Speedo pinions, help!

[copacetickid]

Hey everybody, I did a tranny swap on my recently purchased '79 620 (bought it from DatDoug :D) to give it some better highway mileage. Now I bought a 5-speed from a kid in Renton that came out of an '81 280zx. I'm trying to figure out which is the right pinion to find so that the speedometer reads correctly and where/how to find it. The speedometer pinion that came out of it the 5-speed, totally mangled for some reason, is yellow and has 16 teeth. The pinion that came out of the stock(?) 620 4-speed is white and has 19 teeth.

 

From what little I've been able to find on the subject, what pinion you use depends only on the ratio in the rear diff. However, since (and I could be wrong here) 620s only came out of the factory with a ratio of 4.38:1, the white cog in my 4-speed doesn't even make sense. According to that site, I should have a 21 tooth purple one in my 4-speed. So at first I thought, 2 teeth, that's a small enough error for me, let's put 'er in! Except it turns out that the white pinion has a larger diameter than the yellow one, and it won't fit. BAH.

 

So here's where I need all you guy's help. Why are the pinions different diameters? Does this mean that if I call up the nissan dealership I'll get a purple pinion that is even bigger than the white one? Is purple really the one I need anyway??? Help!

[datsunaholic]

No, 620s came with several rear gear ratios. You have a '79, which came with 4.111s.

 

Use the speedo cog assembly straight out of your 4-speed in the 5-speed. It fits. It works. You have to put the whole assembly in though, not just the cog (the assembly sets the preload and offset). I'd be more concerned with why the original was mangled though.

 

I've done this, used an '82 ZX tranny in my '76 using the '76 pinion assembly.

[]2eDeYe]

Nothing more to add, but that's a great link...

 

Thanks :D

[copacetickid]

Thanks for the reply Datsunaholic. Good to know about my rear diff too. But I'm telling you, the white pinion doesn't fit! I tried this originally and what happens is you plug it in rotated 180 degrees and turn it to engage the gear on the output shaft, but the gears hit prematurely and the slot won't line up correctly with the retainer.

 

I thought it was the pinion housing, so I called up the guy I bought it from and he sent me the original housing out of the 5-speed (might be why it was mangled). But when I got it, it turns out that the housings are completely identical. That's when I looked at the pinions and discovered that there's about a millimeter difference in the diameters.

 

Before I even got the original housing though, I figured that the pinion would engage well enough if I just cut a new slot in the housing that is offset at the right angle for the retainer to fit. Well, that might have worked, but the f*ing machinist who did it f*ed it up royally (on top of treating me like a 12 year old). Should I just try this again with the other housing I have?

 

(what I'm thinking is that there may be two different sets of colored pinions -- one for 5-speeds and one for 4-speeds with different diameters. I don't know)

 

Thanks for your help guys!

[Bleach]

The link shows cogs for Z-car applications. Keep in mind your truck came stock with taller tires.

 

So a tall tire + 4.375 is basically the same as the Z-car's smaller tire and 4.11 diff. (or really close to it) That is why there is a difference. Use the 4-speed one from the original 1979 transmission or get another cog the same. That is a slightly rare axel ratio you have. Available only that year in the 620.

 

Most 620's had a 4.375 ratio but not all.

 

I don't actually think the white one is correct for your truck. Purple is more likely to be the correct cog to use.

Go to the Tacoma wrecking yard and pull the cog from a truck there. Make not of its year. If its a 1975-78 truck then they are all 4.375 rearends.

Maybe just use that cog or at least it will tell you what color the 4.375 comes with. Use that page's scale to move to the next higher ratio cog. If the truck in the yard has a purple cog, then you definitely need a red one.

[Bleach]

when I swapped to a Z transmision in my 620 I couldn't get the truck cog to fit in there correct either. I just turned it so the teeth engauged and put the retaining screw in next to the tab but not through the proper hole. It worked... but I also never found the correcrt cog to make the speedo read right. I had smaller than stock tires and a 4.11 rearend swap. I was trying to calculate what cog I needed to make the speedo read correct with all things factored in. I figured the stock one should have worked. (taller rear gear but smaller tires should equal each other out) ...but somthing wasn't right. oh well, the truck is gone now anyway

[240ADAM]

You should go talk to Ollie at Zspeciaties in Clearveiw he should have all the speedo gears in stock and he knows witch one you will need.

[copacetickid]

Thanks for the posts guys. It didn't even occur to me to hold the retaining tab on without the screw through the hole. I'm going to go try that now :)

 

I also have a feeling that trying to find a pinion that fits AND is right for my kinda-rare-rear-diff is futile. Lets hope that smaller tires and a larger pinion will cancel out and not produce *too* much error.

[JWilk_SD]

On the subject. I have a 520 with 3.89 rear end. What gear would I need and any ideas of where to get it Thanks

[Bleach]

On the subject. I have a 520 with 3.89 rear end. What gear would I need and any ideas of where to get it Thanks

 

What is the factory tire size for a 520? That's the first thing I need to know.

If its close to the Z-car size (195/70/14 or 205/60/15 ) then look for a 1981-83 280ZX non-turbo. (common) those came stock with a 3.9 rear gear. Pull the cog from there. I think it will fit your transmission but I'm not sure. Yours would not have an L-series engine or tranny. (unless you swapped one in there)

 

:)

[datzenmike]

2eDeYe;20175']Nothing more to add' date=' but that's a great link...

 

Thanks :D[/quote']

 

Agreed! I knew there was a difference in the 5spd cogs but it was never clear to me until now. The light is on!

[JWilk_SD]

Thanks. I believe the stock tires are 500 /14's so that pinion should work. Looks like I'm going to the local yards tomorrow.