakazi and Mike! This is just what I needed to know, the important thing , is , I dont have to go ripping the tranny apart in the scrap yard, I just need to pull out the pinion part, right? Once its home I will compare them , I think Mikes pics will then make sense to me , it almost does now.I am really laughing at myself today as I 've owned my truck from brand new, and went 30 years totally oblivious to the fact the 720 rear ends

would interchange, I must have walked by dozens or hundreds of them in the scrap yard through the years, if only I had known! I have another 620 which is a 77 with automatic ( probably a 4.375 rear end) I am contemplating putting in a 3.9 or 3.7 into it to boost the highway mileage , I wonder if anyone has ever tried it? I have never been a racer , so being sluggish off the line would not bother me, unless it was totally anemic..thanks once again for the info,Kev

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Quick question please Mike, can you drive the roll pin out of both pinions & swap the gears & while there replace the seal part #32709-14600 & reassemble? Also if you want to replace the o-ring the part # is 32710-14600.

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And that is the question!!!

 

Different gears have different diameters so if you go to a smaller gear the teeth may not mesh. A larger gear may crush the gear. Everything I've read and a couple of discussions say you can but the above statements say it won't work.

 

This may be worth trying, putting the 720 cog into the 620 pinion housing. I didn't suggest this because of breakage. I have always changed as a complete unite.

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I am going to try it on a 3N71B automatic. 1976 620 transmission with a 1985 720 plastic gear (Blue in color). I will let you know the out come when I get mike truck on the road. I will do a write up on this whether it works or not.

 

This what I found for Z cars listed By Dale on Desert Datsuns Remember tire Diameter pays a part on the ratio Ex. Z cars run smalleer outside diameter tires stock than trucks do.

 

Yellow - 16 Teeth 3.36 gears

Black - 17 Teeth 3.55 gears

Blue - 18 Teeth 3.70 gears

White - 19 Teeth 3.90 gears

Red - 20 Teeth 4.11 gears

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There are also 21 and 22 tooth used in the 720, don't know the colors. BTW always count the teeth to be sure.

 

Good idea Charlie, let me know. The auto and standards look alike.

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Hi all. I am extremely new to the Datsun world and the Ratsun community. I was wondering if the pinion gear in the transmission has to match the rear end gear ratio for the speedometer to be accurate? Any help is welcomed and appreciated. Thanks.

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No. The rear end gear and the speedometer gears in the transmission will always be different.

 

You can change the speedometer driven gear to adjust the speedometer. This is necessary when changing 1) Tire size 2) final gear ratio [ring & pinion]

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We're probably talking about the same thing here. There is a removable speedometer drive inside the rear of the transmission in the tail stock that is available with various number of teeth on the driven gear. It is matched to the tire and gear ratio to have the correct speedometer reading.

 

Your '76 should have a 20 tooth pinion drive in it to match your 4.375 rear end. If you were to swap a '79 4.11 rear end in you should  pull the pinion out of that transmission (a 19 tooth) to keep your speedometer correct.

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Yes, grab the gear if you get a different pumpkin.

 

But it only works if the tires are stock size (and who knows what size tires a vehicle in a junkyard used), and if the speedometer was close. Many are off enough to go to the next size speedometer gear.

 

Best is to check the speedometer indicated speed against a GPS speed, and then you know which gear you'll actually need.

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  • my 75 620 has a 5spd swap, but the swapper didn't swap the speedo gear. I pulled one from a 74 620, the gear is red, and much larger than my white gear. It wont go in, and I didn't want to force it. help!!!

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Does the transmission have a normal

1.3.5

2.4.R pattern?

 

Is the bolt holding the speedo pinion in at the top

280zzxFS5W71Btail003Large.jpg

 

Or at the bottom???

post80sFS5W71BtailLarge-2.jpg

 

.

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Good, then this is an easy fix...

 

Get or borrow an angle grinder with a metal cutting blade or find a cut off saw or equivalent. I suppose a few passes with a hack saw would also work.

 

If you look on the body of the pinion there is a slot for the locating and lock key that is held by the bolt.

 

pre80pinionint0post80ransmods002.jpg

 

Simply cut another on the other side 180 degrees around. I did mine with a 6" hand held angle grinder.

 

pre80pinionint0post80ransmods001.jpg

 

This will allow the pinion to be inverted 180 degrees and engage the worm gear on the mainshaft properly.

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Well since this thread was awakened from a long nap before, I figure why not. ( at least I used the search thingy)

 

I just put 3.7 gears in the 620, where there was 4.37's. The truck is an auto and the diff came out of a stick 720. I neglected to grab the drive gear from the trans. Should I waste gas and go back to get it, or will a stick drive gear not fit a Jatco anyways?

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The sleeve that you pull out is different I believe but if you knock out the roll pin you can extract the pinion with the plastic gear on it. This spindle will swap into the '77 sleeve.

 

3.7 is about 18% different. The '77 auto should have a RED 20 tooth pinion. A BLACK 17 tooth pinion is 17.6% different so that would be close enough. You could try the dealer for part #32703-P2617 about $12

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I realizes this morning I have a jatco from an 81 ZXT. I am pretty sure they had 3.54's. But when I pulled the drive assy out, it was a red 20 tooth. I pulled the trans myself too. Weird.

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Yes this is possible. It's not only just the rear gears but tire size also affects it slightly.

 

So you are saying that the 620 has a zx automatic in it with a 20 tooth pinion gear? When it had the 4.375 differential was the speedometer fairly correct?

 

Now that you have the 3.70 differential Can I assume that the speedo reads lower than actual speed?

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The 620 has the original trans. The ZX jatco was in my scrap pile. I thought I had the issue solved till I saw the red 20 tooth gear. I haven't removed the gear from the 620 yet. And yes, the speedo is way off. I'd say 10 to 12 off on I5.

The ZX was a Colorado car. I wonder if it had a lower R and P. I still have the 3rd member. but it's buried.

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If the speedometer is off a % at all speeds, changing the speedo driven gear (at the transmission) will affect it. But if the speedometer is off the same amount at 30 mph as it is at 60 mph, then a gear change won't fix it.

 

At 60mph, 10 mph off is 1/6 (18%). Next question: Is it reading 10 mph too fast, or 10 mph too slow? Changing the gear to one closest to 18% difference in the number teeth is the way to go.

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If the 20 tooth transmission gear was correct for the 4.375 diff, then changing to a 3.70 is an 18.2% change. A 17 tooth gear is 17.6% different than a 20 tooth and should be close enough.

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Reads lower than actual speed. Gets progressively worse the faster you go.

 

Truck is 137k original. worked fine last Friday. I swapped the diff but forgot to grab the drive gear. I found 2 black 17 tooth gears but they are both bad. They are also quite a but smaller than the red 20 tooth. not sure if one would still mesh in the housing that the red came off of. The red housing appears to be offset more than the black gear housing. Might have to hunt down a 720 with a jatco and gears that are close. 

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