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Datsun 521 L20B

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Im not  diff expert on the trucks but assume it would fit but don't know if that is SAE threads or Metric going from 521 to the 620.  example the leaf spring U bolt/ nuts might be different


as for the head gasket you put a new one on there and will assume its good.Not really hard to fuck that one up unless you actually see water coming out from there.


One really should have the head pressure tested but I have put heads on and they worked.


I used to ck my plugs and if the gasket is bad on the intake side the spark plug will be really clean. If bad on exhaust the radiator will get hot.  Or start truck when cold and actuall see the leak happen.

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...good to know, is this left when you are at the front of the car?


For a vehicle, left and right is always as looked at from the driver's seat.


I will definitely stick with the L series motor for now, lots of variables and I'm pretty invested at this point.


I did a compression test on cylinder 4 and I got a reading around 126, and I would have liked to believe it came from the heater hose but I double checked. (leak was coming from spark plug side)


Will take some photos tonight


Are 620 diff's interchangeable? I have access to one that came with a 4-speed... did a quick search and it's seems it will work but I'm not 100% certain


What would you guys do? Should I pull the head and inspect? Pull the engine and throw in a different one? (will have to source one)


No lectures on doing it right the first time, I really thought I did it right..


I am running a 5-speed out of a 280z




It's very possible that the gasket could leak fluid but the cylinder is still sealed.


Your 521 will have a 4.375 ratio differential. The 620 differentials will fit and were....


'73-'78 manual.................. 4.375

'79 manual........................ 4.11

'73 automatic.................... 4.625

'74-'79 automatic.............. 4.375

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When u did compression check, did you use plug in type or screw in type? Plug in type is not that accurate and you get lower reading.

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I like the 3.899 rear end pumpkins out of the 80 720 regular cab 5 speeds.  All you need is the third member (pumpkin).

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The 720 is even easier. The differential ratio is stamped on the credit card sized engine tag located on the passenger side inner fender below the hood hinge. Bottom line will say ...


TRANS/AXLE............. FS5W71B.......... HL38  (for a 3.889, HL41 for a 4.11, HL37 for a 3.70 etc...)

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I found the motivation to pull the head, get it resurfaced and slapped it back together and I couldn't be happier. I drove the truck all week back and forth to work and as far as the engine and transmission goes, it's seems to be very healthy. It went back under the knife this weekend to pull apart the rear end as it has a whine. I originally thought it was my differential but it's looking like the main culprit is my wheel bearing. This is my first time taking this apart and the manual isn't very descriptive. How do i separate the bearing from the shaft?







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Are you doing bearings and seals?

I'm curious about what parts you get and how they fit.... I need to do my axle seals and bearings soon....

So did the axle just slide out after removing those 4 bolts, did you have to remove the actual differential?


I'll be following this repair....

Looks like you need a spanner wrench or maybe a hammer and punch can loosen it....

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I've found that wheel bearings have a rough growl or rumble sound. My 620 (which is similar) did it only on right turns. Whines can be the tires

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The axle bearing is pressed off and the new will be pressed on.  Be sure that the new bearing is packed with good quality high temp wheel bearing grease.  These rear ends are semi floating which means the rear end lube may not always get to the axle bearings so the axle bearings need to be greased like your front wheel bearings.


Does the axle bearing feel rough when you turn it?


Did you happen to check for excessive play before pulling the axle?




Here is a link to CarPartsManual.com



I hope this helps.

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I pulled apart the rear end, it was surprisingly simple. Access to an accommodating press made it easy. The bearings were in good shape, repacked, seals replaced and installed. Something failed in the diff. Installed the one from the 620 and the rear end has never sounded better. We made a trip out to Calgary and made a vacation out of it. The truck did it's longest trip, over 200km round trip and it drove well. As like any project, it still isn't finished. But there has been lots of challenges with this build and I'm happy to say it's at a "turn key" point and I will just enjoy it for the time-being. Thanks for all the support and advice.
















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Nice steering wheel!

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Seeing as this is my only build thread, and this is era correct I thought I would share with everyone. This winter I built a 71' Corolla and installed a 3TC and 5-speed.










































As far as the 521, I installed some badges, cleaned up some loose ends and put it back on the road for the season.
Installed this fitting license plate



Does anyone have a connector for the rear turn signal available? I threw mine out and they aren't easy to track down.



Edited by Donedreamin
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Just found this thread and went thru it all. Bad luck with the motors, but guess that happens. You seem to have no fear tackling stuff. Like the truck. Cool vids as well. Gotta 1300 521 myself. Any recent work done on it?.......seen the plates, you didn’t happen to sell a 521 bumper recently did you?

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