thinking about a diff swap for my '74 620

[pr3yanimal]

having scrounged around this forum for a good few weeks while researching a truck i was about to buy (for my first vehicle at that), and having finally pulled the coord... i'm hoping to make some small modifications to it already. i've been having a hell of a time finding a 4.11 or even 3.889 rear differential out of a later year 620/720 online anywhere, n i figured this place might be one of my best bets.

 

i've got potentially a move cross-country in the future, so the big thing up first for me would likely be shifting the RPM down when at highway speeds as much as possible, with a better final drive gearing. i've had a hell of a time even finding the appropriate ring & pinion online and even if i could, i doubt being able to find someone who can do that job for me— and i'd be a little nervous about that being one of my earlier jobs myself.

 

presumably the easiest/best option would be to find an H190 in the 4.11 so i'm wondering if anyone on here has one i'd be able to buy off em, n' if people had some ideas otherwise...

 

i read that the 3.889 would be a little rough given the top speed of the truck already being on the lower end of things, so hopefully the 4.11 is a decent middle-ground for not grenading/overheating the l18 still in it on a long road trip if i'm headed back to the south eventually. 😛

[datzenmike]

A 4:11 won't make you go faster. Top speed is determined by engine RPM and the power it makes vs. wind resistance. If your top speed is 60 MPH with the gas floored with your 4.375 differential your RPMs will be 3,392. With a 4.11 differential the engine will only be turning 3,186 RPM at the same speed. At 3,186 RPMs you will not be making enough power to maintain 60 MPH and the speed will drop lower.  

 

I don't know how many times people have complained about how high their engine rev when driving on the highway. The L18 can rev to the moon easily and without problems if kept in tune and filled with oil and water. Nissan designed the '74 so it would make enough power to maintain highway speeds and carry 1,100 pounds of cargo. To maintain highway speeds it has to rev, it won't hurt anything. The L18 engine is not a power house but it can rev. You should be able to maintain over 70 with it. If top speed is limited to less than this maybe a good tune up is needed or the engine is just tired. Perhaps the secondary in the carburetor is not opening? Valve clearances need to be checked and set, timing needs adjusting, points are worn and need replacing, cap rotor and wires are in need of replacing. All these can cause lack of top end power.

 

I would not be replacing the differential. 

[ggzilla]

I drove my Datsun @5000 RPM for hours at a time and it did not grenade or overheat. That's far below the red line

 

On the other hand, if you want to reduce noise and increase fuel economy on the highway, install a 5-speed. Just be ready to pull it back into 4th on every hill or when passing

 

 

[datzenmike]

Agreed! I climbed a mountain road for 15-20 min. in second. Wouldn't quite hold speed in third. Kept pushing on the gas but it was already floored. That was close to 5K. I knew it was in tune, had full oil and water and temperature was just over half where it always runs so just sat back. 

 

The red line for a stock L18 is just a hair under 8,000 RPMs. Below this you can run all day (not that you should) and not exceed the metallurgical strengths of the rotating components as long as in tune and full of oil and water. That's twice what regular highway speeds endure.  

[pr3yanimal]

7 hours ago, datzenmike said:

A 4:11 won't make you go faster. Top speed is determined by engine RPM and the power it makes vs. wind resistance. If your top speed is 60 MPH with the gas floored with your 4.375 differential your RPMs will be 3,392. With a 4.11 differential the engine will only be turning 3,186 RPM at the same speed. At 3,186 RPMs you will not be making enough power to maintain 60 MPH and the speed will drop lower.  

 

I don't know how many times people have complained about how high their engine rev when driving on the highway. The L18 can rev to the moon easily and without problems if kept in tune and filled with oil and water. Nissan designed the '74 so it would make enough power to maintain highway speeds and carry 1,100 pounds of cargo. To maintain highway speeds it has to rev, it won't hurt anything. The L18 engine is not a power house but it can rev. You should be able to maintain over 70 with it. If top speed is limited to less than this maybe a good tune up is needed or the engine is just tired. Perhaps the secondary in the carburetor is not opening? Valve clearances need to be checked and set, timing needs adjusting, points are worn and need replacing, cap rotor and wires are in need of replacing. All these can cause lack of top end power.

 

I would not be replacing the differential. 

oh i didn't think it would make me go faster, i'd read elsewhere on here that the differing final drive would make it generally /slower/ actually, but thankfully i've not had to worry about top speed issues. my biggest issue rn is my fuel gauge/temp gauge not working, but i've been sick with covid so i couldn't get to it quite yet lol

 

but noted, i always tend to play it a bit cautious with things but having the actual RPM difference at that speed shown/explained reassures me quite a bit. i appreciate it! 

 

i def need to go get the carb checked though, but i'm waiting on a reducer ring to fit a 76mm carb filter to 70mm since i wasn't able to find a properly sized one online at ALL under like, $200 for the original OEM one. v.v

[pr3yanimal]

5 hours ago, ggzilla said:

I drove my Datsun @5000 RPM for hours at a time and it did not grenade or overheat. That's far below the red line

 

On the other hand, if you want to reduce noise and increase fuel economy on the highway, install a 5-speed. Just be ready to pull it back into 4th on every hill or when passing

 

 

5-speed + KA24 swap is something i have planned for the future for sure! it was either a 350 or a 370 transmission that the original seller told me is a good option for those.

 

also yeah re: the RPMs, my first few drives had me a bit paranoid just from engine noise. i'm guessing the motor mounts have never seen replacement, and the cabin being completely bare i imagine that's transferring a lot of what i'm hearing directly there.

[banzai510(hainz)]

28 minutes ago, pr3yanimal said:

it was either a 350 or a 370 transmission that the original seller told me is a good option

So good why didnt he do it

 

 

dont you have to go thru emissions in Calf?

 

I would just leave it alone as the prev owner didnt do it either and actually kep this still alive. This is your 1st car/truck??????????????new to vehicles? best to have a garage and time and another vehicle . swapping a axle is kinda ez but Hooking up the parking brake can be a big deal as I have found out then a wheeel cylinder bad and then the lowerering blocks worn out. You know drum brakes? How to adjust them ect.???

 

Just enjoy the vehicle if its running and beautiful stock condittion!!!!!!!!!!!. My 521 hates the hwy but I drive rural roads and retired so no rush going anywhere. Just my opinion.

Edited July 29, 2025 by banzai510(hainz)

[pr3yanimal]

21 minutes ago, banzai510(hainz) said:

So good why didnt he do it

 

 

dont you have to go thru emissions in Calf?

 

I would just leave it alone as the prev owner didnt do it either and actually kep this still alive. This is your 1st car/truck??????????????new to vehicles? best to have a garage and time and another vehicle . swapping a axle is kinda ez but Hooking up the parking brake can be a big deal as I have found out then a wheeel cylinder bad and then the lowerering blocks worn out. You know drum brakes? How to adjust them ect.???

 

Just enjoy the vehicle if its running and beautiful stock condittion!!!!!!!!!!!. My 521 hates the hwy but I drive rural roads and retired so no rush going anywhere. Just my opinion.

guy who sold it to me was a trucker, didn't have time to fully work on it anymore and was happy to give it to a girl who actually had the passion and a bit of know-how. as for emissions, the truck's old enough now i ain't gotta worry about passing smog. it juuuust passes the mark (1975 and earlier, though next year, thanks to Jay Leno, it's any car older than 35 years.)

 

first vehicle yeah, but not new to em. my grandfather worked on old harleys and trucks post-2005 before he passed, and my dad's gone through a bunch of JDM cars. s13 240sx, 2012 WRX STI, now working on a sleeper build 9th gen Civic. never really had hands-on experience with his specifically, but been able to watch him work on it. he's been a help so far, but i might be on this forum a good bit trying to figure out solutions for a buncha (i assume) wiring troubles, so i hope i don't get too annoying lol

 

also hoping to just keep it to routine maintenance + catchup on stuff left to sit for nearly 50 years. it's only had two owners and was mostly a ranch putter or daily. miiiiight swap out the drum brakes ASAP tho. they're feeling a bit dead rn.

[banzai510(hainz)]

when it comes to Datsun wiring its the previuos owner who fucks it up If orginal its mostly corrossion dirty fuse box bad grounds. All fixable quickly. So no hearing I want to put a Painless harness in cause its More than Painfull!!!!!!!!!!!!!!!!!

 

I just find a bumber for the truck and get a electronic ignition and lower it and call it good, Get soem Belltech 6202 2deg(3inch) blocks and turn down the torsion bar. all can be done in a day and still drive it.

 

You get just as many thumbs up with a L motor than a KA install that never gets done.

[pr3yanimal]

2 minutes ago, banzai510(hainz) said:

when it comes to Datsun wiring its the previuos owner who fucks it up If orginal its mostly corrossion dirty fuse box bad grounds. All fixable quickly. So no hearing I want to put a Painless harness in cause its More than Painfull!!!!!!!!!!!!!!!!!

 

I just find a bumber for the truck and get a electronic ignition and lower it and call it good, Get soem Belltech 6202 2deg(3inch) blocks and turn down the torsion bar. all can be done in a day and still drive it.

 

You get just as many thumbs up with a L motor than a KA install that never gets done.

okay see i was thinking that with the wiring!! i found an unmodified dash harness on ebay i was tempted to grab just as a way to start from scratch from the rats nest thats in it currently... but i'm only doing that if i can't handle whats in there now. thankfully my roommate did electrical stuff back in the military, so i can get their help with soldering anything i might need.

 

already lowered it 3" and she's sitting pretty now, and ordered a new ignition for it that im waiting to come in. 

 

engine swap would def only come if i got another vehicle... this is going to be my daily for awhile so not doing any jobs that have it outta commission for more than a weekend.

[datzenmike]

 

This is the go to bible of Datsun 620 manuals for fixing things... https://www.ebay.ca/itm/286097243289?

 

 

 

At $20 it's a steal. This one manual deals exclusively with the '74 nothing about the '73 or '75-'79 trucks, only the '74. Has all the specs and how to fix everything else, diagrams and best of all a complete electrical diagram for manual and automatic transmission trucks. Highly recommend you get one and keep in bathroom. Read every morning. Info in... garbage out. It will answer almost all your questions.

 

Forget the Haynes manual. It's half the size and condensed down to fit mostly useless stuff from the other 6 years of production. Yours is a '74 the only year with the L18 engine.

 

All you can do for drum brakes is replace the shoes, or any leaking wheel cylinders, bleed them and adjust them. Once set up four 10" drums will stop you pretty fast. It's designed to stop carrying 1,100 pounds in the back.

[banzai510(hainz)]

lamps dont work? but sidemark.parking work. you ck fuses wiggle them. does lamps work w highs.then look at the relay. Why you need a harness?blinkers cold be broken bulbs or rusted sockets.

[ggzilla]

21 hours ago, pr3yanimal said:

i found an unmodified dash harness on ebay i was tempted to grab just as a way to start from scratch

 

there were three different dash harnesses for 1974 alone, and every year they are different

 

1974 HARNESS-INSTRUMENT

 24013-B6502 AUTO early

 24013-B7910 AUTO late

 24013-B8000 MANUAL