Rear Diff ratio options for my 620

[wildmaninid]

Now that the KA swap is complete in my 77 620 longbox, I really would like some taller gears to take advantage of my new power. I would like to get down to 3.70 or thereabouts, but anything below 4.00 would be great.
Thanks for reading and for any ideas!

[DailyDato]

I have a 77 shortbed that I am finishing up, I put 3.70s in mine, from a 84 720KC um Z24 five speed, they will swap, just pull urs and stick another in, same amount of bolts, but be careful, i kinda had to hammer both of mine apart, kinda press fit into each other, but you can also find ratios within 3.37, 3.70, 3.89, and stock 4.38, but yea 720 parts, and i would also assume other H190 diffs would work, if the spider gears and axle splines are the same, and not too sure about driveshaft yokes, but always a thought.....

[wayno]

Almost all the 720 2wd carriers will bolt in, there are a few in the later years that are designed like the hardbody that will not. 389 and 370 gear sets are the most plentyful, with 411 being in some of the early 4X4s, and a few 350 somethings in automatics?

[wildmaninid]

Awesome info guys! So when at the junkyard looking at 720's is there a clear mark on the tag as to what ratio the rearend is, or do I need to just pull it and do the math so to speak? I can't wait to get this going.

[DRIVEN]

Check the tag on the firewall. The axle code would read "38" for a 3.89 and so on. I know that there is more info on here somewhere.

[SR20-620]

wont you also need one of these?

 

http://community.ratsun.net/topic/34706-precision-gear-power-brute-lsd-for-your-h190-truck/

 

 

[wildmaninid]

wont you also need one of these?

 

http://community.ratsun.net/topic/34706-precision-gear-power-brute-lsd-for-your-h190-truck/

Negative, ghostrider. If they were current production I would take a chance, but here is what will happen: IF they start reproduction, it will be in limited numbers. WHEN there is a problem down the road, replacement parts will be NLA, just as they are now. I also don't want to spend that kind of coin on a diff, as it is more than my entire KA swap cost me. I am fine with a higher ratio and a one tire fire.....never let me down yet, and we have been using datsuns here in the northwoods as long as I have been on this earth, overloading, abusing and thrashing the drivetrains, and I have never come across a datto diff that has been bad that wasn't properly serviced. We used to put a cord of wood on our kingcab (between truck and trailer) 4 times a year for 16 years, 22 miles up in the mountains. Thanks for trying to get more options for those that are looking for them, but I will be just fine!

[yello620]

Wild, the 720's have a tag on the right side fenderwell near the hood hinge. The tag list the axle code. The 720 had several gear options too, 3.54(have not seen this tag yet), 3.70, 3.90, and maybe 4.11. The 79 620's had a 4.11 gearset.

 

I have had all of these in my 620 in the past few years with my L20b. I swapped out the 4.37 gears for 4.11, not enough change(waste of my time). Then i went for a big change and swapped a Set of 3.70 gears, that was a huge change, yet a little too much for the L20b/4speed. So i traded those gears for a 3.90 gearset, perfect. They were great, later i added the 5spd too. Still could cruise down the highway without a problem, and accelerate uphill at highway speed in 5th gear. Excellent combination. I believe i was at about 3000 rpm at 70 mph. And 4000 rpm put me a little over 90 mph.

[yello620]

Negative, ghostrider. If they were current production I would take a chance, but here is what will happen: IF they start reproduction, it will be in limited numbers. WHEN there is a problem down the road, replacement parts will be NLA, just as they are now. I also don't want to spend that kind of coin on a diff, as it is more than my entire KA swap cost me. I am fine with a higher ratio and a one tire fire.....never let me down yet, and we have been using datsuns here in the northwoods as long as I have been on this earth, overloading, abusing and thrashing the drivetrains, and I have never come across a datto diff that has been bad that wasn't properly serviced. We used to put a cord of wood on our kingcab (between truck and trailer) 4 times a year for 16 years, 22 miles up in the mountains. Thanks for trying to get more options for those that are looking for them, but I will be just fine!

 

 

Agreed, no need to swap in a posi that will be NLA before we even receive them in the mail.

[wildmaninid]

Yello, thanks for the insight! I think I am going to go with the 3.70 since I now have the ka, and plan on a lot of road trips. And hey if it don't work I can go with something out.

[yello620]

I bet the 3.70's work really well with the extra power of the KA, but they were a little too much for the L20b. I may end up changing gears again too. Time will tell, need to see what it does with the VG in there. Should be running soon.

[Z-train]

I have a set of 3.89s ready to go behind the stroker.Can't wait.

 

The ratio is stamped on all the ring gears.

[yello620]

The ratio is stamped on all the ring gears.

 

Good point, checking the fenderwell tag allows you to decide whether to pull a gear set or not to pull, but when you do get them pulled, make sure that you verify the ratio. As Z-train said, it is stamped on the ring gear. But it is stamped like 37:10. Divide 37 by 10 to find you actually have a set of 3.70 gears.

[datzenmike]

They had 3.364 in the later Mileage Option trucks and the 4.11in the '81-'82 Z22 2wd long box automatic.

[wildmaninid]

Wow, 3.364! I think that may be a bit too high for my application I think....I need to find out what ratio came out of the hardbody I used for my drivetrain donor, I wouuld like to match that as close as possible, as dad was getting 30 mpg on the hiway with his truck. Thanks for the info Mike.

[yello620]

Your HB donor just might have the H190 axle in it. If it does, you should be able to use it. However some of the later H190's had a longer pinion flange. Have read that you can swap the short pinion flange on, but never tried it myself.

 

You didnt throw away the donor truck yet did you. :D

[wildmaninid]

No I didn't, I still have it. I should probably check on that today!

[yello620]

If the diff looks similar, it is the h190. The ones that do not fit have a removable cover on the rear of the housing, like a GM 10 or 12 bolt.

[wildmaninid]

Well I went and checked the donor truck. The tag on the door says the axle is an "hf35". The rear end looks like the 620, no bolt on cover in the rear. So is/can this be an option for my truck?

[wildmaninid]

Well today I went ahead and pulled the 3.50 carrier from my donor hardbody and swapped it into my truck today. I went from 3000 rpm@ 60mph to 2300 rpm@ 60mph! Exactly what I was looking for! And my speeds is now dead on, since I have the complete drivetrain from that hardbody front to back! Thanks for the info and advice, Ratsun wins once again!

[Wide14u]

my truck idles higher then that :D

[wildmaninid]

LOL how high does it get Wide? You can just let out the clutch and roll with that!!

[Wide14u]

its more like a cruise control :rofl: im so lazy

[josh817]

Hey Wildmaninid, was curious if you had any pictures of the swap, if you did it in the truck, if not was it easy to drop out from the truck?

 

Someone put the bug in my ear to re-gear my 521 but I'm not sure if it is economical, or if I can get it done in a weekend; all of this of course is if I have the same interchangeability that you have...

[Z-train]

Re-gearing is not a good idea if you don't have a corresponding increase in HP.The factory gears were matched to the HP the motor was making.Putting taller(numerically lower)gears in a stocked motored truck will turn it into a PIG.